Topic: Climate Considered Especially in Relation to Man 1908/1918

[Prometheus]

From https://archive.org/details/climateconsider03wardgoog/page/n381/mode/1up
by Ward, Robert DeCourcy, 1867-1931
1918





PREFACE

TTHE preparation of a volume on Climate for The
?   * Science Series was suggested to me by the

|-   Editors in Octóber, 1904. I was asked to prepare a

j   book “ which can he read by an intelligent person who

has not had special or extended training in the tech-
<   nicalities of the Science, . . . the book to be such

as would not compete with strictly meteorological
text-books, but to handle the broad questions of
climate.” It so happened that it was then already in
my mind to prepare a book dealing with certain large
relations of climate, which might serve as supple-
0 mentary reading for the students in my course on
General Climatology in Harvard University. The
present volume is an attempt on my part to write
a book which shall meet the wishes of the Editors of
The Science Series and at the same time fit the needs
of my students.

Climate is based on lecture-notes which have been
accumulating for the past ten years. It does not
attempt to present any very new or original material,
but it does aim to co-ordinate and to set forth clearly
and systematically the broader facts of climate in
such a way that, as desired by the Editors, the gen-
eral reader, although not trained “ in the technicali-
ties of the sdence,” may find it easy to appreciate

iii

I
  iv

PBEFACE

»

them. At the same time, the needs of the teacher
and student have been kept constantly in mind, and
the subject-matter has been arranged in such a way
as seems best to adapt it for purposes of thorough
study.

Climate may be considered in a way as supplement-
ing the first volume of Dr. Julius Hann’s Handbuch
der Klimatologie, an English translation of which
was prepared by me and published in 1903. In that
book, the Standard work of its kind in the world, the
principles of climatology are clearly set forth. My
present volume deals with matters which are either
omitted altogether in the Handbook, or else are very
briefly treated therein. Climate is wholly independ-
ent of Hann’s splendid work, except in so far as my
study of that book inspired me to prepare this one.

The general scope and purpose of the different sec-
tions in Climate are as follows. The Introduction
is essentially a very condensed synopsis of the first
six chapters of Hann’s first volume, with the addition
of some other matter. Chapter I gives a sketch of
the classification of the zones. Chapters II and III
give a brief summary of the general climatic types
which result from the control of land and water, and
of altitude, over the more important elements of
climate. Chapters IV, V, and VI are intended to
give an outline of the climatic characteristics of the
zones in a simple and vivid form, with the least pos-
sible use of tabular matter. For further general in-
formation on this subject, reference may be made to
  PBEFACE

T

the world-charts of temperature, winds, cloudiness,
rainfall, etc., given with greater or less completeness
in the various text-books of meteorology, and, very
fully, in the Atlas of Meteorology. In Chapter VII
the attempt is made to give a survey of some of the re-
lations between weather and climate and a few of the
more important diseases. Little information on this
subject is readily accessible to the general reader.
The life of man in the tropics, the temperate zones,
and the polar zones is considered in Chapters VIII
to X. No attempt has been made to discuss this
subject in detail, for to do so would far exceed the
limits set for this book. It has rather been my plan
to piek out typical illustrations here and there, as
suggestions. Many of the cases referred to will
probably be familiar to teachers and students of
geography, but the co-ordination of all the examples
by climatic zones and by the natural climatic sub-
divisions of these zones will, it is hoped, tend to give
adequate emphasis to the climatic factor, which has
hitherto been much neglected. The final chapter, on
changes of climate, deals with historie and periodic,
and not with geologie changes. The last phase
of the subject has been fully discussed in many books,
while the former, which are of more interest to most
persons, have received much less attention. The ques-
tion of the influence of forests on climate, which many
readers may expect to find considered in this book, is
omitted because it is adequately taken up in Hann’s
Ilandbook (Vol. I).
  vi

PREFACE

I have drawn very freely upon Hann’s Handboek
der Klimatologie, Vols. II and III (2d ed., Stuttgart,
1897), as well as upon his Lehrbuch der Meteorologie
(2d ed., Leipzig, 1906), two books which are so com-
plete in all details that every writer on meteorological
or climatological subjects is inevitably very depend-
ent upon them. The curves in Chapters IV, V, and
VI were all drawn from data given in the Lehrbuch.
In the chapters on the life of man in the different
zones, I have made liberal use of RatzeTs Anthropo-
geographie (2d ed., Stuttgart, 1899). The Princi-
pal references other than these are the following:
W. M. Davis: Elementary Meteorology (Boston,
1902); A. J. and F. D. Herbertson: Man and His
Work (London, 1899); W. Koppen: Klimakunde.

I.   Allgemeine KUmalehre (2d ed., Leipzig, 1906);

A.   Supan: Grundzüge der physischen Erdkunde (3d
ed., Leipzig, 1908); W. Trabert: Meteorologie und
Klimatologie (Leipzig and Vienna, 1905); W. J.
van Bebber: Hygiënische Meteorologie (Stuttgart,
1895); A. Woeikof: Die Klimate der Erde (Jena,
1887); Atlas of Meteorology (Edinburgh, 1899).

I am indebted to the publishers, Messrs. 6. P.
Putnam’s Sons, for their generous permission to me
to use certain parts of this book in an article pre-
pared for the Encyclopeedia Britannica in 1906, as
well as for the privilege which they willingly accorded
me of publishing as separate articles many of the
chapters induded in this book. Chapters I to III
have appeared in the BuÜetin of the American
  PREFACE

VÜ

Geographical Society; Chapters IV to VI in the
Journal of Geography; Chapter VII in the Bulletin
of the Geographical Society of Philadelphia, and
Chapter XI in the Popular Science Monthly. My
thanks are also due to my fellow-workers, Professors
Hann, Mohn, Supan, Koppen, Angot, and W. M.
Davis, and also to Dr. Fridtjof Nansen, for permis-
sion to reproduce some of their maps and diagrams in
the present volume. Mr. Henry S. Mackintosh, of
Keene, N. H., has very kindly helped me in the proof-
reading.

ROBERT DE C. WARD.

Harvard University,

Cambridge, Mass.,

December, 1907.
 
  CONTENTS.

PAGB

Intboduction.....................................1

Meaning and scope of climatology—Relation of
meteorology and climatology—Literature of climatol-
ogy—The climatic elements and their treatment—

Solar climate—Physical climate.

CHAPTER I.

The Climatic Zones and theib Subdivisions   .   19

Classification by latitude circles: the five classic
zones; klima as used by the Greeks; Ptolemy’s cli-
mates; Parmenides; Polybius; Posidonius; Aristotle;
Eudoxus; Strabo; Hippocrates—Temperature zones:

Supan ; Koppen; Gebelin—Wind zones: Davis;

Woeikof — Summary and conclusions—Necessary
subdivisions of the zones.

CHAPTER IL

The Classification of Climates ....   35

Need of a classification of climates—Relation of
Continental andocean areas to temperature: reasons
for the slow change in the temperature of ocean
waters—Marine or oceanio climate—Continental cli-
mate—Desert climate—Coast or littoral climate—
Monsoon climate—Mountain and plateau climate—
Mountains as climatic divides.

CHAPTER HL

Tiie Classification of Climates (Continued) .   .   55

Supan’s climatic provinces—Köppen’s classifica-
ix
  X

CONTENTS

tion of climates—Ravenstein’s hygrothermal types—
Classification of rainfall systems—Herbertson’s nat-
ural geographical regions—Summary and conclu-
sions.

CHAPTER IV.

The Chabacteristics of the Zones. I. The Tbopics
General: climate and weather—Temperature—The
seasons—Physiologioal effects of heat and humidity
—Pressure—Winde and rainfall—Land and sea
breezes —Thunderstorms—Clondiness—In ten si ty of
sky-light and twilight—Climatic subdivisions: L
The equatorial belt—II. Trade wind beits—UI. Mon-
soon beits—IV. Mountain climate.

CHAPTER V.

The Chabacteristics of the Zones. IL The Tem-

pebate Zones..............................

General : “ Temperate” zones—Temperature —
Pressure and winde—Rainfall—Humidity and cloud-
iness—Seasons: their effects on man—Weather—
Climatic subdivisions—South temperate zone—Sub-
tropical beits: Mediterranean climates—North tem-
perate zone : Western coasts—Interiors—Eastera
coasts—Mountain climates.

CHAPTER VL

The Chabacteristics of the Zones. ni. The Polar

Zones ....................................

General: relation to man, animals, and plants—
Temperature—Pressure and winds—Rain and snow
—Humidity, doudiness and fog — Cyclones and
weather—Twilight and optical phenomena—Physi-
ological effects.

CHAPTER VII.

The Hygiene of the Zone£......................

Introduction: some general relations of climate and
health—A complex subject—Climate, micro-organ-
  CONTENTS

PAGB

isme, and disease—Geographical distribution of dis-
ease—Tropics: general physiological effects—Trop-
ical death rates—Hygiene in the tropics—Tropical
diseases—Malaria—Yellow fever—Dysentery: diar-
rhceal disorders—Tropical abscess of the liver—
Cholera—Plagne—Sunstroke and related conditions
—Dengue—Beri-beri—Other minor diseases—Gen-
eral conclusions: tropics—Temperate zones: gen-
eral—Winter and summer diseases—Tuberculosis—
Pnenmonia—Diphtheria—Influenza— Bronchitis—
Rheumatism—Measles and scarlet fever—Typhoid
fever—Whooping cough —Cholera infantum—Hay
fever—Polar zones: general—Scurvy—Climate and
health: general conclusion.

CHAPTER VUL

The Life of Man in the Tbopics ....   220

Climate and man: general—Some old views re-
garding the effects of climate on man—Factors in
the problem other than climate—Climate and habit-
ability—The development of the tropics—The labour
problem in the tropics—The government of tropical
possessions—Primitive civilisation and the tropics—
Dwellings in the tropics—Clothing in the tropics—

Food in the tropics—Agriculture, arts, and industries
in the tropics—Some physiological effects of tropical
climates—The equatorial forests—The open grass-
lands of the tropics: savannas—Trade wind beits on
land: the deserts—Trade wind beits at sea—Mon-
soon districts—Tropical mountains.

CHAPTER IX.

The Life of Man in the Tempebate Zones .   . 272

Climate and man in the temperate zones: general /

—Northward movement of civilisation in the north
temperate zone—Present-day migrations within the
  xu

CONTENTS

PAGB

temperate zones—Tlie continents and the temperate
zone—Differences between northerners and south-
erners—Variety of conditions in the temperate zones:
classification—Life of man in the forests of the tera-
perate zone—Forest clearings—The steppes —Cli-
mates and crops in the temperate zones—The deserts
—Mountains—Climate and weather: some mental
effects—Climate and weather and military operations
—Railroads — Tran sportation by water—Various
effects of the weather.

CHAPTER X.

The Life of Max ix the Polak Zoxes .   .   .   322

General: a minimum of life—Culture—Subdivisions
of the Arctic zone—Characteristics of the tundra—

The reindeer—Population and occupations—Dwell-
ings—Food and clothing—Iceland—The polar ice
cap: the Eskimo—Dwellings—Food and clothing—
Travel and transportation—Occupations and arts—
Customs—Deserts of sand and deserts of snow.

CHAPTER XI.

Changes of Climate.............................338

Popular belief in climatic change—Evidence of
climatic changes within historie times—What mete-
orological records show—Why the popular belief in
climatic changes is untrustworthy—Value of evi-
dence concerning changes of climate—Periodic oscil-
lations of climate: the sunspot period—Brückner’s
35-year cycle—Climatic cycles of longer period—
Geological changes in climate—Conclusion.

Index

3G5
  PAGB

8

10

14

22

25

27

39

48

50

56

63

64

65

66

67

G8

69

ILLUSTRATIONS.

Distribution op Insolation over the Earth

Annual Variation op Insolation at Different

Latitudes..............................

Insolation Received at Different Latitudes
on Junk 21...............................

The Zones in the Time op Parmenidks
Supan’s Temperature Zones .   .   .   .

Temperature Zones after Koppen

Influence op Land and Water on the Annual
March of Air Temperature ....

Diurnal Vabiation op Pressure: Influence op
Altitude.................................

Diurnal Yariation op Temperature: Influence
op Altitude..............................

Supan’s Climatic Provinces................

General Distribution op Plant Zones
Sciieme op Climates at Sea-Level
Names op Climates at Sea-Level
Yertical Distribution op Climates
Prr8sure and Winds in Janu art
Pressure and Winds in July .

Köppen’s Classification of Climates in Rela-
tion to Vegetatiox   ................

xiii
  ILLUSTRA TIONS

xiv

FIO.   PAGB

18   Herbertson’s Major Natural Regions .   .   71

19   Annual Marcii of Temperature: Equatorial

Type...................................91

20   Annual March of Rainfall in the Tropics   •   92

21   Annual March of Cloudiness in the Tropics   .   95

22   Annual March of Temperature: Tropical Type   97

23   Monthly Distribution of Rainfall: Sub-Tropi*

cal Winter Rains.......................125

24   Rainy and Rainless Zones on Eastern Atlan-

tic Co ast.............................128

25   Annual March of Temperature for Selected

Süb-Tropical Stations .   131

26   Annual March of Cloudiness in a Sub-Tropi-

cal Climate............................133

27   Annual March of Temperature for Selected

Stations in the Temperate   Zones   .   .   .   135

28   Annual March of Rainfall:   Temperate   Zones   139

29   Annual March of Cloudiness in Continental

and Mountain Climates: Temperate Zones .   147

30   January North Polar Isotherms .   .   .   155

31   July North Polar Isotherms .   .   .   .156

32   Mean Annual North Polar Isotherms   .   .   158

33   Annual March of Temperature: Polar Type   .   164

34   Annual March of Cloudiness in the North

Polar Zone: Marine Type ....   173
  ACKNOWLEDGMENT OF ILLUSTRATIONS.

Fig. 1. W. M. Davis: Elementary Meteorology.

“ 2, 8, 7, 8, 9. A. Angot: Traité élémentaire de Météorologie.

“ 4. H. Berger: Oeschichte der wissenschaftiichen Erdkunde der
Qriechen.

“ 5,10, 24. A. Supan: QrundzÜge der physischen Erdkunde. 8d
edition.

“ 6. W. Koppen: Die Wdrmezonen der Erde, nach der Daver der
beween, gemdssigten und katten Jahreezeit, und nach der
Wirkung der Wdrme auf die organische Wélt betrachtet.
Met. Zeitschr., i, 1884.

“ 11,12,18,14,15,16,17. W. Kóppen: Versuch einer Klassiflkation
der Klimate, vorzugsweise nach ihren Beziehungen zur
Pflanzenwelt. Hettner’s Oeogr. Zeitschr., vi, 1900.

“ 18. A. J. Herbertson: The Major Natural Regions. Oeogr. Jour.,
zxv, 1905.

“ 80, 81, 82. Scientiflc Results of the Nonvegian North Polar Expedi-
tian. Vol. vi, Meteorology.

xv
 
  CLIMATE

INTRODU CTION

Meaning and Scope of Climatology—Relation of Meteorology and
Climatology—Literature of Climatology—The Climatic Ele-
ments and their Treatment—Solar Climate—Physical Climate.

Meaning and Scope of Climatology. The word
klima (from xMvetv, to incline), as used by the
Greeks, originally referred to the supposed slope of
the earth toward the pole, or to the inclination of the
earth’s axis or of the sun’s rays. It may, perhaps,
have had reference to the different exposures of
mountain slopes. Later, probably after Aristotle’s
time, it came to be used as about equivalent to our
zone, but at first it was simply a mathematica! or an
astronomical term, not associated with any idea of
physical climate. A change of latitude in those days
meant a change of climate. Such a change was
gradually seen to mean a change of atmospheric con-
ditions as well as a change in length of day. Thus
klima came to have its present meaning.

An excellent illustration of the ancient meaning of
  2

INTRODUCTJON

the word klima is found in the system of climates pro-
posed by the famous geographer, Ptolemy. This
was a division of the earth’s surface between equator
and north pole into a series of climates, or parallel
zones, separated by latitude circles and diifering from
one another simply in the length of their longest day.
Ptolemy’s subdivision of the earth’s surface was really
nothing but an astronomical climatic table.

Climate, as we use the term, is the resultant of the
average atmospheric conditions, or, more simply, it
is the average condition of the atmosphere. Weather
is a single occurrence, or event, in the series of condi-
tions which make up the climate. The climate of a
place is in a sense its average weather. The average
values of these atmospheric conditions can be deter-
mined only by means of careful observations, con-
tinued for a period sufficiently long to give accurate
results. Climatology is the study or Science of
climates.

Relation of Meteorology and Climatology. Mete-
orology and climatology are interdependent. It is
impossible to distinguish very sharply between them.
Each needs the results obtained by the other. In a
strict sense, meteorology deals with the physics of
the atmosphere. It considers the various atmo-
spheric phenomena individually, and seeks to deter-
mine their physical causes and relations. lts view is
largely theoretical. The aspect of meteorology which
is of most immediate practical importance to man is
that which concerns weather-forecasting.
  INTRODÜCTION

3

When the term meteorology is used in its broadest
meaning, climatology is a subdivision of meteorology.
Climatology is largely descriptive. It aims to give
as clear a picture as possible of the interaction of the
various atmospheric phenomena at any place on the
earth’s surface. It rests upon physics and geogra-
phy, the latter being a very prominent factor. Cli-
matology may almost be defined as geographical
meteorology. lts main object is to be of practical
service to man. lts method of treatment lays most
emphasis on the eleinents which are of the most im-
portance- to life. Climate and crops, climate and
industry, climate and health, are subjects of vital
interest to man. No other science concerns man more
closely in his daily life.

Literature of Climatology. Scientific climatology
is based upon numerical results obtained by system-
atic, long-continued, and accurate meteorological
observations. The essential part of its literature is
therefore found in the collections of data published
by the various meteorological services and observator-
ies. In addition, large numbers of short sketches and
notes on climate, partly the more or less haphazard
accounts of travellers, partly the more careful studies
of scientific observers, are scattered through a wide
range of geographical and other publications. The
only comprehensive text-book of climatology is the
Handbuch der Klimatologie of Professor Julius
Hann, of the University of Vienna. This is the
Standard book on the subject, and upon it is based
  4

[Prometheus]

JNTR0DÜCT10N

much of the present volume, and of other recent
discussions of climate. The second edition of this
work, in three volumes, was published in 1897 (Stutt-
gart, Engelhorn). The first volume deals with gen-
eral climatology, and has been translated into
English.1 The second and third volumes are de-
yoted to the climates of the different countries of the
world. Woeikof’s Die Klimate der Erde (Jena,
Costenoble, 1887) is also a valuable reference book.
The first part concerns general relations of climate,
particularly to rivers and lakes, to vegetation, and to
snow-cover, while the second part deals with the
climates of special areas. The Standard meteorologi-
cal journal of the world, the Meteorologische Zeit-
schrift (Braunschweig, Vieweg, monthly), is indis-
pensable to anyone who wishes to keep in touch with
the latest publications on climatology, for it contains
the most complete record of such literature, as well
as a large number of original notes and discussions.
The newest and most complete collection of charts is
that in the Atlas of Meteorology (London, Con-
stable, 1899), in which also there is an excellent
bibliography. For the titles of more recent pub-
lications reference may be made to the Interna-
tional Catalogue of Scientific Literature (annual
volume on Meteorology); or to the more frequent
bibliographical lists in the Meteorologische Zeit-
schrift; the Monthly Weather Review (Washington.
U. S. Weather Bureau); the Quarterly Journal of
lBy R. De C. Ward. London and New York, Macmillan, 1903.
  INTRODÜCTION

6

the Royal Meteorological Society (London), and the
Halbmonatüches IAtteraturverzeichnus der “ Fort-
schritte der Physik ” (B raunschweig, Vieweg, twice
a month).

The Climatic Element8 and their Treatment.
Climatology has to deal with the same groups of at-
mospheric conditions as those with which meteorology
is concerned, viz.: temperature (including radiation);
moisture (including humidity, precipitation, and
cloudiness); wind (including storms); pressure;
evaporation, and also, but of less importance, the
composition and the Chemical, optical, and electrical
phenomena of the atmosphere. The characteristics
of each of these so-called climatic element$ are set
forth in a Standard series of numerical values, based
on careful, systematic, and long-continued meteoro-
logical records, corrected and compared by well-
known methods. Various forms of graphic presen-
tation, by curves, or by wind roses, etc., are employed
to emphasise and simplify the numerical results.
Instructions concerning the use, exposure, hours of
observation, and corrections of the ordinary meteoro-
logical instruments; as well as for obtaining the
usual numerical results, are published by the various
govemmental meteorological services. In Hann’s
Handbook of Climatology, Vol. I, will be found a
general discussion of the methods of presenting the
different climatic elements, and of the reasons for
adopting the accepted scheme of presentation. The
most complete guide in the numerical, mathematical,
  6

INTR0DÜCT10N

and graphic treatment of meteorological data for
climatological purposes is Hugo Meyer’s Anleitung
zur Bearbeitung meteorologücher Beobachtungen
für die Klimatologie (Berlin, Springer, 1891).

Climate deals first of all with average conditions,
as is apparent from the definition given above. But
means may be made up of very different values of the
elements which go into them, and therefore a satis-
factory presentation of a climate must include more
than mere averages. It must take account, also*, of
regular and irregular daily, monthly, and annual
changes, and of the departures, mean and extreme,
from the average conditions which may occur at the
same place in the course of time. The mean mini-
mum and the mean maximum temperature or rainfall
of a month, or a season, are important data, nót in any
way replaced by a knowledge of the mean monthly
or seasonal temperature and rainfall. Further, a
determination of thé frequency of occurrence of a
given condition, or of certain values of that condition,
is important, for periods of a day, month, or year, as,
for example, the frequency of winds according to
direction or velocity; or of different amounts of
cloudiness; or of temperature changes of 5, or 10,
or more degrees; the number of days with and with-
out rain or snow in any month, or year, or with rain
of a certain amount, etc. The probability of occur-
rence of any condition, as of rain in a certain month;
or of a temperature of 82°, for example, is also a
useful thing to know conceming a climate. In the
  INTBODÜCTION

7

past, climatology has been too much concemed with
monthly, seasonal, and annual averages. An im-
portant addition to the usual climatic summaries
would be the introduction, for all regions in which the
cyclonic or storm control of weather conditions is
characteristic, of the cyclonic unit, so that, for ex-
ample, the average duration and value of cyclonic
ranges of temperature in the several months, or the
proportion of the annual rain and snowfall received
from cyclonic storms and from local thunderstorms,
might be determined.1

Solar Climate. The sun is clearly the principal
control of climates on the earth’s surface. The gen-
eral distribution of temperature, as well as the sea-
sonal and diurnal changes, all depend upon changes
in the intensity of sunshine. Hence a brief considera-
tion of the distribution of insolation over the earth’s
surface is essential to a proper understanding of cli-
mates. Climate, in so far as it is controlled solely by
the amount of solar radiation which any place receives
by reason of its latitude, is called solar climate.
Clearly, all places on the same latitude drcle would
have the same solar climate, for the intensity and
amount of insolation depend upon the angle of in-
cidence of the sun’s rays, and upon the length of day,
and both of these depend upon latitude. Solar cli-
mate alone would prevail if the earth had a homo-

1 See R. DeC. Ward: Suggestions Conceming a More Rationa!
Treatment of Climatology. Report Eighth International Geo-
graphic Congress, Washington, D. C., 1904, pp. 277-298.
  8

IXTRODUCTION

geneous land surface, and if there were no
atmosphere. For under these conditions, and with-
out air or ocean currents, the distribution of tem-
perature at any place would depend solely on the
amount of energy received from the sun, and upon
the loss of heat by radiation. And these two factors
would have the same value at all points on the same
latitude circle.

The relative amounts of insolation received at dif-
ferent latitudes and at different times have been care-
fully determined. The values all refer to conditions
at the upper limit of the earth’s atmosphere, i. e.,

Fig. i. Distribution of Insolation over the Earth

without the effect of absorption by the atmosphere.
The accompanying diagram (see Fig. I) shows very
clearly the distribution of insolation in both hemi-
  INTRODÜCTION

9

spheres at different latitudes and at different times in
the year. The latitudes are given at the left margin
and the time of year at the right margin. The values
of insolation are shown by the vertical distance above
the plane of the two margins.

At the equator, where the day is always twelve
hours long, there are two maxima of insolation at the
equinoxes, when the sun is vertical at noon, and two
minima at the solstices, when the sun is farthest off
the equator. The annual curves show that the values
do not vary much through the year, because the sun
is never very far from the zenith, and day and night
are always equal. There is a slight difference in the
insolation at the two maxima, owing to a difference
in the sun’s distance, the earth’s orbit being an ellipse
and not a circle. The earth is nearer the sun in the
winter of the northem hemisphere, and therefore the
spring maximum is somewhat greater than the au-
tumn maximum. The varying distance from the sun
also explains the fact that the maxima of insolation
do not come exactly on the dates of the equinoxes.

These conditions are clearly brought out in curve
1 of Fig. 2, which shows the annual march of insola-
tion on the equator. The law of the distribution of
insolation would be simple if the sun were always on
the equator, for the angle of insolation and the length
of day and night would then always remain the same.
But under existing conditions, both the angle of in-
solation and the length of day are constantly chang-
ing, and the interaction between these two Controls •
  10

INTRODÜCTION

becomes very complex. As the latitude increases, the
angle of insolation becomes more oblique, and the
intensity of insolation decreases, but at the same time
the length of day rapidly increases during the sum-
mer, and towards the pole of the hemisphere which is
having its summer the gain in insolation from the

Jan. Feb. Mar. Apr. MayJuneJuly Aug. SeptOct. Nov. Dec.Jan.,

Fig. 2. Annual Variation of Insolation at Different Latitudes

latter cause more than compensates for the loss by
the former. The doublé period of insolation, above
noted for the equator, prevails as far as about lat. 12®
N. and S.; at lat. 15° the two maxima have united,
  1NTRÓDÜCTIÓN

11

and the same is true of the minima. Take the
case of an intermediate latitude, like 45° N. (see
curve 2, Fig. 2). Here there is one minimum, in
December, when the sim is south of the equator, and
one maximum, in June, when the sim is north. The
slight displacement of this maximum and minimum
from the exact date ct£ the two solstices is due to the
difference in the sun’s distance. At the north pole
(curve 8, Fig. 2), there is one maximum at the
summer solstice, and no insolation at all while the sun
is below the horizon. The distribution of insolation
at different latitudes on the same day is also interest-
ing. On June 21, for example (see Fig. 1), the
equator has a day twelve hours long, but the sun’s
maximum altitude is only 66^°, t. e.t it does not
reach the zenith, and the amount of insolation
is less than at the equinox. On the northem
tropic, however, the sun is vertical at noon, and the
day is between thirteen and fourteen hours long.
Hence the amount of insolation received at this lati-
tude on June 21 is greater than that received on the
equinox at the equator. As one passes from the
tropic to the pole the sun stands lower and lower
at noon, and the value of insolation would
steadily decrease with latitude if it were not
for the increase in the length of day. Going pole-
wards from the northem tropic on June 21, the
value of insolation increases for a time, because, al-
though the sun is lower, the number of hours during
which it shines is greater. A maximum value is
  12

INTR0DUCT10X

reached at about lat. 43%° N. The decreasing alti-
tude of the sun then more than compensates for the
increasing length of day, and the value of insolation
diminishes, a minimum being reached at about lat.
62°. Then the rapidly increasing length of day to-
wards the pole (the day being twenty-four hours long
beyond the Arctic circle) again brings about an in-
crease in the value of insolation, until a maximum is
reached at the pole which is greater than the value re-
ceived at the equator at any time. (See Fig. 2, in
which the curves are all drawn on the same scale).
The length of day is the same on the Arctic circle
as at the pole itself, but while the altitude of the sun
varies during the day on the former, being at the hori-
zon at midnight and highest at noon, the altitude at
the pole remains 231/2° throughout the twenty-four
hours. The result is to give the pole a maximum
(See Fig. 8, curve marked 1.00.). On June 21, there
are therefore two maxima of insolation, one at lat.
481/2° and one at the north pole. From lat. 481-4°
N., insolation decreases to zero on the Antarctic
circle, for sunshine falls more and more obliquely,
and the day becomes shorter and shorter. Beyond
lat. 66%° S. the night lasts twenty-four hours. On
December 21 (see Fig. 1), the conditions in southem
latitudes are similar to those in the northem hemi-
sphere on June 21, but the southem latitudes have
higher values of insolation because the earth is then
nearer the sun.

At the equinox, the days are equal everywhere, but
  INTRODUCTION

13

the noon sun is lower and lower with increasing lati-
tude in both hemispheres until the rays are tangent to
the earth’s surface at the poles (except for the effect
of refraction). Therefore, the values of insolation
diminish from a maximum at the equator to a mini-
mum at both poles. From the fact that the Southern
hemisphere has its summer in perihelion and its win-
ter in aphelion, it follows that there is a greater dif-
ference between the seasonal values of insolation south
of the equator than north of it. In other words, the
solar climate of the Southern hemisphere is more se-
vere than that of the northem. Nevertheless, owing
to the fact that the earth moves more rapidly around
its orbit when nearest the sim, both hemispheres re-
ceive equal amounts of insolation at the same lati-
tudes, and in the mean of the year, both have the same
amount of insolation.

The values of insolation thus far considered have
reference to the upper limit of the earth’s atmosphere,
or to the earth’s surface assuming that no atmosphere
exists. The effect of the atmosphere is to weaken the
sun’s rays. The more nearly vertical the sun, the less
the thickness of atmosphere traversed by the rays.
The values of insolation at the earth’s surface, after
passage through the atmosphere, have been calcu-
lated. They vary much with the condition of the air,
as to dust, clouds, water vapour, etc. In Fig. 2, the
broken lines, 4, 5, and 6, show the values of insolation
at the equator, lat. 45° N., and the north pole, allow-
ing for a loss of 25% during the passage through the
  14

INTRODÜCTION

atmosphere, i. e., with a coëfficiënt of transmission
0.75. This is higher than that usually observed, even
under very favourable conditions, with the sun in the
zenith. As a rule, even when the sky is clear, about
one-half of the solar radiation is lost during the day

Fig. 3. Insolation Received at Different Latitudes on June 21

by atmospheric absorption. The great weakening of
insolation at the pole, where the sun is very low, is
especially noticeable. The effect of the atmosphere
is also shown in Fig. 8. The upper curve represents
  INTRODUCTION

15

the total quantity of insolation received at the earth’s
surface with a coëfficiënt of transmission of 1.00
(t. e.j no loss). Under such conditions, as already
noted, there are two maxima on June 21, at lat. 431/2°

N.   and at the north pole. The second curve cor-
responds to a coëfficiënt of transmission of 0.75, which
is also used in the broken curves of Fig. 2. Under
these conditions, there is but one maximum, at about
lat. 86° N., and the north pole has only 49% of the
total radiation emitted by the sun. The third curve
is based on a coëfficiënt of transmission of 0.50, and
shows one maximum at lat. 32° N., the pole receiving
only 18% of the total amount which reaches the upper
limit of the atmosphere at that point. The curves

O.   75 and 0.50 show that, taking the atmosphere into
account, even in midsummer the amount of insolation
decreases from between lats. 80° and 40° to the pole.
The following table (after Angot) shows the effect
of the earth’s atmosphere (coëfficiënt of transmis-
sion 0.7) upon the value of insolation received at sea
level.

VALUES OF DAILY INSOLATION AT THE UPPEB LIMIT OF THE

eabth’s atmosphere and at sea level.

Lat.   Upper limit of atmosphere         Earth’s surface      
   Equator   40°   N. Pole   Equator   40°   N. Pole
Winter Solstice .   948   360   0   552   124   0
Equinoxes .   1000   773   0   612   411   0
Summer Solstice.   888   1115   1210   517   660   494
  16

INTROD ÜCT10N

The following table gives, according to Zenker,
the relative thickness of the atmosphere at different
altitudes of the sun, and also the amount of trans-
mitted insolation.

BELATIVB DI8TANCES TBAVEH8ED BT SOLAR RAYS THROUGH THE
ATHOSPHERB, AND INTENSITIES OF RADIATION PER UNIT AREAS

Altitude of Sun.

o° I 5° I io° I 20° I 30° I 40° I 50° | 6o° | 70° | 8o° | 90°
Relative Lengths of Path through the Atmosphere.

44.7110.8 I 5.7 I 2.92 I 2.00 I 1.56 11.31 I 1.15 ] 1.06 I 1.02 I 1.00
Intensity of Radiation on a Surface Nor mal to the Rays.
0.0 | 0.15 | 0.31 10.51 | 0.62 | 0.68 | 0.72 | 0.75 | 0.76 | 0.77 | 0.78
Intensity of Radiation on a Horizontal Surface.

0.0 I 0.01 I 0.05 I 0.17 I 0.31 I 0.44 I 0.55 I 0.65 I 0.72 1 0.76 I 0.78

Physical Climate. It is clear that the distribution of
insolation, just considered, explains many of the large
facts of the distribution of temperature—for example,
the decrease of temperature from equator to poles;
the doublé maximum of temperature on and near the
equator; the increasing seasonal contrasts with in-
creasing latitude, etc. But it is equally apparent that
the distribution of temperature often does not follow
the distribution of insolation closely, for, if it did
so, the two poles would be warm at the times of their
respective maxima of insolation. The high values of
insolation at the poles do not correspond to high tem-
peratures, as will be seen in a later chapter (VI).
  IXTRODUCTION

17

The old view which thus explained an “open polar
sea” was erroneous. The distribution of insolation
suggests a subdivision of the earth’s surface into
three distinct beits. In one, within about 12° of the
equator, there are two maxima and two minima. In
a second, there is one maximum; and for part of the
year the absence of the sun reduces the amount to
zero. In a third, the conditions are intermediate;
there is one maximum and one minimum, but there is
no time when the value of insolation decreases to
zero. Of the second and third of these beits, there are
two divisions, one in the northern and one in the
southem hemisphere. It will be noted that the
tropics, the polar, and the temperate zones roughly
correspond to these insolation beits.

The regular distribution of solar climate between
equator and poles which would exist on a homogene-
ous earth, whereby similar conditions prevail along
each latitude circle, is very much modified by the un-
equal distribution of land and water; by differences
of altitude; by air and ocean currents; by varying
conditions of cloudiness, and so on. Hence the cli-
mates met with along the same latitude circle are no
longer all alike. Solar climate is greatly modified by
atmospheric conditions and by the surface features of
the earth, and what is known as physical climate is
the result. The uniform latitudinal arrangement of
solar climatic beits is interfered with. Physical cli-
mate results from the reaction of the earth’s surface
features upón the atmosphere. According to the
  18

[Prometheus]

INTRODUCTION

dominant control, in each case, we have solar, Conti-
nental, marine, and mountain climates. In the first
named, latitude is the essential; in the second and
third, the effect of land or water; in the fourth, the
effect of altitude.
  CHAPTER I

THE CLIMATIC ZONES AND THEIR SUBDIVISIONS

Classification by Latitude Circles: The Five Classic Zones; Klima
as Used by the Greeks; Ptolemy’s Climates; Parmenides;
Polybius; Posidonius; Aristotle; Eudoxus; Strabo; Hippoc-
rates—Temperature Zones: Supan; Koppen; Gebelin—Wind
Zones: Davis; Woeikof—Summary and Conclusions—Neces-
sary Subdivisions of the Zones.

Classification by Latitude Circles. So great is the
variety of climates to be found in different parts of
the world that it has long been customary to classify
these climates roughly into certain broad beits.
These are the climatic zones. A simple grouping of
this kind can, however, obviously take account only
of the most general characteristics of the climates
which are included within each zone. The five zones
with which we are most familiar are the so-called tor-
rid, the two temperate, and the two frigid zones. The
torrid, or, better, the tropical zone, naming it by its
boundaries, is limited on the north and south by the
two tropics of Cancer and Capricorn, the equator
dividing the zone into two equal parts. The temper-
ate zones are limited towards the equator by the
tropics, and towards the poles by the Arctic and Ant-
arctic circles. The two frigid, or, better, the two polar

*9
  20

CLIMATE

zones, are caps covering both polar regions, and
bounded on the side towards the equator by the Arctic
and Antarctic circles.

These five zones are classified on purely astronomi-
cal or mathematical grounds. They are really zones
of sunshine, or of solar climate. Within the tropical
zone, the sim reaches the zenith at two different times
in the year; its greatest possible zenith distance is
47°; the day is never less than ten and a half hours
long. On the tropics themselves, the sun reaches the
zenith but once a year. In the polar zones, the sun is
below the horizon for twenty-four hours at least once
in winter, and is above the horizon for the same length
of time at least once in summer. On the polar circles,
the noon altitude of the sun decreases to 0° on the
shortest day. The temperate zone has conditions be-
tween these two extremes. At no point can the sun
be in the zenith; nor, except on the polar circles, is
there ever anywhere a twenty-four-hour day or night.

The tropical zone has the least annual variation of
insolation. It has the maximum annual amount of
insolation. lts annual range of temperature is very
slight. It is the summer zone. Beyond the tropics
the contrasts between the seasons rapidly become
more marked. The polar zones have the greatest
variation in insolation between summer and win-
ter. They also have the minimum amount of insola-
tion for the whole year. They may well be called the
winter zones, for their summer is so short and cool
that the heat is insufficiënt for most forms of vegeta-
  CLIMATIC ZONES AND SUBDIVISIONS 21

tion, especially for trees. The temperate zones are
intermediate between the tropical and the polar in
the matter of annual amount and of annual variation
of insolation. Temperate conditions do not char-
acterise these zones as a whole. They are rather the
seasonal beits of the world. These five zones further
differ more or less from one another in the character
of their animals and plants, and in the conditions of
human life within their boundaries.

Taking the area of a hemisphere as unity, the rela-
tive areas of these zones are as follows:

Tropical .................. 0.40

Temperate ................. 0.52

Polar ..................... 0.08

This subdivision of the earth’s surface on the basis
of the geometrical distribution of sunshine dates
from the time of the early Greek philosophers and
geographers, but it is impossible to determine with
certainty just when and by whom the various sugges-
tions in this connection were made. The famous
geographer Ptolemy, who lived in the second cent-
ury a.d., used different schemes at different times. In
the lower latitudes the breadth of a klima, or zone,
was fixed by the difference of a quarter of an hour in
the length of the longest day, but in higher latitudes
differences of half an hour, an hour, and finally a
month were the determining factors.

Parmenides, who flourished about the middle of
the fifth century b. c., proposed a five-zone division
of the earth’s surface not very unlike our present sys-
  22

CLIMATE

tem. These zones were a torrid zone, uninhabitable
because of heat; two frigid zones, uninhabitable be-
cause of cold; and two intermediate zones, of moder-
ate temperature, suitable for man. The exact limits
assigned to these zones are not known with certainty;
but it is reasonable to suppose that the Arctic circle
was even then recognised as a natural boundary for
the north polar zone, and it is pretty clear that the
temperate zone was much smaller, and the torrid zone
much larger, than in our present classification. (See
Fig. é.)

/^idïiiïV

1   fcrrfd Zon* j

Fig. 4. Thb Zones ui
the Time of Parmenides

The exact boundaries of the different zones varied
more or less for some time, as astronomical know-
ledge became more and more exact, and as the habit-
able area of the earth’s surface was gradually ex-
tended, but the scheme was generally adopted by
later writers. Polybius (bom about B.c. 204), how-
ever, divided his torrid zone into two parts by the
equator, and Posidonius (bom about B.c. 135) di-
vided his torrid zone into three parts, making six and
seven zones respectively. Aristotle (bom b.c. 884)
  CLIMATIC ZONES AND SUBDIVISIONS 23

limited the torrid zone by the tropics, and the north
temperate zone by the Arctic circle; but there is doubt
whether he really meant the fixed Arctic circle which
we know. He believed both temperate zones habit-
able, thus limiting the uninhabitable area to the
astronomical tropical zone. Eudoxus, of Cnidus,
who lived about b.c. 866, used a division of a quadrant
of the earth’s circumference into fifteen parts, of
which four belonged to the torrid, five to the temper-
ate, and six to the frigid zone. The tropics were
thus fixed at latitude 24°. Strabo (bom about b.c.
54), opposed the prevailing view that the whole of
the belt between the two tropics was uninhabitable,
and also first clearly set forth the opinion that the
temperature decreases with increasing altitude above
sea-level, as well as with increasing latitude. Strabo
also had some fairly distinct ideas regarding local
differences of climate resulting from the influence of
land and water and of mountain barriers, and noted
several effects of climate upon man and upon vegeta-
tion. He appreciated the fact that the zones were
zones of temperature as well as zones of sunshine.
As early as about 400 B.C., Hippocrates had endeav-
oured to show a causal relation between sunshine and
the topography of a district on the one hand and the
characteristics of its inhabitants on the other. He
also gave an outline of geographical pathology.1

1 The older views regarding the climates and the habitability of
the five zones were thus stated by Virgil (Georgica, i, 283.239»
translation by Davidson): ** Five zones embrace the heavens;
  24

CLIMATE

Temperature Zones. The classification of the
climatic zones on the basis of the geometrical distribu-
tion of sunshine serves very well for purposes of
simple description, but a glance at any isothermal
chart shows at once that the isotherms do not coincide
with the latitude lines. In fact, in the higher lati-
tudes, the former often follow the meridians more
closely than they do the parallels of latitude. The
astronomical zones—». e., the zones of light—there-
fore differ a good deal from the zones of heat. Hence
it has naturally been suggested that the zones be lim-
ited by isotherms rather than by parallels of latitude,
and that a closer approach be thus made to the actual
conditions of climate.

Supan (see Fig. 5) has suggested limiting the hot
belt, which corresponds to the old torrid zone, but is
slightly greater, by the two mean annual isotherms
of 68°—a temperature which approximately coin-
cides with the polar limit of the trade winds and
with the polar limit of palms. The latter is consid-
ered by Grisebach to be the truest expression of a
tropical climate. The hot belt widens somewhat.
over the continents, chiefly because of the mobility of
the ocean waters, whereby there is a tendency towards
an equalisation of the temperature between equator

whereof one is ever glowing with the bright sun, and scorched
forever by his fire; round which the two farthest ones to the right
and left are extended, stiff with cerulean ice and horrid showers.
Between these and the middle zones, two by the bounty of the gods
are given to weak mortals; and a path is cut through both, where
the series of the signs might revolve obliquely.”
  CLIMATIC ZONES AND SUBDIVISIONS

25

and poles in the oceans, while the stable lands acquire
a temperature suitable to their own latitude. Fur-
thermore, the unsymmetrical distribution of land in
the low latitudes of the northern and southem hemi-
spheres results in an unsymmetrical position of the hot
belt with reference to the equator, the belt extending

Fig. 5. Supan*s Temperature Zones

farther north than south of the equator. The polar
limits of the temperate zones are fixed by the isotherm
of 50° for the warmest month. This is a much more
satisfactory limit than the mean annual isotherm of
82°, which has also been suggested; for climates dif-
fering very widely from one another are found to
have the same mean annual temperature of 32°. The
latter value has chiefly a theoretical interest, but is
of some practical importance in its relation to the
regions of frozen ground. Summer heat is more im-
  26

CLIMATE

portant for vegetation than winter cold; and where
the warmest month has a temperature below 50°,
cereals and forest trees do not grow, and man has to
adjust himself to the conditions in a very special way.
The two polar caps are not symmetrical as regards the
latitudes which they occupy. The presence of ex-
tended land masses in the high northem latitudes
carries the temperature of 50° in the warmest month
farther poleward there than is the case in the corre-
sponding latitudes occupied by the oceans of the
southem hemisphere, which warm less easily and are
constantly in motion. Hence the Southern cold cap,
which has its equatorial limits at about lat. 50° S., is
of much greater extent than the northern polar cap.
So far as this south polar zone is concemed, the pres-
ence or absence of an Antarctic continent is imma-
terial; for such a land mass must be ice-covered, and
hence cannot operate to raise the temperature as in
the case of a land surface to which the sun’s rays have
immediate access. The northern temperate belt, in
which the great land areas lie, is much broader than
the southem, especially over the continents. These
temperature zones have real significance. They em-
phasise the natural conditions of climate more than
can be the case in any subdivision by latitude circles,
and they bear a fairly close resemblance to the old
zonal classification of the Greeks.

In high latitudes, neither the mean annual tempera-
ture nor the temperature of the coldest month is
nearly as important a climatic control over vegetation
  27

Fig. 6.

Temperature Zones after Köppen
  28

CLIMATE

as is the temperature of summer, from the point of
view of climate as a whole, and especially in relation
to organic life. The summer temperatures deter-
mine habitability, the limits of plant growth, and the
general conditions of human life. Hence, in the
higher latitudes, zones bounded by mean annual
isotherms are no great improvement over zones limited
by latitude circles.

Another classification of temperature zones has
been suggested by Koppen (see Fig. 6). In this,
the length of time during which the tempera-
ture remains within certain fixed limits, these limits
having well-marked relations to organic life, is taken
into account. Two critical daily mean temperatures,
68° and 50°, and the duration of these temperatures
for periods of one, four, and twelve months, are the
factors in this classification. These temperatures are
not reduced to sea-level. A normal duration of a
temperature of 50° for less than a month fixes very
well the polar limit of trees and the limits of agricul-
ture. Near this line are found the last groups of
trees in the tundras. A temperature of 50° for four
months marks the limit of the oak, and also closely
coincides with the limits of wheat cultivation. North
of the tree limit, agriculture ceases, and man’s food is
to be sought very largely in the sea. With the ap-
proach to this line, the period of plant growth is
shortened more ar)d more, agricultural operations be-
come restricted, and occupations of other kinds are
followed. These critical temperatures and their
  CLIMATIC ZONES AND SUBDIVISIONS 29

vaiying periods of duration from the basis of the fol-
lowing classification:

1.   Tropical belt: all months hot (over 68°). This
is almost altogether within the tropics; it reaches, in
round numbers, from latitude 20° N. to 16 S.

2.   Sub-tropical beits: 4 to 11 months hot (over
68°); 1 to 8 months temperate (50°-68°.)

8. Temperate beits: 4 to 12 months temperate.

4.   Cold beits: 1 to 4 months temperate; the rest
cold (below 50°).

5.   Polar beits: all months cold.

The temperate beits of both hemispheres are
further subdivided into three districts1—the steadily
temperate belt2 is found only on the oceans; the belt
of hot summers8 only on the continents; and the third,
with moderate summers and cold winters,4 extends
around the world, with the exception of a notable in-
terruption over Siberia.

In the second of these subdivisions, except in east-
em North America and Asia, the rainfall is generally
deficiënt; irrigation is more or less necessary, and
deserts and steppes characterise the Continental por-
tions. Only in the monsoon districts of southem and
eastern Asia, of Brazil, and of south-eastem North
America, do we find high temperatures combined with

1AU characterised by having at least four months temperate
(50°-68°)v and not more than four months hot (over 68°).

2   No month over 68° or below 50°.

2 Has temperatures below 50° for one or more months.

4 Has less than four months, but not less than one month,
temperate (50°-68°).
  30

CLIMATE

high relative humidity. The third subdivision above
noted is now the chief seat of human development.
Over a large part of the cold belt of the northern
hemisphere, the ground is permanently frozen, thaw-
ing only a little on the surface in summer. Never-
theless, in portions of it trees and hardy cereals grow.
The polar beits are, as a whole, outside the limits of
tree growth.

Another suggestion has been made by Gebelin, who
has proposed to select, as limits of the temperate zone,
certain visible geographical boundaries, in contrast
with the ideal climatic limits based upon the distribu-
tion of sunshine. On the oceans, the tropical circles
serve as acceptable boundaries on the sides towards
the equator, but on the continents the desert beits on
both sides of the tropics are reasonable limits, although
these deserts do not reach the eastem coasts of the
continents. For the polar limits of the temperate
zone, the tundras are chosen on the continents, and the
summer ice-masses on the oceans.

Wind Zone». While a simple classification of the
zones on the basis of temperature is an improvement
upon any rigid scheme of division by latitude circles,
the heat zones emphasise the element of temperature
to the exclusion of such important elements as winds
and rainfall. S>o distinctive are the larger climatic
features of the great wind beits of the world, that a
classification of climates according to wind systems
has been suggested by Davis. As the rain beits of
the world are closely associated with these wind sys-
  CLIMATIC ZONES AND SUBDIVISIONS 31

tems, a classification of the zones by winds also em-
phasises the conditions of rainfall. In such a scheme,
the torrid, or tropical zone, with its regularity of
weather through the year, and the comparative sim-
plicity of its climatic features, is bounded on the north
and south by the margins of the trade wind beits, and
is therefore larger than the classic torrid zone. This
trade wind zone is somewhat wider on the eastern side
of the oceans, and properly includes within its limits
the equable marine climates of the eastern margins
of the ocean basins, even as far north as latitude 30°
or 35°.

Most of the eastern coasts of China and of the
United States are thus left in the more rigorous and
more variable conditions of the north temperate zone.
Through the middle of the trade wind zone extends
the sub-equatorial belt, with its migrating calms, rains,
and monsoons. On the polar margins of the trade
wind zone lie the sub-tropical beits, of altemating
trades and westerlies. The temperate zones, with the
great irregularity of their weather phenomena and
their marked seasonal changes, embrace the latitudes
of the stormy westerly winds, having on the equator-
ward margins the sub-tropical beits, and being some-
what narrower than the classic temperate zones.
Towards the poles, there is no obvious limit to the tem-
perate zones, for the prevailing westerlies extend
beyond the polar circles. These circles may, how-
ever, serve fairly well as boundaries, because of their
importance from the point of view of insolation. The
  32

CLIMATE

polar zones in the wind classification, therefore, re-
main just as in the older five-zone scheme.

A compromise between the rigid division by lati-
tude circles and the isothermal and wind classifica-
tions has been suggested by Woeikof, who objects to
limiting the torrid zone by the tropics on the ground
that the high temperatures of that zone, as welL as
its characteristic winds, extend beyond these parallels.
Latitude 30° would be a more natural boundary; but
as the westerlies, which are characteristic of the tem-
perate zones, prevail there in winter, latitude 25° is
chosen as a compromise between 23%° and 80°. The
polar zones are bounded by latitude 65°. When
bounded by these several limits, the areas of the dif-
ferent zones are as follows:

Tropical Zone...................... 417

Temperate Zones.....................490

Polar Zones.......................... 93

1000

[Prometheus]

Summary and Conclusions. Reviewing what has
been said regarding the climatic zones, it would seem
that, all things considered, a simple division by iso-
therms, such as that suggested by Supan (1896), is
the best for general use. The early division by lati-
tude circles, while it has the merits of great simplicity,
and emphasises the all-important element of sunshine,
is too arbitrary, and hence does not accord sufficiently
well with the facts of actual climate. Nevertheless, we
should not discard the classic zones without recog-
  CLIMATIC ZONES AND SUBDIVISIONS 33

nising that they have a real meaning in relation to
solar climate. The grouping of the climatic zones
according to wind systems has much to recommend it
from a meteorological standpoint, but is not quite
simple enough for general use. Its adoption involves
an understanding of the great wind and calm beits of
the world, and of the migration of these beits. The
shifting of the boundaries of the torrid zone also
brings in an element of uncertainty which is some-
what confusing, although, as a place in the sub-tropi-
cal belt really changes its climate with the seasonal
change from westerlies to trades, and vice versa, it
may reasonably be expected to change its zone. In
other words, actual climatic conditions are recognised;
and in any case, this is a more reasonable plan than to
limit the torrid zone by means of the tropics, which
arbitrarily cut across the trade wind beits and sepa-
rate areas which are climatically the same. The tem-
perature zones proposed by Koppen, while useful in
special studies of plant distribution, are too detailed
for general adoption.

Whatever climatic zones we adopt, we should cer-
tainly abandon the word temperate altogether as the
designation of the middle zone in each hemisphere,
and substitute some such adjective as intermediate
for it. The words torrid and frigid should likewise
disappear, and be replaced by tropical or equatorial,
and polar.

Necessary Subdivisions of the Zones. However
we may classify them, the climatic zones are far from

3
  34

CLIMATE

being uniform in character throughout their whole
extent. Hence, no brief, simple description of the
climate of a zone can be given. For this reason, sug-
gestions have been made regarding subdivisions of
the different zones. Thus, in the case of the classic
north temperate zone, it has been proposed to subdi-
vide it into sub-tropical, temperate, and sub-arctic, but
the question how to limit these subdivisions is difficult
to settle. A more rational scheme is that which, in
view of the great differences in the climatic relations
of land and water, recognises a first large subdivision
of each zone into land and water areas. Then, as Con-
tinental interiors diflfer from coasts, and as windward
coasts have climates unlike those of leeward coasts, a
further natural subdivision would separate these dif-
ferent areas. Finally, the control of altitude over
climate is so marked that plateaus and mountains
may well be set apart by themselves as separate clima-
tic districts. If each of the zones, whether bounded
by latitude circles, or by isotherms, or by wind Sys-
tems, be considered under these general subdivisions,
as close an approach to actual conditions of climate
will be made as is possible in general description. Ob-
viously, however, when the larger zones are subdi-
vided to such an extent as is here suggested, we are
dealing with a classification of climates rather than
with climatic zones.
  \

l

CHAPTER II

THE CLASSIFICATION OF CLIMATES

Need of a Classification of Climates—Relation of Continental and
Ocean Areas to Temperature: Reasons for the Slow Change in
the Temperature of Ocean Waters—Marine or Oceanic Cli-
mate—Continental Climate—Desert Climate—Coast or Lit-
toral Climate—Monsoon Climate—Mountain and Plateau
Climate—Mountains as Climatic Divides.

Need of a Classification' of Climates. A broad di-
vision of the earth’s surface into zones is necessary as
a first step in any systematic study of climate, but it
is not satisfactory when a more detailed discussion is
undertaken. The reaction of the physical features
of the earth’s surface upon the atmosphere compli-
cates the climatic conditions found in each of the
zones, and makes further subdivision desirable. Un-
der the control of these different physical conditions,
the climatic elements unite to produce certain fairly
distinct types of climate, and these may be classified
in various ways. The usual method is to separate
the Continental (near sea-level) and the marine. An
extreme variety of the Continental is the desert; a
modified form, the littoral; while altitude is so im-
portant a control that mountain and plateau climates
are further grouped by themselves.

35
  36

CLIMATE

Relation of Continental and Ocean Areas to Tem-
perature. Land and water differ greatly in their be-
haviour regarding absorption and radiation. The
former warms and cools readily, and to a considerable
degree; the latter, slowly and but little. (1) Of the
insolation which falls upon the ocean, a good deal is
at once reflected, and is therefore not available for
warming the water. Land surfaces, on the other
hand, are poor reflectors; but little insolation is lost
in that way; hence more energy is available for raising
their temperature. (2) Most of the insolation which
enters the water is transmitted to some depth, and,
therefore, is not effectively applied to warming the
surface. Land is opaque and does not allow the in-
cident insolation to pass beyond a comparatively thin
surface stratum; hence this surface can be well
warmed. (3) The evaporation of water requires a
large amount of energy, which changes the state of
the water without raising its temperature (latent
heat). Land, although often moist, is itself non-
volatile; therefore the loss of energy in the process of
evaporation is usually very slight. (4) Water is
more diflicult to warm than any other natural sub-
stance, while land is warmed easily and quickly. If
equal amounts of heat are received by equal areas of
land and water, the former warms about twice as much
as the latter. (5) The mobility of water keeps the
warmer and the colder portions well mixed, and there-
fore greatly retards the process of warming any one
portion of the surface. Land cannot thus equalise
  THE CLASSIFICATION OF CLIMATES

87

its temperature. (6) The cloudiness over the oceans
is usually greater than that over the lands, and this
operates to shade the former more than the latter, re-
ducing the energy available for w&rming the water
surface. For these various reasons, ocean surfaces
can warm but little during the day, or in summer,
and can cool but little during the night, or in winter.
They, and the air over them, are therefore conserva-
tive as regards their temperatures. Land areas, and
the air over the lands, on the other hand, warm and
cool readily. The influence of latitude, as seen in
solar climate, is not infrequently wholly overcome by
the influence of land and water.

Marine or Oceanic Climate. Conservatism in its
temperature conditions is the most distinctive feature
of a marine climate. The results of the Chatten-
ger Expedition show that the diurnal range of air
temperature over the ocean between latitudes 0° and
40° averages only 2° or 8°. Further, the slow
changes in temperature of the ocean waters involve
a retardation in the times of occurrence of the maxima
and minima, and a marine climate, therefore, has
characteristically a cold spring and a warm autumn,
the seasonal changes of temperature being but slight.
The surface waters of oceans and lakes average some-
what warmer than the air over them, and for this
reason all considerable bodies of water which remain
unfrozen in winter become sources of warmth for the
adjacent lands during the colder months. Character-
istic, also, of marine climates is a prevailingly higher
  38

CLIMATE

\

relative humidity, a larger amount of cloudiness, and
a heavier rainfall than is found over Continental inter-
iors. All of these features have their explanation in
the abundant evaporation from the ocean surfaces.
In the middle latitudes, again, there is this contrast
between the oceans and the Continental interiors, that
the former have distinctly rainy winters, while over
the latter the colder months have a minimum of pre-
cipitation. Ocean air is cleaner and purer than land
air, and ocean air is, on the whole, in more active mo-
tion, because friction of air on water is less than
friction of air on land.

It is obvious that an equable, damp, and cloudy
climate, such as that which is, on the whole, typical of
the oceans and of their leeward coasts, must affect
vegetation in a way quite different from that notecl
in a hotter and drier climate, with greater variations
of temperature. Thus Schindler has shown that
wheat contains less protein in a marine climate, and
hence more meat, leguminous plants, and other nitro-
genous foods are necessarily eaten. An interior
climate, like that of Southern Russia and Hungary,
produces wheat which is richer in protein; the need of
other nitrogenous foods is consequently decreased.
The proportion of starch is decreased, and that of
gluten is increased, in a hot, dry climate. The size
of the erop is also affected by the climate.

Continental Climate. Marine climate is equable;
Continental, is severe. The annual temperature
ranges increase, as a whole, with increasing distance
  THE CLASSIFICATION OF CLIMATES

39

from the ocean; the regular diurnal ranges are also
large, reaching 85° or 40°, and even more, in the arid

J. F. M. A. M. J. J. A. S. 0. N. D. J.

Fig. 7. Influence of Land and Water on the
Annual March of Air Temperature

Continental interiors. The coldest and warmest
months are usually January and July, the times of
  40

CLIMATE

maximum and minimum temperatures being less re-
tarded than in the case of marine climates. April is
usually warmer than October, unless spring warm-
ing is delayed by the melting of a snow-cover. In
the latter case, the snow-covered land surface tem-
porarily takes on the characteristics of a water
surface, and has a retarded spring. The greater sea-
sonal contrasts in temperature over the continents
than over the oceans are furthered by the less cloudi-
ness over the former. The clearer Continental skies
of high latitudes favour a lowering of the winter, but
a slight rise of the summer temperatures, while in
lower latitudes the clearer summer skies favour a
higher mean annual temperature. Diurnal and an-
nual changes of nearly all the elements of climate are
greater over continents than over oceans; and this
holds true of irregular, as well as of regular, varia-
tions. The contrast between marine and Continental
climates in the matter of the annual march of tem-
perature is shown in Figure 7. In low latitudes, the
curve for Funchal, on the island of Madeira (M),
represents the marine type, and that for Bagdad, in
Asia Minor (Bd), the Continental. For higher lati-
tudes, the curves for Valentia (V), a coast station
in the south-west of Ireland, and for Nerchinsk (N),
in eastern Siberia, are representatives of the two
types.

Owing to the distance from the chief source of
supply of water-vapour—the oceans—the air over
the larger land areas is naturally drier and dustier
  THE CLASSIFICATION OF CLIMATES

41

than that over the oceans. Yet even in the arid Con-
tinental interiors in summer, the absolute vapour con-
tent is surprisingly large, although the air is still far
from being saturated. In the hottest months the
percentages of relative humidity may reach 20% or
30%. At the low temperatures which prevail in the
winter of the higher latitudes, the absolute humidity
is very low, but, owing to the cold, the air is often
damp. Cloudiness, as a rule, decreases inland, reach-
ing its minimum in deserts. And with this lower
relative humidity, more abundant sunshine and higher
temperature, the evaporating power of a Continental
climate is much greater than that of the more humid,
cloudier, and cooler marine climate. Actual evapo-
ration is, however, under these conditions, usually
much less than the possible evaporation which would
take place were there more water present to be
evaporated. Both amount and frequency of rain-
fall, as a rule, decrease inland, but the conditions are
very largely controlled by local topography and by
the prevailing winds. The decreased frequency of
rainfall on the lowlands is especially marked in win-
ter. Winds average somewhat lower in velocity,
and calms are more frequent, over continents than
over oceans. The seasonal changes of pressure over
the former give rise to systems of inflowing and out-
flowing, so-called Continental, winds, sometimes so
well developed as to become true monsoons. Usu-
ally, however, the changes in direction and the de-
velopment are not very marked.
  42

CLIMATE

In winter, clear, crisp days, which are followed by
cold, calm nights, and interrupted from time to time
by spells of cloudy, windy weather, with or without
light precipitation; in summer, clear, calm nights,
followed by hot days with increasing wind velodty
and heavy clouds towards noon, and often by thun-
derstorms later in the aftemoon—these are typical
weather conditions of Continental interiors in the
higher latitudes; and they are of much interest to
man. The extreme temperature changes which oc-
cur over the continents are the more easily bome be-
cause of the dryness of the air; because the minimum
temperatures of winter occur when there is little or
no wind, and because, during the warmer hours of the
summer, there is the most air movement.

Desert Climate. An extreme type of Continental
climate may be found in deserts. It is a curious fact
that desert and marine climates—the two extremes of
the climatic scale—resemble one another in some re-
spects. Desert air, though often dusty by day, is
notably free from micro-organisms; the purity of
ocean air is well known. Again, deserts and oceans
alike have high wind velocities. The large diurnal
temperature ranges of inland regions, which are
most marked where there is little or no vegetation,
give rise to active convectional currents during
the warmer hours of the day. Hence high winds,
disagreeable because of the dust and sand which they
carry, are common by day, while the nights are apt
to be calm and relatively cool. Travelling by day is
  THE CLASSIFICATION OF CLIMATES 43

unpleasant under such conditions. Diurnal cumu-
lus clouds, often absent because of the excessive dry-
ness of the air, are thus replaced by clouds of blowing
dust and sand. This sand, often carried afar, may
find a resting-place on the moister lands to leeward.
Thus beds of loess are formed. Indeed, many geo-
logical phenomena, and special physiographic types
of varied kinds, are associated with the peculiar con-
ditions of desert climate. The excessive diurnal
ranges of temperature cause rocks to split and break
up. Wind-driven sand erodes and polishes the rocks.
When the separate fragments become small enough,
they, in their turn, are transported by the winds and
further eroded by friction during their joumey. The
ground is often swept clean by the winds. Curious
conditions of drainage result from the deficiency in
rainfall. Rivers “ wither ” away, or end in sinks or
brackish lakes. Desert plants protect themselves
against the attacks of animals by means of thorns,
and against evaporation by means of hard surfaces
and an absence of leaves. The life of man in the des-
ert is likewise strikingly controlled by the climatic
peculiarities of strong sunshine, of heat, and of dust.
Occasionally heavy downpours of rain (cloud-bursts)
over mountains or on the borders of deserts, cause
sudden floods. Even slight rainfalls in deserts
awaken multitudes of dormant plant seeds.

Coast or Littoral Climate. Between the pure
marine and the pure Continental types, the coasts fur-
iysh almost every grade of transition. Hence coast
  u

CLIMATE

or littoral climates may well be placed in a group by
themselves. Prevailing winds are here important
Controls. When these blow from the ocean, as on the
western coasts of the temperate zones, the climates
are more marine in character; but when they are off-
shore, as on the eastern coasts of these same zones, a
somewhat modified type of Continental climate pre-
vails, even up to the immediate sea-coast. Hence the
former have a much smaller range of temperature;
their summers are more moderate and their winters
milder; extreme temperatures are very rare; the air
is damp; there is much cloud. All these marine feat-
ures diminish with increasing distance from the ocean,
especially when there are mountain ranges near the
coast, as is the case in the western United States and
in Scandinavia. In the tropics, windward coasts are
usually well supplied with rainfall, and the tempera-
tures are modified by sea breezes. Leeward coasts
in the trade wind beits offer special conditions. Here
the deserts often reach the sea, as on the western coasts
of South America, Africa, and Australia. Cold ocean
currents, with prevailing winds along shore rather
than onshore, are here hostile to rainfall, although
the lower air is often damp, and fog and cloud are
not uncommon.

Monsoon Climate. Exceptions to the general rule
of rainier eastern coasts in trade wind latitudes are
found in the monsoon regions, as in India, for ex-
ample, where the western coast of the peninsula is
abundantly watered by the wet south-west monsoon.
  THE CLASSIFICATION OF CLIMATES

46

As monsoons often sweep over large districts, not
only coast but interior, a separate group of monsoon
climates is desirable. In India, there are really three
seasons—one cold, during the winter monsoon; one
hot, in the transition season; and one wet, during the
summer monsoon. Little precipitation occurs in
winter, and that chiefly in the northern provinces.
The high temperatures of the transition periods are
most oppressive when the air is most damp. In India
this is the case in the autumn. In low latitudes, mon-
soon and non-monsoon climates differ hut little, for
summer monsoons and regular trade winds both give
rains, and wind direction has slight effect upon
temperature.

The winter monsoon is offshore, and the summer
monsoon onshore, under typical conditions, as in
India. But exceptional cases are found where the
opposite is true. Thus, on the north-westem coast of
Japan, the north-eastern coasts of Formosa and of the
Philippines, and the eastern coasts of the Southern
Deccan and of Ceylon, the prevailing offshore, winter,
dry monsoon becomes an onshore, rainy wind. Many
complicated cases of this kind are not easily co-ordi-
nated. In higher latitudes, the seasonal changes of
the winds, although not truly monsoonal, involve dif-
ferences in temperature and in other climatic de-
ments. The eastern coast of the United States has
prevailing cold, dry, clear winds from the Continental
interior in winter, while the prevailing winds of sum-
mer are south-west, and hence warm and often moist.
  46

[Prometheus]

CLIMATE

The only well-developed monsoons on the coast of the
continents of higher latitudes are those of eastern
Asia. These are offshore during the winter, giving
dry, clear, and cold weather; while the onshore move-
ment in summer gives cool, damp, and cloudy
weather. Without these seasonal winds the winters
would have the maximum amount of rain and cloud.

Mountain and Plateau Climate. Both by reason
of their actual height and because of their obstructive
effects, mountains influence climate similarly in all
the zones. Hence mountain and plateau climates
are placed in a group by themselves, as distinguished
from those of lowlands. The former, as contrasted
with the latter, are characterised by a decrease in
pressure, temperature, and absolute humidity; an in-
creased intensity of insolation and radiation; larger
ranges in soil temperature; usually a greater fre-
quency of percipitation, and, up to a certain altitude,
more of it.

At an altitude of 16,000 ft., more or less, pressure
is reduced to about one-half of its sea-level value.
The highest human habitations are found under these
conditions. While the pressures and the pressure
changes at sea-level have no marked effect upon man,
the physiological effects of the decreased pressure
aloft (faintness, nausea, headache, weakness) are ex-
perienced by a majority of people at altitudes above
12,000 to 15,000 ft. The symptoms, and the height
at which they appear, vary much in different cases,
and depend upon the physical condition of the indi-
  THE CLASSIFICATION OF CLIMATES

47

vidual, the weather, bodily exertion, and so on. The
greatest altitudes attained by man were reached by
balloon, and in such cases a supply of oxygen is usu-
ally taken up by the aëronaut. Man endures the
rapid pressure changes during balloon ascents with
difficulty, and often only with considerable suffering.
The eagle and the condor, however, suffer no incon-
venience during their high flights.

It has been suggested by Jourdanet that mountain
and plateau climates be divided into groups, climats
de montagne, below 6500 feet, and climats d’altitude,
above that height. The former are beneficial because
of the stimulating quality of their clean, cool air; the
latter may be injurious because of the low pressure.
The variations in pressure, as well as the actual press-
ures, diminish aloft. On high mountains and plat-
eaus, the pressure is lower in winter than in summer,
owing to the fact that the atmosphere is compressed
by cold to lower levels in the winter, and is exp&nded
upwards in summer by heat. The morning minimum
pressure on mountains is usually the primary mini-
mum, the aftemoon minimum being less marked and
coming later than on lowlands. Figure 8 shows the
diurnal variation of pressure at Geneva (408 meters,
G), Beme (578 meters, B), on the Santis (2467
meters, S), and on the summit of Mont Blanc (4811
meters, MB), and illustrates well the general char-
acteristics of the curves found at different altitudes.
Local topography, however, is an important control-
ling influence, and modifies such curves very much.
  48

CLIMATE

The intensity of insolation and of radiation both
increase aloft in the cleaner, purer, drier, and thinner
air of mountain climates. The sun usually shines more
often and more powerfully at high altitudes. The

0!*   4t>   8t> NOON I6(?   2011   24*!

Fig. 8. Diurnal Variation of Pressure : Influence of Altitude

intensity of the sun’s rays attracts the attention of
mountain-climbers at great altitudes. The excess of
surface temperature over air temperature also in-
creases aloft, and is a favourable element in plant
growth. There is likewise an increase in the range of
surface temperature, although this is much influenced
  THE C LAESIE WAT ION OF CLIUATEE   49

by exposure. The vertical decrease of temperature,
which is also much affected by local conditions, is es-
pecially rapid during the warmer months and hours;
mountains are then cooler than lowlands. The in-
versions of temperature characteristic of the colder
months, and of the night, give mountains the advan-
tage of higher temperature then, a fact of importance
in connection with the use of mountains as winter re-
sorts. At such times, the cold air flows down the
mountain sides and collects in the valleys below, be-
ing replaced hy warmer air aloft. Hence diurnal
and annual ranges of temperature on the mountain
tops of middle and higher latitudes are lessened, and
the climate in this respect resembles a marine condi-
tion; but topography and the conditions of local
clouds and winds are here important Controls. The
times of occurrence of the maximum and minimum
are also much influenced by local conditions. Figure
9 shows the diurnal march of temperature for Paris
(solid) and the Eiffel Tower (broken) in January
and July. It will be noted that the times of maxi-
mum and minimum are retarded on the Eiffel Tower,
and that the range is less than at the earth’s surface.
These are characteristics of mountain climates. Ele-
vated, well-endosed valleys, with strong sunshine,
often resemble Continental conditions of large tem-
perature range; and plateaus, as compared with
mountains at the same altitude, have relatively higher
temperatures and larger temperature ranges. Alti-
tude tempers the heat of the low latitudes. High
  50

CLIMATE

mountain peaks, even on the equator, can remain
snow-covered the year around; the plateau of south-

Fig. 9. Diurnal Variation of Temperature : Influence of
Altitude

em India, at 6000 to 7000 ft. above sea-level, always
has moderate mean temperature, and from the dense
  THE CLASSIFICATION OF CLIMATES

51

jungle of the tropical lowland to the snowy moun-
tain top, successive zones of vegetation are en-
countered.

Nine-tenths of the water vapour in the atmosphere
are below 21,000 feet. Hence mountains are im-
portant vapour barriers, and one side may be damp
while the other is dry. Curiously mistaken ideas of
distance often result from the remarkable clearness
and dryness of the air on high mountains. No gen-
eral law govems the variations of relative humidity
with altitude, but on the mountains of Europe the
winter is the driest season, and the summer the
dampest. At well-exposed stations there is a rapid
increase in the vapour content soon after noon, espe-
cially in summer. The same is true of cloudiness,
which is often greater on mountains than at lower
levels, and is usually at a maximum in summer, while
the opposite is true of the lowlands in the temperate
latitudes. One of the great advantages of the higher
Alpine valleys in winter is their small amount of
cloud. This, combined with their low wind velocity
and strong insolation, makes them desirable winter
health resorts. Latitude, altitude, topography, and
winds are determining factors in controlling the
cloudiness on mountains. In intermediate latitudes
there is a seasonal migration of the level of maximum
cloudiness, and of maximum relative humidity, from
the lowlands in winter to higher altitudes in the
warmer months, in association with the diurnal con-
vectional movements of the warmer season. Frequent
  52

CLIMATE

rapid local changes also occur. In the rare, often
dry, air of mountains and plateaus, evaporation is
rapid, the skin dries and cracks, and thirst is increased.

Rainfall usually increases with increasing altitude
up to a certain point, beyond which, owing to the loss
of water vapour, this increase stops. The zone of
maximum rainfall averages about 6000 to 7000 feet
in altitude, more or less, in intermediate latitudes,
being lower in winter and higher in summer. Moun-
tains usually have a rainy and a drier side; the con-
trast between the two is greatest when a prevailing
damp wind crosses the mountain, or when one slope
faces seaward and the other landward. When the
prevailing winds differ little in dampness, this con-
trast is lessened, and there may then be a very close
corréspondence between the rainfall and the topo-
graphic map of a region. Mountains often provoke
rainfall, and local “ islands,” or, better, “ lakes,” of
heavier precipitation result. Such are found on the
mountains of the Sahara, and of other deserts. This
local precipitation favours the growth of vegetation;
small streams and oases are found, and temporary
camps, or more permanent settlements, of the no-
madic tribes of the desert are there established. Well-
marked zones of vegetation are noted under such
conditions, as in the transition from the dry Califor-
nian lowlands up through the deciduous, and then the
coniferous, forests of the Siërra Nevada to the snows
on the summits. Similarly, the high plateaus of
southem Utah and of Arizona are high enough to re-
  THE CLASSIFICATION OF CLIMATES

53

ceive fairly abundant rainfall, while the lowlands are
arid.

Mountains resemble marine climates in having
higher wind velocities than Continental lowlands;
mountain summits have a noctumal maximum of
wind velodty, while plateaus usually have a diurnal
maximum. Mountains both modify the general, and
give rise to local, winds. Among the latter, the well-
known mountain and valley winds are often of con-
siderable hygienic importance in their control of the
diurnal period of humidity, cloudiness, and rainfall,
the ascending wind of daytime tending to give clouds
and rain aloft, while the opposite conditions prevail
at night. The high temperature and dryness of the
foehn, which is of immense benefit to man by reason
of its melting and evaporating powers, although of-
ten enervating and depressing, result from the fact of
a descent of the air from a mountain slope or summit.
The bora, with its cold gust, is a wind in whose de-
velopment a mountain or plateau is essential. And
the mistral of Southern France owes some of its cold
to radiation over the interior plateaus.

Mountains as Climatic Divides. Very different
conditions of temperature, pressure, and humidity
may be found on the opposite sides of a well-defined
mountain range, because such a range interferes with
the free horizontal interchange of the lower air.
Mountain ranges which trend east and west, like the
Alps and the Himalayas, separate more severe from
less severe climates; those which follow a coast-line, as
  54

CLIMATE

in California, Scandinavia, or eastern Siberia, separ-
ate marine from Continental. Large differences of
pressure on the two sides may be equalised by a flow
of air across the mountain, as in the foehn.
  CHAPTER III

THE CLASSIFICATION OF CLIMATES (Continued)

Supan’s Climatic Provinces—Köppen’s Classification of Climates—
Raven stein’s Hygrothermal Types—Classification of Rainfall
Systems—Herbertson’s Natural Geographical Regions—Sum-
mary and Condusions.

Supan’s Climatic Provinces. The ordinary classi-
fication into Continental, marine, and mountain cli-
mates is too general. Some scheme of classification is
needed in which the geographical factor plays an im-
portant part, and which recognises the types of
climate, possessing common characteristics of tem-
perature, rainfall, and winds, that occur over areas
having similar topographic conditions. A fairly sim-
ple scheme of this kind has been suggested by Supan,
who recognises thirty-five so-called climatic prov-
inces, but any such rigid subdivision is obviously sus-
ceptible of almost infinite modification. Twenty-one
of these provinces are in the eastern hemisphere, in-
cluding Polynesia; twelve are in the western, and
two in the polar zones. The description of thesk
provinces is as follows:1

1. Arctic Province. This coincides with the

i Free translation of original, following Bartholomew's Atlas of
Meteorology, p. 7.

55
  56

CLIMATE

north polar cold cap, the area wherein the mean tem-
perature of the warmest summer month is never over
50° F., and within which trees do not grow.

2. West European Province. Mild winters, ow-
ing to influence of the westerly winds and Gulf
Stream. Yearly temperature range under 59° F.

Fig. io. Supan’s Climatic Provinces

(15° C.). Plentiful rainfall, fairly well distributed
throughout the year, but varying in quantity owing
to great diversity of land contours. The climatic
conditions often vary in short distances, and hence the
region can be divided into many subdivisions.

3.   East European Province. Here the evidences
of a land climate begin to be observed; but as most of
the region is a plain, differences depend mainly on
latitude. The rainfall is smaller than in Province 2,
  CLASSIFICATION OF CLIMATES

57

and gradually diminishes towards the southeast, and
has a marked summer maximum.

4.   West Siberian Province. This is separated
from 8 by the limit of the positive annual isanomal-
ous lines, which practically coincide with the Urals.
The characteristic peculiarities of 8 are found here
greatly emphasised, and the greater variability of
temperature is to be noted.

5.   East Siberian Province. A gradual rising of
the ground is found east of the Yenisei, with low
plains only along the rivers. The winter cold pole
is here, and thé yearly range of temperature is a
maximum. As a rule, the rainfall is small.

6.   Kamchatkan Province. The sea diminishes
the temperature extremes noted in Province 5, and
much rain falls.

7.   Sino-Japanese Province. On the continent,
relatively well-marked winter cold, and strong peri-
odical rains. In Japan, these peculiarities are less
extreme.

8.   Asiatic Mountain and Plateau Province.
This includes all the lofty plateaus bounded by
mountain ranges, which shield it on every side, and
so render it very dry. The great height makes the
winter temperature severe; but the summer heat is
great, owing to the Continental position. The daily,
as well as the yearly, range of temperature is very
marked.

9.   Aral Province. Dry, low-lying plain, with the
greatest rainfall in the north in summer, and in the
  58

CLIMATE

south in winter. The plains of western Turkestan
have severe winters and very hot summers.

10.   Indus Province. A plain remarkable for
great dryness and heat.

11.   Mediterranean Province. Very varied in
climate, owing to its great irregularity of outline,
both horizontal and vertical. Mild, except on high
plateaus. Winter rains.

12.   Saharan Province. Reaches to Mesopota-
mia. Region of dry, north winds, and probably the
one receiving least rain. Its Continental position and
lack of vegetation increase the heat of summer ex-
traordinarily; both annual and daily ranges of tem-
perature are considerable.

18.   Tropical African Province. Owing to the
height of the central plateau, the heat is less intense,
but it is very great on the narrow Coastal plains.
Tropical rains decreasing towards the jvest.

14.   Kalahari Province. Includes all the almost
rainless region of Southwest Africa.

15.   Cape Province. Sub-tropical.

16.   Indo-Australian Monsoon Province. Strong,
periodical rains are brought with the Southwest and
northwest monsoons, except at a few places in the
ardhipelago. The temperature is fairly uniform,
despite the great extent of the province, and the
yearly range is very small.

17.   Inner Australian Province. With great ex-
tremes of temperature. Irregular and rare rains.

18.   Southwest Australian Province. Sub-tropical.
  CLASSIFICATION OF CLIMATES

59

19.   East Australian Province. It extends to the
water-parting and includes the southeast coast and
Tasmania. Plentiful and fairly regular rains. Mod-
erate range of temperature.

20.   New Zealand Province. Probably includes
the small neighbouring islands. Mild climate, with
fairly regular rains.

21.   Tropical Polynesian Province. Tropical cli-
mate, ameliorated by the ocean, so that mild sum-
mer weather prevails throughout the year. On the
loftier islands, the rain is abundant, and has a tropi-
cal periodicity.

22.   Hawaiian Province. Also a mild climate,
but with sub-tropical rains.

28. Hudson (North Canadian) Province. Great
extremes of temperature and little precipitation.

24.   Northwest American Coastal Province. Mild,
equable, rainy climate.

25.   Californian Province. Relatively cool, es-
pecially in summer. Marked sub-tropical rainy
seasons.

26.   North American Mountain and Plateau
Province. Great yearly and daily ranges. Dry.

27.   Atlantic (East North American) Province.
Great contrast in temperature conditions of north
and south in winter. Extreme climate even on the
coast. Plentiful rains, evenly distributed through-
out the year. Great variability.

28.   West Indian Province. This also includes
the soulhem rim of North America. Equable tem-
  60

CLIMATE

perature. Rain at all seasons, but with a marked
summer maximum.

29. Tropical Cordilleran Province. On the in-
terior plateau, perpetual spring, owing to consider-
able height above sea-level. In Mexico and Central
America, marked zenithal rains; in South America,
more regular precipitation.

80.   South American Tropical Province. Little
that is certain is known about this province, which in-
cludes mountainous regions and plains, and ought,
therefore, to possess considerable variety of climate.

81.   Peruvian Province. This province extends
as far south as 80° S., and so includes the northem
part of Chile. Abnormally cool. Rainless.

82. North Chilean Province. Sub-tropical.

88. South Chilean Province. Equable tempera-
tures, with cool summers. Extraordinarily rainy.

84. Pampa Province. Range of temperature
fairly large, especially in the north. Rain not
plentiful.

35. Antarctic Province. Resembles the Arctic, so
far as can at present be determined, in winter cold
but differs in having a very low summer temperature
and a very regular distribution of pressure and winds.

Fig. 10 shows the geographical distribution of
these climatic provinces.

Köppen’s Classification of Climates. An interest-
ing classification of climates, from a botanical stand-
point, is that proposed by Koppen. This rests upon
certain critical values of the temperature and rain-
  CLASSIFICATION OF CLIMATES

61

fall of the warmest or coldest, or of the wettest and
driest month. The plant classification proposed by
A. de Candolle in 1874, and later adopted by Drude,
is accepted. This is a division into five principal
biological groups under the control of temperature
and moisture, as follows:

A.   Megatherms: plants which need continuously
high temperature without much annual range, and
also abundant moisture. There is no cool season; the
temperature of the coolest month is over 64.5°
(18° C.), and there is at least one month of heavy rain.
When there are marked dry seasons, the principal one
comes in winter and spring. In parts of this belt
there are two rainy seasons. In this belt are found
the lofty tropical forests intertwined with vines and
creepers—sago, betel, pepper, cacao, bread-fruit,
baobab, coffee, sugar-cane, banana, ginger, and so on.

B.   Xerophytes: plants which like dryness and .
need high temperatures, at least for a short season.
These are found in tropical districts which have a
long dry season, and in the steppes and deserts of the
tropics and of the warmer parts of the temperate
zones. They are adapted in various ways for life in
a dry climate; they rest during the dry time, and, in
extreme cases, where rain may not fall for years, they
survive as seeds. The vegetation varies with the soil.
In this group we find the date, mesquite, acacia, cac-
tus, agave, and similar plants.

C.   Mesoiherms: need moderate heat (59°-68°)
and a moderate amount of moisture; some require
  62

[Prometheus]

CLIMATE

high summer temperatures; others shun low win-
ter temperatures; others shun the dryness which of-
ten accompanies high summer temperatures. These
plants inhabit latitudes between 22° and 45° N. or
40° S., so long as the moisture continues sufficiënt.
There is a cool season—coldest month below 64.5°
(18° C.)—and a hot summer—warmest month over
72° (22° C.),—or a mild winter—coldest month over
48° (6° C.)—or both. The classic Mediterranean
climate is found in this belt. The mesotherm belt
jcontains the tea, maté, rice, cotton, magnolia, hickoiy,
arbor vit®, hemlock, wheat, com, olive, fig, grape,
heath, cinchona, etc.

D.   Mikrotherm.8: need less heat, lower mean an-
nual temperature, cooler and shorter summers, and
colder winters. The warmest month is at least 50°
(10° C.) and not over 72° (22° C.); the coldest is be-
low 48° (6° C.), with at least an occasional snow-
cover in winter and sufficiënt rainfall in the warmer
season. Evergreen and deciduous forests, grains,
and, in the warmer portions, fruit and corn are found.

E.   Heki8totherm8: plants of the Arctic zone, be-
yond the limits of tree growth and of the zone of
scrubby Antarctic vegetation. These need the least
heat. Mosses, lichens, and similar lowly forms are
typical.

A simple scheme of distribution of these five groups
of plants may first be developed with reference to an
ideal continent, stretching from pole to pole, with
oceans on both sides and without mountains (Fig.
  CLASSIFICATION OF CLIMATES

63

11). Here a a is the western and b b the eastern
coast. The approximate latitudes are given at the
margins. The groups of de Candolle’s System are
arranged as shown, if the xerophytes are limited to

Fig. ii. General Distribution op
Plant Zones

the deserts and steppes, and if those woody plants
of the megatherm and mesotherm zones which are
adapted to a dry climate are included within these
zones. The typical zonal arrangement is interrupted
in latitudes 20° to 50° by the fact that the arid dis-
  64

CLIMATE

trict of the xerophytes (B) is wedged in on the west
coast between A and C. Farther east, zone B broad-
ens poleward, cuts through the middle of the meso-
therm zone, and usually ends without reaching the
east coast.

The five principal types are further subdivided un-
til the whole number of climates reaches twenty-four.
The special conditions which characterise each cli-
mate are carefully determined, and each sub-climate
is named after one of its characteristic plants or ani-
  CLASSIFICATION OF CLIMATES

66

mals; or af ter some distinctive meteorological pheno-
menon; or, again, af ter the general character of its
vegetation. Fig 12 gives the limits of the different

Fig. 13. Na mes of Climates
at Sea-Level

sub-climates, and also the characteristic conditions of
temperature and precipitation.1 Fig. 18 gives the

1 Figures are degrees Fahr. C = coldest month. W = warmest
month. 4 M = 4 months. dr. 1.2 in. = driest month rainfall 1.2 in-
ches. D. 18° and D 36° = difference between extreme months
18° and 36°. q = quotiënt obtained by dividing the amount of
rainfall in the wettest month (in mm.) by the maximum vapour'
tension (in mm.) at the mean temperature of the same month, an

5
  66

CLIMATE

scheme of the sub-climates for the lowlands, with their
names. Four climates which do not occur at sea-
leve! are here lacking (C7, E8, E4, F)The verti-

Alti-  tude  m.  4000   Vertical Diatrfb   Continental lype   Alti-  tude  m.
9000         4000
9000      ic*-« e   9000
1000   I      9000
         ÏOOO

Fïo. 14. Vertical Distribution op Climates

cal distribution of these climates, much simplified, is
shown in Fig 14. The descent of the climatic strata
from equator to higher latitudes is shown on the right
for the Continental, and on the left for the marine type,
as far as about latitude 57°. Climates Cl to C4,
and Dl and D2, have large temperature ranges, and
are therefore lacking at the equator and on the ocean;
while C5 to C7, and D3, have small ranges, and are
not found on the continents in higher latitudes. The
general control of pressure, winds, and ocean currents
over the climatic types is shown in the two following
ideal diagrams, in which the two vertical lines indi-
cate the west and east coasts of the ideal continent,
and the area included reaches to the middle of the ad- * 1

expression which combines the effect of rainfall and evaporating
power. r=rain probability of rainiest month.

1C7, High savanna climate; E3, Yak, or Pamir climate; E4,
Chamois or high alpine climate; F, perpetual frost, without life.
  CLASSIFICATION OF CLIMATES

67

jacent ideal oceans. The line 0°-0° is the equator
(Figs. 15 and 16). The short arrows give the wind
direction 500-1000 metres above the surface; calms
are represented by the sign o ; the long broken arrows

Fig. 15. Prbssurb and Winds in January

indicate the prevailing surface ocean currents. At a a
there is a rise of cold water from beneath the surface
of the ocean. The curving lines are sea-level isobars;
the lower pressures are shaded. The letters and
  68

CLIMATE

boundaries, drawn in short, slanting lines in Fig. 16
indicate the climatic districts of Fig. 11. Fig. 15 is
similar to Fig. 16, as far as these climatic districts are

Fig. 16. Pressure and Winds in July

concemed. Therefore the letters and boundaries
are omitted. Fig. 17 shows the geographical distri-
bution of the climatic types and sub-types.
Ravenstein’s Hygrothermal Types. Recognising
  69

Fig. 17. Köppen’s Classification of Climates in Relation to Vegetation
  70

CLIMATE

the importance of relative humidity as a climatic
factor in its influence upon life, upon agriculture and
upon industry, and basing his grouping of climates
upon certain relations between temperature and rela-
tive humidity, Ravenstein proposes a subdivision of
the earth’s surface into sixteen hygrothermal climatic
types. The general characteristics and examples of
these types are as foiïows:

1.   Hot (73° and over) and very damp (humidity
81% or more): Batavia, Cameroons, Mombasa.

2.   Hot and moderately damp (66-80%): Ha-
vana, Calcutta.

8.   Hot and dry (51-65%): Bagdad, Lahore,
Khartum.

4.   Hot and very dry (50% or less): Disa, Wadi
Halfa, Kuka.

5.   Warm (58° to 72°) and very damp: Walfish
Bay, Arica.

6.   Warm and moderately damp: Lisbon, Rome,
Damascus, Tokio, New Orleans.

7.   Warm and dry: Cairo, Algiers, Kimberley.

8.   Warm and very dry: Mexico, Teheran.

9.   Cool (88° to 57°) and very damp: Greenwich,
Cochabamba.

10.   Coól and moderately damp: Vienna, Mel-
boume, Toronto, Chicago.

11.   Cool and dry: Tashkent, Simla, Cheyenne.

12.   Cool and very dry: Yarkand, Denver.

18. Cold (32° or less) and very damp: Ben Nevis,
Sagastyr, Godthaab.
  CLASSIFICATION OF CLIMATES

71

14.   Cold and moderately damp: Tomsk, Pike’s
Peak, Polaris House.

15.   Cold and dry: (No example given).

16.   Cold and very dry: Pamir.

Classification of Rainfall Systems.—The seasonal

occurrence of rainfall has suggested a classification
of the rainfall systems of the world into types. While
these schemes are useful in climatological study, they
are hardly to be considered as classifications of cli-

Fig. 18. Herbkrtson’s Major Natural Regions

mate. Mühry1 2 gave a rigid scheme of rainfall types
in six beits for each hemisphere, these beits being
divided by latitude lines; and Koppen has prepared
a useful map of the hyetal regions of the world, based
on the seasonal distribution of rainfall types.3

1A. Mühry: Klimatographische Uebersicht der Erde, Leipzig
and Hfeidelberg, 1862, 741-744. Also: AUgemeine geographische
Meteorologie, 1860, 145, and note 28, 199. Containing chart, as
well as the scheme of rainfall types.

2 See Atlas of Meteorology, Plate 19.
  72

CLIMATE

Herbert8orÏ8 Natural Geographical Region*.—A
scheme of “ natural geographical regions ” has been
suggested by Herbertson,1 the basis of classifica-
tion being a certain unity of temperature, rainfall
seasons, configuration and vegetation (Fig. 18).

The different types of natural regions recur in
fairly systematic order on the different continents,
being chiefly controlled by marine and Continental in-
fiuences, and each type, wherever found, has certain
similar general relations to human life and develop-
ment, as well as to animals and plants. The types
are as follows:

1.   Polar. (a) Lowlands (Tundra type); (b)
Highlands (Ice-cap type).

2.   The cool temperate regions. (a) Western
margin (West European type); (b) Eastern mar-
gin (Quebec type); (c) Interior lowlands (Siberian
type); (d) Interior mountain area (Altai type).

8. The warm temperate regions. (o) Western
margin with winter rains (Mediterranean type);
(b). The eastern margin, with summer rains (China
type); (c) The interior lowlands (Turan type); (d)
Plateau (Iran type).

4.   (a) The west tropical deserts (Sahara type);

(b) East tropical lands (Monsoon type); (c) Inter-
tropical table-lands (Sudan type).

1 A. J. Herbertson: “ The Major Natural Regions: An Essay in
Systematic Geography.” Geogr. Joum. xxv., 1965, 300-309. A
revised chart has been published in Herbertson’s The Senior
Geography, Oxford, 1907. (The Oxford Geographies, Vol. III.)
  CLASSIFICATION OF CLIMATES

73

5.   Lofty tropical or sub-tropical mountains (Tib-
etan type).

6.   Equatorial lowlands (Amazon type).

Summary and Conchmom. The broad classifica-
tion of climates into the three general groups of
marine, Continental, and mountain, with the subor-
dinate divisions of desert, littoral, and monsoon, is
convenient for purposes of summarising the interac-
tion of the climatic elements under the Controls of
land, water, and altitude. But in any detailed study,
some scheme of classification is needed in which simi-
lar climates in different parts of the world are
grouped together, and in which their geographic dis-
tribution receives particular consideration. It is ob-
vious from the preceding paragraphs that an almost
infinite number of classifications might be proposed;
for we may take as the basis of subdivision either the
special conditions of one climatic element, as, for ex-
ample, the same mean annual temperature, or mean
annual range of temperature, or the same rainfall, or
rainy seasons, or humidity, and so on. Or again,
similar conditions of the combination of two or more
elements of climate may be made the basis of classifi-
cation. Or we may take a botanical, or a zoölogical
basis. Of the classifications which have been pro-
posed, special reference is here made to those of
Supan, Koppen, and Herbertson. That of Supan,
taken as a whole, gives a rational, simple, and satis-
factory scheme of grouping, whose frequent use in
climatic descriptions would tend toward system, sim-
  74

CLIMATE

plicity, and facility of comparison. It emphasises
the essentials of each climate, and serves to impress
these essentials upon the mind by means of the com-
pact, well-considered summary which is given in the
case of each province described. Obviously, no clas-
sification of climates which is at all complete can ap-
proach the simplicity of the ordinary classification of
the zones.

Köppen’s admirable scheme of subdividing climates,
with the emphasis on the botanical side, is perhaps
better adapted to the use of students of plant geogra-
phy than of general climatology. But it has the
great merit of recognising the existing differences of
climate between east and west coasts, and between
coasts and interiors. The co-ordination of districts
of vegetation and of climate, which this scheme so
strikingly emphasises, is a noteworthy fact in clima-
tology. The subdivision could obviously be continued
almost indefinitely.

Herbertson’s classification of the natural geo-
graphical regions is, on the whole, not very unlike that
adopted in Supan’s climatic provinces, but is less com-
plete. It is obvious that no scheme of subdivision
of this kind can be regarded as being rigid or as sat-
isfying all students of questions of distribution.
Nevertheless, some general grouping of climatic re-
gions with reference to similar features of tempera-
ture and rainfall and configuration, is a distinct help
in most geographical studies. The larger types
naturally recur on the several continents, in a fairly
  CLASSIFICATION OF CLIMATES

75

systematic fashion. It results from this fact that
there is a recurrence, in a large way, of somewhat
similar conditions of life. This is a particularly help-
ful consideration in investigations of the economie
and political history of mankind. The chief pecul-
iarities of the important types can be readily leamed;
the special variations in individual areas may be in-
vestigated for each case by itself.

Ravenstein’s hygrothermal types rest upon unsatis-
factory data, and regions of very different climatic
conditions are grouped together because they happen
to have the same mean annual temperature and rela-
tive humidity.
  CHAPTER IV

THE CHABACTERISTICS OF THE ZONES: I.—THE

TROPICS

General: Climate and Weather—Temperature—The Seasons—
Physiological Effects of Heat and Humidity—Pressure—Winds
and Rainfall—Land and Sea Breezes—Thunderstorms—Cloudi-
ness—Intensity of Skylight and Twilight—Climatic Subdivi-
sions:—I. The Equatorial Belt.—II. Trade Wind Beits.—III.
Monsoon Beits.—IV. Mountain Climate.

General: Climate and Weather. The so-called
“ torrid zone ” has been variously bounded. lts
limits have been set at the tropics (lat. 23^°); at the
mean annual isotherms of 68°, which also correspond
closely with the poleward extension of palms; and at
the polar margins of the trade winds. The dominant
characteristic of this great belt, embracing but a little
less than one-half of the earth’s surface, is the re-
markable simplicity and unifórmity of its climatic
features. This simplicity is reflected in the striking
regularity in the recurrence of the ordinary weather
phenomena. The tropics lack the proverbial uncer-
tainty and changeableness which characterise the
weather of the higher latitudes. In the torrid zone,
weather and climate are essentially synonymous
terms. Periodic phenomena, depending upon the
daily and annual march of the sun, are dominant.
Non-periodic weather changes are wholly subordi-

76
  CHARAVTERISTICS OF ZONES—TROPICS 77

nate. The succession of daily weather changes is even
more regular, and the distribution of the climatic ele-
ments is even more uniform over the tropical oceans
than over the lands. In special regions only, and at
special seasons, is the regular sequence of weather
temporarily interrupted by an occasional tropical
cyclone. These cyclones, although comparatively in-
frequent, are notable features of the climate of the
areas in which they occur. Generally bringing very
heavy rains, and thus locally increasing the total an-
nual precipitation by a considerable amount, they yet
cause no marked temperature changes such as those
which are the common accompaniments of extra-
tropical cyclones. The devastation produced by one
of these storms often affects the economie condition
of the people in the district of its occurrence for many
years.

Temperature. The sun is always well up in the
sky. The length of day and night varies little.
Hence the mean temperature is high, it is very uni-
form over the whole zone, and there is little variation
during the year. The mean annual isotherm of 68°
is a rational limit at the polar margins of the zone,
and the mean annual isotherm of 80° encloses the
greater portion of the land areas, as well as much of
the tropicaK oceans. The isotherms are thus far
apart. The warmest latitude circle for the year is
not the equator, but latitude 10° north. The highest
mean annual temperatures, shown by the isotherm
of 85°, are in central Africa, in India, the north of
  78

CLIMATE

Australia and Central America, but, with the excep-
tion of the first, these areas are small. Massowah, on
the Red Sea, has an annual mean of over 86°. The
temperatures average highest where there is little
rain, and not in the belt of heavy equatorial rains,
where the clouds afford some protection from the
sun’s rays. In June, July, and August there are
large districts in the south of Asia, and in northern
Africa, with temperatures over 90°. Winds blowing
out from these heated deserts are uncomfortably hot
and dusty.

Over nearly all of the zone the mean range of tem-
perature is less than 10°, and over much of it, especi-
ally the oceans, it is less than 5°. At Equatorville,
in the interior of Africa, on the Congo, the mean
annual range is only a little over 2°; at Iquitos (lat.
8.7° S.), in Peru, it is 4.8°* Even near the margins
of the zone, where the seasonal di ff erences are great-
est, the ranges are less than 25°, as at Calcutta, Hong
Kong, Rio de Janeiro and Khartum. The mean
daily range is usually larger than the mean annual.
Thus at Equatorville the former is about 14.5°. It
has been well said that “night is the winter of the
tropics.” The differences between the maximum and
minimum temperatures of the year near the equator
are not much greater than the daily range. Over an
area covering parts of the Pacific and Indian Oceans,
from Arabia to the Caroline Islands and from Zan-
zibar to New Guinea, as well as on the Guiana coast,
  CHABACTERISTICS OF ZONES—TROPICS 79

the minimum temperatures do not normally fall be-
low 68°,. and over much of the torrid zone as a whole
they do not fall below 59°. Towards the margins of
the zone, however, the minima on the continents fall
to, or even below, 82°. Maxima of 115°, and even over
120° (122°), occur over the deserts of northern
Africa. A district where the mean maxima exceed
118° extends from the western Sahara to northwest-
ern India, and over central Australia. Near the
equator the maxima are therefore not as high as those
in many so-called “ temperate ” climates. The
greater portion of the torrid zone is a water surface,
and marine conditions are therefore typical for most
of it. These tropical oceans show remarkably small
variations in temperature. The ChaUenger re-
sults showed a daily range of hardly 0.7° in the sur-
face water temperature on the equator, and Schott de-
termined the annual range as 4.1° on the equator; 4.3°
at latitude 10°, and 6.5° at latitude 20°. It has been
clearly pointed out by Hann that the uniform dis-
tribution of temperature throughout the year—the
dominant feature of the tropics—results not only
from (1) the small variation in insolation and in the
length of the day; but also (2) from the great extent
of the zone, which makes it impossible for cold winds
from higher latitudes to penetrate into the lower lati-
tudes; ( the oblique course of the trades, which are
well warmed on their indirect road towards the equa-
tor; (4) the slight noctumal cooling, where the air
  80

[Prometheus]

CLIMATE

is damp and vapour is readily condensed; and (5) the
great extent of the tropical oceans, which gives so
much of the zone a marine climate.

The Seasons. In a true tropical climate, seasons,
in the temperate zone sense, do not exist. The varia-
tions in temperature throughout the yeai' are so slight
that the seasons are not classified according to tem-
perature, but depend on rainfall and the prevailing
winds. The life of animals and plants in the tropics,
and of man himself, is regulated very largely, in some
cases almost wholly, by rainfall. Agriculture pros-
pers, or fails, according to the sufficiency and punct-
ual appearance of the rains. After a long dry season,
when the rains come, there is an extraordinarily
sudden awakening of the parched and dusty vegeta-
tion. Where, on the other hand, there is abundant
moisture throughout the year, a tree may at the same
time be carrying buds, blossoms, and ripe fruit.
Vegetation under these conditions has been well called
non-periodic. Although the tropical rainy season is
characteristically associated with a vertical sun (i. e.,
summer), that season is not necessarily the hottest
time of the year. The temperature is usually some-
what lower under the clouds. The rainy season often
goes by the name of winter for this reason, and also
because the weather is dull. The time of the maxi-
mum temperature is also controlled by the rainy sea-
son. Towards the margins of the zone, with increas-
ing annual ranges of temperature, seasons in the ex-
tra-tropical sense gradually appear.
  CHARACTERISTICS OF ZONES—TROPICS 81

Physiological Effects of .Heat and Humidity.
Tropical monotony of heat is associated with high
relative humidity, except over deserts and in dry sea-
sons. The air is therefore muggy and oppressive.
The high temperatures are disagreeable and hard to
bear. The “ hot-house air ” has an enervating effect.
Energetic physical and mental action are often diffi-
cult, or even impossible. The tonic effect of a cold
winter is lacking. The most humid districts in the
tropics are the least desirable for persons coming from
higher latitudes; the driest are the healthiest. The
most energetic natives are the desert-dwellers. The
monotonously enervating heat of the humid tropics
weakens, so that man becomes sensitive to slight tem-
perature changes. A fall of temperature to within
a few degrees of 70° seems to some tropical natives
almost unbearably cold, and certain African tribes
sleep on clay banks heated inside by fires, although
the mean temperature of the coldest month is over
70°. In drier climates such changes are more easily
home. The intensity of direct insolation, as well as
of radiation from the earth’s surface, may produce
sunstroke and heat prostration. “Beware of the
sun ” is a good rule in the tropics.

Pressure. The uniform temperature distribution
in the tropics involves uniform pressure distribution.
Pressure gradients are weak. The annual fluctuations
are slight, even on the continents. The diurnal varia-
tion of the barometer is so regular and so marked
that, as von Humboldt said, the time of day can be
  82

CLIMATE

told withui about fifteen minutes if the reading of the
barometer be known. Even severe thunderstorms do
not overcome the regular diurnal march of the press-
ure, but the approach of tropical cyclones can be
foretold by the pressure changes.

Winds and Rainfall. Within the tropics, there are
both heavy rains and large districts of very deficiënt
precipitation. Along the barometric equator, where
the pressure gradients are weakest, is the equatorial
belt of calms, variable winds and rains—the dol-
drums. This belt, with its actively ascending, damp,
hot air, offers exceptionally favourable conditions for
abundant rainfall, and belongs among the rainiest
regions of the world, averaging probably about one
hundred inches. The rainfall is so heavy that the
salinity of the surface waters of the oceans is actu-
ally less than in the latitudes of the trades. The
sky is prevailingly cloudy, especially in the early
afternoon hours; the air is hot and oppressive; heavy
showers and thunderstorms are frequent, chiefly in
the afternoon and evening—conditions not very un-
like those which exist during certain spells of sum-
mer weather in the north temperate zone. There are
the dense tropical forests of the Amazon and of equa-
torial Africa. There frost and drought need not be
feared. This belt of calms and rains, of variable
width and rather indefinite limits, shifts north and
south of the equator after the sun. It is dreaded by
seamen because sailing vessels are apt to have long
delays in Crossing it. The calm belt is generally
  CHABACTERISTICS OF ZONES—TROPICS 83

somewhat narrower than the belt of rains, the warm
ascending air being carried north and south, and giv-
ing precipitation beyond the limits of the calm zone.
In striking contrast are the easterly trade winds,
blowing between the tropical high pressure beits and
the equatorial belt of low pressure, and supplying to
the doldrum belt a constant flow of warm air which
already contains a large amount of water vapour,
evaporated from the oceans by the trades, and needs
only a moderate cooling in order to give abundant
rainfall. Of great regularity, embracing about one-
half of the earth’s surface, and adding greatly to the
uniformity of tropical climates, the trades have long
been favourite sailing routes because of the steadi-
ness of their winds, the infrequency of storms, the
brightness of their skies, and the freshness of the air,
all of which are in pleasing contrast with the muggy
and oppressive calms of the doldrums. The most de-
sirable house-sites in the tropics are very commonly
on the top of some elevation, exposed to the trade
wind. The trades are subject to many variations.
Their northern and southem margins shift north and
south af ter the sun; at certain seasons they are
interrupted, often over wide areas near their equator-
ward margins, by the migrating belt of equa-
torial rains and by monsoons; near lands, they
are often interfered with by land and sea breezes;
in certain regions, they are invaded by violent
cyclonic storms. The trades, except where they blow
onto windward coasts, or over mountains, are natu-
  84

CLIMATE

rally drying winds, for they blow from higher to lower
latitudes. Some facts seem to show that there is a
descending component in the trades. They cause the
deserts of northern and Southern Africa, eastern Asia,
Australia, and Southern South America. Over the
oceans, the only rains in the trade wind beits are in
the form of passing showers.

The monsoons on the southem and eastern coasts
of Asia are the best known winds of their class. In
the northern summer, the south-west monsoon, warm
and sultry, blows over the latitudes from about 10°
north to and beyond the northem tropic, between
Africa and the Philippines, giving rains over India,
the East Indian Archipelago, and the east coasts of
China. These winds reach a storm velocity over the
Arabian sea. In winter, the south-east monsoon, the
normal, cold-season, Continental outflow from Asia,
combined with the north-east trade, generally cool and
dry, covers the same district, extending as far north
as latitude 30°. Crossing the equator, these winds
reach northern Australia, and the western islands of
the South Pacific, as a north-west rainy monsoon,
while this region in the opposite season has the normal
south-east trade. Other monsoons are found
in the Gulf of Guinea and in equatorial Africa.
Wherever they occur, they control the seasonal
changes.

The most important climatic phenomenon of the
year in the tropics is the rainy season. Tropical
rains are, in the main, summer rains, i. e., they follow,
  CHARACTERI8T1C8 OF Z0NE8—TROPICS 85

as a general rule, soon after the “ vertical sun,” 1 the
rainy season coming when the normal trade gives way
to the equatorial belt of rains, or when the summer
monsoon sets in. There are, however, many cases
of a rainy season when the sun is low, especially on
windward coasts in the trades. Tropical rains come
usually in the form of heavy downpours and with
a well-marked diurnal period, the maximum varying
with the locality between noon and midnight. The
conditions at Calcutta, as shown in the accompany-
ing data, are fairly typical.2

DIURNAL DISTRIBUTION OF RAINFALL AT CALCUTTA.

12 P.M.   2 A.M.   50   12 M.—2 P.M.   111
2-4 A.M.   71   2-4 P.M   116
4-6 A.M.   65   4-6 P.M.   120
6-8 A.M.   71   6-8 p.m.   128
£-10 A.M.   58   8-10 p.m.   73
10 A.M.   12 M.   92   10 P.M.   12 P.M.   45

Local influences are, however, very important, and
in many places night rainfall maxima are found.

The tropical rainy season is therefore not to be
thought of as a period of continuous rains, falling
steadily day and night for week after week. The
momings are often fine, with clean air, well washed
by the rains of the preceding afternoon or night.
Woeikof’s detailed studies of tropical rainfalls, as a
whole, lead him to the conclusion that (1) the inten-
sity of tropical rains averages higher than in middle

1   It will be remembered that at all places within the tropics the
sun is vertical twice in the year.

2   Seven year record; expressed in thousandths of the daily mean.
  86

CLIMATE

latitudes, but the difference is not great; (2) the
heaviest short downpours have, so far as observation
now goes, occurred in middle latitudes; ( general,
moderate rainfalls lasting continuously for many
hours, which are common in the temperate zones, are
known in many parts of the tropics and have even
been given special names; (4) the heaviest daily rain-
falls have been noted outside the tropics, as at Cherra-
punji, for example; and (5) it is likely that the most
intense rains in the tropics fall during large tropical
cyclones.

Land and Sea Breezes. The sea breeze is an im-
portant climatic feature on many tropical coasts.
With its regular occurrence, and its cool, clean air,
it serves to make many districts habitable for white
settlers, and has deservedly won the name of “ the
doctor.” On not a few coasts, the sea breeze is a
true prevailing wind. The location of dwellings is
often determined by the exposure of a site to the sea
breeze. For this reason, many native villages are put
as near the sea as possible. The houses of well-to-do
foreigners often occupy the healthiest and most de-
sirable locations, where the sea breeze has a free en-
trance, while the poorer native classes live in the lower,
less exposed and less desirable places. A social
stratification is thus determined by the sea breeze.

Thunderstorms. Local thunderstorms are fre-
quent in the humid portions of the tropics. They
have a marked diurnal periodicity; find their best
opportunity in the equatorial belt of weak pressure
  CHARACTERISTICS OF ZONES—TROPICS 87

gradients and high temperature, and- are commonly
associated with the rainy season, being most common
at the beginning and end of the regular rains. In
many places, thunderstorms occur daily throughout
tljeir season, with extraordinary regularity and great
intensity. Lightning is, however, reported as very
seldom doing any damage. Attention has been
called to the fact that the frequent electrical dis-
charges cause the rain water to be relatively rich in
nitric acid. This condition, together with the carbon
dioxide in the rain water and the high temperature
of the same, promotes active and deep rock decom-
position. In higher latitudes, where the ground may
be frozen part of the year, and where the decompos-
ing action of rain water is less, there is less of this
effect. In northem India, hail-storms of great
violence occur, and persons have been killed by
them.

Cloudiness. Taken as a whole, the tropics are not
favoured with such clear skies as is often supposed.
Cloudiness varies about as does the rainfall. The
maximum is in the equatorial belt of calms and rains,
where the sky is always more or less cloudy. The
minimum is in the trade latitudes, where fair skies as
a whole prevail.1 The equatorial cloud belt moves
north and south after the sun. Wholly clear days
are very rare in the tropics generally, especially near

1 Supan, Grundzüge der Physischen ErdJcunde, 3d ed., 1903, Fig.
18, page 53, gives a diagram showing the distribution of rainfall
and cloudiness (also of other elements) according to latitude.
  88

CLIMATE

the equator, and during the rainy season heavy clouds
usually cover the sky.

Tropical clouds and rainfall, as a whole, repeat, in
an exaggerated form, the summer conditions of much
of the north temperate zone. Broken skies; cumulus
and cumulo-nimbus clouds; heavy showers or thun-
derstorms—these usually characterise the rainy sea-
son. Skies clear, or flecked with scattered small
cumuli, are typical of the dry season. Wholly over-
cast, dull days, such as are common in the winter of
the temperate zone, occur frequently only on tropical
coasts in the vicinity of cold ocean currents, as in
Peru and on parts of the west coast of Africa. In
these same regions ocean fogs are common.

Intensity of Skylight and Twilight. The inten-
sity of the light from tropical skies by day is trying,
even almost unbearable, to newcomers. The intense
insolation, together with the reflection from the
ground, increases the general dazzling glare under a
tropical sun, necessitating protection of some sort.
The far-famed deep blue of the tropical sky is much
exaggerated. During much of the time, smoke from
forest and prairie fires (in the dry season); dust (in
deserts), and water vapour give the sky a pale, whit-
ish appearance. In the heart of the trade wind beits
at sea, the sky is much more of a deep blue. The
beauties of tropical sunrise and sunset, and of the
tropical night, have, however, not been overrated.
Twilight within the tropics is shorter than in higher
latitudes, but the coming on of night is less sudden
  CHABACTERISTICS OF ZONES—TROPICS 89

than is generally assumed. Pechuel-Lösche and
others have shown that it is possible, on the Loango
coast, to read ordinary print twenty to thirty minutes
after sunset.

Climatic Subdivision*. The rational basis for a
classification of the larger climatic provinces of the
torrid zone is found in the general wind systems and
in their control over rainfall. Following this scheme
there are these subdivisions: I. The equatorial belt;
II. The trade wind beits; III. The monsoon beits.
In each of these subdivisions there are modifications,
due to ocean and Continental influences. In general
both seasonal and diurnal phenomena and changes
are more marked in Continental interiors than on the
oceans, islands, and windward coasts. Further, the
effect of altitude is so important that another subdi-
vision should be added to indude IV. Mountain
climates.

I. The Equatorial Belt. Within a few degrees
of the equator, and when not interfered with by other
Controls, the annual curve of temperature has two
maxima following the two zenithal positions of the
sun, and two minima at about the time of the solstices.
This, which is known as the equatorial type of annual
march of temperature, is illustrated in the data
and curves for the interior of Africa, Batavia, and
Jaluit. (Fig. 19).

The greatest range is shown in the curve for the in-
terior of Africa; the curve for Batavia illustrates in-
sular conditions with less range; and the oceanic type,
  90

CLIMATE

for Jaluit, Marshall Islands, gives the least range.
At Jaluit, the daily maxima for the entire year are
between 88° and 91.5° and the daily minima between
75° and 77°. This doublé maximum is not a
universal phenomenon, there being many cases where
but a single maximum occurs, as will be seen
later.

TAB LH OF MEAN MONTHLY TEMPERATURES FOR SELECTED
TROPICAL STATIONS1

   I.   Equatorial Type         II.   Tropical Type      
   Conti-  nental   Insular   Marine   Continental      Monsoon   Insular   
   Africa  interior   Batavia   Jaluit,  Marshall  Islands   Wadi  Halfa   Alice  Springs   Nagpur   Hono-  lulu   James-  town
Lat.   8.1° N.   8° 11' S.   5° 65' N.   21° 63' N.   23° 38' S.   21°9' N.   21° 18' N.   15° 66' S.
Loog.   23.6° E.   106° 50' E.   169° 40' E.   31° 20' E.   133° 37'E.   79° 11' E.   157° 60'N.   5° 43'W.
Altitude   1837 ft.   23 ft.   10 ft.   426 ft.   1926 ft.   1093 ft.   49 ft.   39 ft.
Jan.   73.4°   77.5°   80.8°   81.3°   85.6°   68.2°   70.0°   74.7°
Feb.   77.2°   77.7°   81.0°   68.6°   83.3°   73 8°   70.3°   75.9°
Mar.   83.8°   78.4°   80.6°   73.0°   77.9°   83 7°   70.9°   73.8°
April  May   85.3°   79.3°   80.4°   81.0°   68.5°   90.3°   72.9°   75.0°
   83.7°   79.5°   80.4°   87.1®   80.6°   94.3°   74.3®   88.9°
Tune   81.5°   78.8°   80.2°   91.4°   54.0°   85.6°   76.1°   70.5°
July   78.4°   78.3°   80.2°   93.4°   51.8°   80.1°   77.2°   71.8°
Aug.   75.7°   78.6°   80.4°   91.6°   59.4°   80.2°   77.5°   69.4°
Sept.   77.7°   79.3°   80.4°   87.1°   66.6°   80.4°   77.2°   87.6°
Oct.   78.1°   79.5°   80.8°   83.1°   73.4°   78.6°   76.5°   86.7°
Nov.   75.7°   79.0°   80.8°   71.4°   79.7®   72.3°   73.8°   67.8°
Dec.   72.0°   78.1°   80.6°   84.8°   82.8°   66.7°   71.4°   71.8°
Mean   78.6°   78.8°   80.6°   79.3°   70.3°   79.5° -   73.9°   71.1°
Range   12.4°   2°   0.8°   32.1°   33.8°   27.6°   7.5   10.2°

As the belt of rains swings back and forth across
the equator after the sun, there should be two rainy
seasons with the sun vertical, and two dry seasons
when the sun is farthest from the zenith, and while

1 Given to nearest tenth of a degree Fahr.
  CBARACTERISTIC8 OF ZONES—TROPICS 91

the trades blow. These conditions prevail on the
equator, and as far north and south of the equator
(about 10°-12°) as sufficiënt time elapses between
the two zenithal positions of the sun for the two rainy

Fig. 19. Annual March of Temperature: Equatorial Type.

A: Africa, interior. B: Batavia. J: Jaluit, Marshall Islands.

seasons to be distinguished from one another. In
this belt, under normal conditions, there is, therefore,
no dry season of any considerable duration. The
doublé rainy season is clearly seen in equatorial
  Fig. 20. Annual March of Rainfall
in the Tropics

S. A: South Africa. Q: Quito. S. P:
Sao Paulo. M: Mexico. H: Hilo.

P. D: Port Darwin.

92
  CHABACTERISTICS OF ZONES—TROPICS   93

Africa and in parts of equatorial South America.
The maxima lag somewhat behind the vertical sun,
coming in April and November, and are unsymmetri-
cally developed, the first maximum being the principal
one. The minima are also unsymmetrically devel-
oped, and the so-called “ dry seasons ” are seldom
wholly rainless. In this equatorial belt, the annual
range of rainfall is generally below 20%; in the west-
ern portion of the Malay Archipelagd and on the
upper Amazon, it is below 10%. In these latitudes,
therefore, the distribution of rainfall is not unlike that
in extra-tropical latitudes which are under the marine
regime of rainfall, there being precipitation at all
seasons.

This rainfall type with doublé maxima and minima
has been called the equatorial type, and is illustrated
in the following data and in the curves for south
Africa and Quito. (Fig. 20).

The mean annual rainfall at Quito is 42.12 inches.
These doublé rainy and dry seasons are easily modi-
fied by other conditions, as by the monsoons of the
Indo-Australiaii area, for example, so there is no
rigid belt of equatorial rains extending around the
world. In South America, east of the Andes, the
distinction between rainy and dry seasons is often
much confused. In this equatorial belt, the cloudi-
ness is high throughout the year, averaging .7 to .8,
with a relativeljj small annual period. The data and
curve following are fairly typical, but the annual
period varies greatly under local Controls. (Fig. 21).
  94

[Prometheus]

CLIMATE

TABLB SHOWING MONTHLY DISTRIBUTION OP RAINPALL POR
SELECTED TROPICAL STATIONS1

         Tropics         
   Doublé Rainy      Single Rainy Season         
   Season  Equatorial      Margin of Tropics      Trade  Rains   Monsoon . Rains
         Southem   Northem      
   Southern  Africa   Quito   S&o  Paulo   Mexico   Hilo   Port  Darwin
Latitude   6° S.   Equator   23.5° S.   19.4° N.   19.7° N.   12.5° S.
Jan.   86   77   195   7   79   241
Feb.   80   92   156   9   94   215
March   123   115   103   26   86   166
April  May   195   165   58   26   94   61
   91   109   60   85   66   23
June   10   35   46   174   55   1
July   7   25   19   180   82   0
Aug.   17   52   31   207   81   2
Sept.   37   60   60   179   73   5
Oct.   61   91   82   79   88   38
Nov.   188   94   74   20   95   72
Dec.   105   85   116   8   107   176

TABLB SHOWING MONTHLY DI8TRIBUTION OP CLOUDINESS IN AN

BQUATORIAL CLIMATE. (CAMEROONS; GABOON. LAT. 3°

N., WEST AFRICA.)

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov Dec. Year
5.4 6.3 7.0 7.2 7.4 7.7 8.9 8.6 8.4 8.0 7.4 6.6 7.4

At greater distances from the equator than about
10° or 12°, the sun is still vertical twice a yearwithin
the tropics, but the interval between these two dates
is so short that the two rainy seasons merge into one,
in summer, and there is also but one dry season, in

1 The figures in this table are thousandths of the mean annual
rainfall. In the first column of the table, the average of a con-
siderable number of stations is given.
  CHARACTERISTICS OF ZONES—TROPICS 95

winter. This is the so-called tropical type of rain-
fall,1 and is found where the trade beits are encroached
upon by the equatorial rains during the migration of
these rains into each hemisphere. It is illustrated in

E: Equatorial type. M: Monsoon type

the data and curves for Sao Paulo, Brazil, and for the
city of Mexico (see rainfall table above and Fig. 20).
The mean annual rainfall at Sao Paulo is 54.18
inches, and at Mexico 22.99 inches.

The districts of tropical rains of this type lie along
the equatorial margins of the torrid zone, outside of
the latitudes of the equatorial type of rainfall. The
rainy season becomes shorter with increasing distance
from the equator. The weather of the opposite sea-
sons is strongly contrasted. The single dry season
lasts longer than either dry season in the equatorial

1 Supan calla it the margimU type of the tropics.
  96

CLIMATE

belt, reaching eight months in typical cases, with the
wet season lasting four months. The lowlands often
become dry and parched during the long, dry trade
wind season (winter), and vegetation withers away,
while grass and flowers grow in great abundance
and all life takes on new activity during the time
when the equatorial rainy belt, with its calms, variable
winds, and heavy rains, is over them (summer). The
Sudan lies between the Sahara and the equatorial
forests of Africa. It receives rains, and its vegeta-
tion grows actively, when the doldrum belt is north
of the equator (May-August). But when the trades
blow (December-Match), the ground is parched and
dusty. The Venezuelan llanos have a dry season in
the northem winter, when the trade blows. The
rains come in May-October. The campos of Brazil,
south of the equator, have their rains in October-
April, and are dry the remainder of the year. The
Nile overflow results from the rainfall on the mount-
ains of Abyssinia during the northward migration
of the belt of equatorial rains.

Simple tropical rainfalls, as shown in the above
curves, are typical of large areas, but they are not in-
frequently complicated by association with trade or
monsoon rains, as in the West Indies, Central Amer-
ica, and India. The true doldrum rains may come
along the polar margin of the equatorial low-pressure
belt, when this belt is moving equatorward, followed
by the trades.

The so-called tropical type of temperature variation,
  Fig. 22. Annual March of Tempera- .
ture:   Tropical Type

W: Wadi Halfa. N: Nagpur. A: Alice
Springs. H: Honolulu. J: Jamestown,

St. Helena

7

97
  98

CLIMATE

with one maximum and one minimum, is illustrated
in the data given in the table on page 90, and
in the accompanying curves for Wadi Halfa, in Up-
per Egypt; Alice Springs, Australia; Nagpur, India;
Honolulu, Hawaii, and Jamestown, St. Helena.
The effect of the rainy season is often shown in a
displacement of the time of maximum temperature to
an earlier month than the usual one. During the
rains, the temperature is apt to remain constant, as
in the case of Nagpur, and of other stations in
India, Mexico, and the interior of Senegambia. This
type of temperature curve is characteristic of most
of the tropics outside of the latitudes reached by the
equatorial belt.

II. Trade Wind Beits. The trade beits near
sea-level are characterised by fair weather, steady
winds, infrequent light rains or. even an almost com-
plete absence of rain; very regular, although slight, *
annual and diurnal ranges of temperature, and a
constancy and regularity of weather which is more
truly “ temperate ” than that of most of the so-called
temperate zone. The climate of the ocean areas in
the trade wind beits is indeed the simplest and most
equable in the world, the greatest extremes—and even
these are moderate—being found to leeward of the
larger lands, where the Continental conditions are
carried offshore by the prevailing winds. On the
lowlands swept over by the trades, beyond the polar
limits of the equatorial rain belt (roughly between
lats. 20° and 80°), are most of the great deserts of the
  CHARACTERI8TICS OF ZONES—TROPICS 9Ö

world. These deserts extend directly to the water’s
edge on the leeward, western coasts of Australia,
south Africa, and South America. In the two lat-
ter regions, the desert conditions are further helped
by the presence of cold ocean currents offshore. Be-
cause of their great extent, these trade wind deserts
constitute one of the most important climatic districts
in the world.

The ranges and extremes of temperature are much
greater over the deserts, especially the Continental in-
teriors, than over the oceans of the trade wind beits.
Minima of 32° or less occur during clear, quiet nights,
and daily ranges of over 50° are common. The mid-
summer mean temperature rises above 90°, with noon
maxima of 110°, or more, in the non-cloudy, dry air
of a desert day. The days, with high, dry winds,
carrying dust and sand, with extreme heat, accent-
uated by the absence of vegetation, are disagreeable
or even dangerous to life; but the calmer nights, with
active radiation under clear skies, are much more
comfortable. The nocturnal temperatures are even
not seldom too low for comfort in the cooler season,
when thin sheets of ice may form. Under the strong
insolation by day and the quick cooling by night,
rocks in the deserts split and break up. On the whole,
however, man is less susceptible to the larger tem-
perature ranges in tropical deserts than to the smaller
ones in the equatorial belt, because of the lower rela-
tive humidity in the former case. In the trade wind
deserts, as in other arid regions, man is nomadic.
  100

CLIMATE

While the trades are drying winds as long as they
blow strong over the oceans, or over lowlands, they
contain a large amount of water vapour, and readily
beoome rainy if they are cooled during an ascent
over a mountain or highland. Hence the windward
(eastern) sides of mountains or bold coasts in the
trade wind beits are well watered, while the leeward
sides, or interiors, are dry. Mountainous islands in
the trades, like the Hawaiian Islands, many of the
East and West Indies, the Philippines, Bomeo, Cey-
lon, Madagascar, Teneriffe, etc., show marked diver-
gences of this sort. The eastern coasts of Guiana,
Central America, south-eastern Brazil, south-eastem
Africa, and eastern Australia are well watered, while
the interiors are very dry in the two last-named coun-
tries. The eastern highland of Australia constitutes
a more effective barrier than that in south Africa;
hence the Australian interior has a more extended
desert. South America in the south-east trade belt is
not well enclosed on the east, and the most and por-
tion is an interior district near the eastern base of
the Andes, where the land is low. Even far inland,
the Andes again provoke precipitation along their
eastern base, and the narrow Pacific Coastal strip, to
leeward of the Andes, is a very pronounced desert
from the equator to about lat. 30° S. The cold ocean
waters, with prevailing southerly (drying) winds
alongshore, are additional factors in causing this
aridity. The Peruvian climatic province is abnorm-
ally cool. Highlands in the trade beits are therefore
  CE ARA CTERISTICS OF ZONES—TROPICS 101

moist on their windward slopes—even in deserts,
mountains provoke local rainfall, tree growth, and
local streams—and becomes oases of luxuriant plant
growth, while close at hand, on the leeward sides, dry
savannas or deserts may be found. The damp, rainy
and forested windward (N.E. trade) side of Central
America was, from the earliest days of European oc-
cupation, left to the natives, while the centre of civili-
sation was naturally established on the more open and
sunny south-western side.

The rainfall associated with the conditions just de-
scribed is known as the trade type. These rains have
a' maximum in winter, when the trades are most ac-
tive, this being a departure from the general rule of
summer rains in the tropics. In cases where the trade
blows steadily throughout the year against mount-
ains or bold coasts, as on the Atlantic coast of Cen-
tral America, there is no really dry season. The data
and curve for Hilo (mean annual rainfall 145.24
inches), on the windward side of the Hawaiian
Islands, show typical conditions (see Fig. 20). The
tropical rains are convectional, and therefore prefer
the warm season; the trade rains are orographic, and
have a winter maximum.

The trade type of rainfall is often much compli-
cated by the combination with it of the tropical type
and of the monsoon type (see next paragraph).
Zanzibar, for example, has its principal maximum of
rainfall in April, which is pure tropical, and has a
secondary maximum in December, which is trade.
  102

CLIMATE

Again, on the lee of highlands which receive a winter
maximum on their windward slopes, summer rains
may occur at the time when the trade is weakest, and
the otherwise long dry season is interrupted by scat-
tering showers. In the Malay archipelago, there are
complications of equatorial and trade rains; likewise
in the West Indies. Trade rains often have a tend-
ency to give precipitation both day and night, while
torrid zone rains generally prefer the day.

III.   Monsoon Beits. In a typical monsoon re-
gion, such as that of India, eastern Asia, and the ad-
jacent islands, the rains follow the vertical sun, and
therefore have a simple annual period much like that
of the tropical type above described, the dry season
coming when the sun is lowest (winter). This mon-
soon type of rainfall is well illustrated in the data
and curve for Port Darwin (mean annual rainfall
62.72 inches), in Australia. This summer monsoon
rainfall results from the inflow of a large body of
warm, moist air from the sea on to a land area; a con-
sequent retardation of the velocity of the air currents,
as the result of friction, and an ascent of the air, the
rainfall being particularly heavy where the winds.
have to climb over high lands. Thus, in India, the
precipitation is heaviest at the head of the Bay of
Bengal, where Cherrapunji, at the height of 4455
feet in the Khasi Hills, has a mean annual rainfall of
between 400 and 500 inches; along the southem base
of the Himalayas (60 to 160 inches); on the bold
western coast of the peninsula (80 to 120 inches and
  CHARACTERISTICS OF ZONES—TROPICS 103

over), and on the mountains of Burma (up to 160
inches). In the rain-shadow of the Western Ghats,
the Deccan often suffers from drought and famine
unless the monsoon rains are abundant and well dis-
tributed, and the decreasing rainfall up the Ganges
valley leaves the Indus plain with a deficiency (less
than five inches). The prevailing direction of the
rainy monsoon wind in India is south-west; on the
Pacific coast of Asia, south-east. This monsoon dis-
trict is very large, including the Indian Ocean, Ara-
bian Sea, Bay of Bengal, and adjoining Continental
areas; the Pacific coast of China, the Yellow and
Japan seas, and numerous islands from Bomeo to
Sakhalin on the north and to the Ladrone Islands on
the east. Where the seasons are clearly defined in
India, they are three in number: a cool, dry season
(winter) when northerly trade winds prevail, and
when there is little or no rainfall except in the north-
em provinces, where moderate cyclonic storms oc-
casionally occur; a wet season, sultry and oppressive,
with the inflowing south-west monsoon of summer;
and a hot, dry season before the beginning of the
rains. The beginning of the monsoon rains usually
comes suddenly (“ burst ”), with heavy thunder-
storms. A typical temperature curve for a monsoon
district is that for Nagpur, in the Indian Deccan
(see Fig. 22), and a typical cloudiness curve is given
in Fig. 21, the maximum coming near the time of the
vertical sun, in the rainy season, and the minimum in
the dry season.
  104

CLIMATE

TABLE SHOWING MONTHLY DISTRIBUTION OP CLOUDINESS IN A
MONSOON CLIMATE (BENGAL, LAT. 23.5° N.).1

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year
1.9 1.8   2.6 3.0 4.5 7.5   8.5   8.4   7.5 4.3   2.5   1.8 4.5

In the Australian monsoon region, which reaches
across New Guinea and the Sunda Islands, and west
of Australia, in the Indian Ocean, over latitudes
0°-10° S., the monsoon rains come with north-west
winds in the period between November and March or
April. The northern portion of Australia is thus
watered by zenithal summer rains, and the Southern
portions of Sumatra, Borneo, and Java are also un-
der the influence of this north-west monsoon. The
essential features of the whole Indo-Australian mon-
soon belt, therefore, are a fairly uniform distribution
and small annual range of temperature; and well-
marked periodic rains, coming with north-west or
south-west winds according to the hemisphere.

The general rule that eastern coasts in the tropics
are the rainiest finds exceptions in the case of the rainy
western coasts in India and other districts with simi-
lar rains. On the coast of the Gulf of Guinea, for
example, there is a small rainy monsoon area during
the summer; heavy rains fall on the seaward slopes of
the Cameroon Mts. Not far inland, Baliburg shows
a doublé maximum of the equatorial type. Gorée,
lat. 15° N., on the coast of Senegambia, gives a fine
example of a rainy (summer) and a dry (winter)
monsoon. A case of a special kind is the Somali

1 Five stations.
  CHABACTERISTICS OF ZONES—TROPICS 105

coast, which trends N.E-S.W., and is therefore
parallel with both monsoons. Hence at no season
can it become very rainy, and mean annual rainfalls
of 40 inches , are not recorded until the coast takes a
turn to the south, at Mombasa.

Numerous combinations of equatorial, trade and
monsoon rainfalls are found, often creating great
complexity. In the case of Port Darwin, the station
is near enough to the equator to have two rainy sea-
sons (equatorial type) when the sun is vertical, as is
frequently the case in the West Indies and Central
America in the same latitude. The rainiest month,
however, is January, between the two times of verti-
cal sun, but during the height of the monsoon, there
being a rainy season of four months and a dry season
of eight months. The monsoon thus interferes with
the typical occurrence of equatorial rains. It is also
true that the dry season in monsoon districts is drier
than the two dry seasons of the equatorial type.
Batavia, on the island of Java, has simple monsoon
rains. Buitenzorg, on the same island, has a mon-
soon maximum in January, two months bef ore the
sun is vertical for the first time, and it has a regular
tropical maximum of rainfall in October, following
the second zenithal position of the sun. The north
coast of Ceram, in the Moluccas, has north-west sum-
mer monsoon rains, with a maximum in February,
while the south coast has winter rains, with the south-
east trade. The rainy and dry seasons thus come
under different conditions and at opposite times on
  106

CLIMATE

the two coasts. These two sets of conditions are
often very close together, with a divide between them.
On the island of Hawaii, Hilo, on the east side, is
exposed to the,north-east trade and has a winter maxi-
mum of rainfall. Kailua, on the lee side, has about
one-third as much rainfall, with a summer maximum.
The islands of the East Indian archipelago furnish
many examples of such curious complications. The
eastern coast of Madagascar has south-east trade
winds fairly uniformly through the year, while the
interior and west coast have a summer maximum—
the normal tropical rainfall season.

IV.   Mountain Climate. Within the tropics, alti-
tude is chiefly important because of its effect in tem-
pering the heat of the lowlands, especially at night.
If tropical mountains are high enough, they carry
snow the year around, even on the equator, and the
zones of vegetation may range from the densest
tropical forest at their base to the snow on their sum-
mits. The highlands and mountains within the
tropics are thus often sharply contrasted with the
lowlands, and offer more agreeable and more healthful
conditions for white settlement. They are therefore
often sought out by residents from colder latitudes
as the most attractive resorts. In India, the hill sta-
tions are crowded during the hot months by civilian
and military officials, and it has been well said that
India is ruled from 7,000 feet above sea-level. The
climate of many tropical plateaus and mountains
has the reputation of being a “ perpetual spring.”
  CHARACTERISTICS OF Z0XE8—TROPICS 107

Thus, on the interior plateau of the tropical Cordil-
leras of South America, and on the central plateau of
tropical Africa, the heat is tempered hy the altitude,
while the lowlands and coasts are very hot. The
rainfall on tropical mountains and highlands often
differs considerably in amount from that on the
lowlands, and other features common to mountain
climates the world over are also noted. But the
main emphasis is rightly laid upon the temperature.
 

[Prometheus]

 CHAPTER V

THE CHARACTERISTICS OF THE ZONES: II.—THE
TEMPERATE ZONES

General: “ Temperate ” Zones—Temperature—Pressure and Winds
—Rainfall—Humidity and Cloudiness—Seasons: Their Effects
on Man—Weather—Climatic Subdivisions—South Temperate
Zone—Sub-tropical Beits:   Mediterranean Climates—North

Temperate Zone: Western Coasts—Interiors—Eastern Coasts
—Mountain Climates.

General: “Temperate" Zone». The so-called
“ temperate ” zones occupy about one-half of the
earth’s surface. As a whole, they are temperate only
in that their mean temperatures and their physiolo-
gical effects are intermediate between those of the
tropics and those of the polar zones. The modifica-
tions of solar climate which result from the distribu-
tion and influence of land and water are greatest in
the temperate zones. The north temperate zone in-
cludes the greatest known extremes of temperature.
If the use of the word “ temperate ” were not so
firmly established it would be well to change the name
to intermediate, or to middle.1

1 North-middle and south-middle would then distinguish the
zones in the two hemispheres. (See W. M. Davis: The Temperate
Zones, Joum. Geogr., vol. i, 1897, pp. 139-143.) “ Temperate ” does,
however, apply fairly well to the south temperate zone.

108


  CHARACTERISTICS OF ZONES—TEMPERATE 109

A marked changeableness of the weather is a strik-
ing characteristic of these zones. Apparently irreg-
ular and haphazard, these continual weather changes
nevertheless run through a fairly systematic series,
although they are essentially non-periodic. Climate
and weather are by no means synonymous over most
of the extra-tropical latitudes.

Temperature. The mean annual temperatures at
the margins of the north temperate zone differ by
more than 70°. The ranges between the mean tem-
peratures of hottest and coldest months reach 120° at
their maximum in north-eastem Siberia, and 80° in
North America. A January mean of —60° and a
July mean of 95°, and maxima of over 120° and
minima of —90°, occur in the same zone. In the dis-
tricts of lowest winter minima, the mean summer tem-
peratures exceed 85°, and in portions of the districts
of highest mean summer maxima, the mean winter
minima fall below 82°. Such great ranges character-
ise the extreme land climates. Under the mild in-
fluence of the oceans, the windward west coasts have
much smaller ranges than the interiors; the seasonal
differences increase inland. The annual ranges in
the middle and higher latitudes exceed the diurnal,
the conditions in much of the torrid zone being ex-
actly reversed. Over much of the oceans of the tem-
perate zones the annual range is less than 10°. In
the south temperate zone there are no extreme ranges,
the maxima, slightly over 80°, being near the margin
of the zone in the interior of South America, south
  110

CLIMATE

Africa, and Australia. In these same localities, the
diurnal ranges, however, rival those of the north
temperate zone.

The north-eastern Atlantic ocean and north-west-
em Europe are about 85° too warm for their latitude
in January, while north-eastem Siberia is 80° too cold.
The lands north of Hudson’s Bay are 25° too cold,
and the waters of the Alaskan Bay 20° too warm.
In July, and in the southem hemisphere, the anoma-
lies are small. The lands which are the centre of
civilisation in Europe average too warm for their lati-
tudes. These lands are the most truly “ temperate ”
portion of the north temperate zone. The north-
west coast of North America is much the same. The
diurnal variability of temperature is greater in the
north temperate zone than elsewhere in the world,
and the same month may differ greatly in its charac-
ter in different years. One winter in higher latitudes
may have much snow, and temperatures below
normal; the next may give much rain instead of snow,
and the ground remain unfrozen. One summer may
be very favourable for crops; the next may give a
poor harvest.

From the point of view of temperature, these zones
may be considered in three divisions: (1) the sub-
tropical, (2) the “temperate” latitudes, and ( the
sub-polar. The annual temperature curve has one
maximum and one minimum. In the Continental
type, the times of maximum and minimum are about
one month behind the maximum and minimum in-
  CHABACTERISTICS OF ZONES—TEMPERATE 111

gnlaftnn dates. In the marine type, the retardation
may amount to nearly two. months. Coasts and
islands have a tendency to a cool spring and warm
autumn; continents, to similar temperatures in both
spring and fall.

Presmre and Winds. The prevailing winds are
the “ westerlies,” which occupy about as much of the
earth’s surface as do the easterly trades. The wester-
lies are, however, much less regular than the trades.
They vary greatly in velocity in different regions and
in different seasons, from a light wind to a gale of
fifty or more miles an hour. They are stronger in
winter than in summer. They are much interfered
with, especially in the higher northem latitudes, by
seasonal changes of temperature and pressure over the
continents, whereby the latter establish, more or less
successfully, a system of obliquely outflowing winds
in winter and of obliquely inflowing winds in sum-
mer. On the eastern coast of Asia there is a com-
plete reversal in wind direction at the opposite
seasons, but usually the seasonal shift is much less
than 180°. In summer, when the lands have low
pressure, the northem oceans are dominated by great
oval areas of high pressure, with outflowing spiral
eddies, while in winter, when the northem lands have
high pressure, the northem portions of the oceans
develop cyclonic systems of inflowing winds over
their warm waters. All these great Continental and
oceanic systems of spiraling winds are important
climatic Controls.
  CLIMATE

112

The westerlies are also much confused and inter-
rupted by storms. Hence their designation of stormy
•westerlies. A constant succession of cyclones, and
the accompanying anticyclones, travelling along with
the prevailing westerlies, causes the latter very fre-
quently to change direction in order to become part
of a cyclonic or an anticyclonic whirl. In these
storms, velocities of eighty or more miles an hour
may be reached at sea. So common are such in-
terruptions that the prevailing westerly wind direc-
tion is often difficult to discem without careful
observation. Cyclonic storms are most numerous
and best developed in winter. The irregular press-
ure changes usually wholly mask the faint diurnal
variation of the barometer which is so characteristic
of the tropics, and which becomes less and less marked
with increasing latitude. Although greatly inter-
fered with near sea-level by Continental changes of
pressure, by cyclonic and anticyclonic whirls, and by
local inequalities of the surface, the eastward move-
ment of the atmosphere remains very constant aloft.
The drift of the higher clouds, and wind observations
on mountains, show clearly that the upper currents
blow with great steadiness from westerly points, the
departures being temporary, and under the control
of passing cyclones or anticyclones. The south tem-
perate zone is chiefly water. Hence the westerlies
are but little distorted by Continental effects. They
are strong and steady, and almost as regular as the
trades. “ Roaring forties ” is a well-known designa-
  CHARACTERISTICS OF ZONES—TEMPERATE 113

tion for the Southern middle latitudes, and between
latitudes 40° and 60° S. the “ brave west winds ” blow
with a constancy and a velocity found in the northern
hemisphere only on the oceans, and then in a modi-
fied form. Storms, frequent and severe, cfyaracter-
ise these southem hemisphere westerlies, and easterly
wind directions are temporarily noted during their
passage. Voyages to the west around Cape Horn
against head gales, and in cold, wet weather, are
much dreaded. South of Africa and Australia, also,
the westerlies are remarkably steady and strong.
The winter in these latitudes is stormier than the
summer, but the seasonal difference is less than that
north of the equator.

Between trades and westerlies lies a debatable belt
of high pressure, shifting seasonally. Within it,
stormy westerlies and drying trades alternately hold
sway. It is the sub-tropical belt, a favoured climatic
region, where invalids seek health, and an escape from
the rigors of a cold winter is found by many who
have time and means to leave their northem homes.

Rainfall. Rainfall is fairly abundant over the
oceans, where evaporation is large, and also over a
considerable part of the lands (30-80 inches, and
more). It comes chiefly in connection with the usual
cyclonic storms, or in thunderstorms, but altitude
often serves locally to increase this precipitation. So
great are the differences, geographic and periodic, in
rainfall, produced by differences in temperature,
topography, cyclonic conditions, etc., that none but
8
  114

CLIMATE

the most general rules can be laid down. The
equatorward margin of the temperate zone rains is
clearly defined on the west coasts, at the points where
the coast deserts are replaced by beits of light or
moderate rainfall. Bold west coasts, on the polar
side of lat. 40°, are very rainy, having 100 inches and
more a year in the most favourable situations. The
hearts of the continents, far from the sea, and especi-
ally when well enclosed by mountains, or when blown
over by cool ocean winds which warm in Crossing the
land, have light rainfall (less than 10-20 inches).
East coasts, receiving rain from moist winds blowing
in from the adjacent oceans as monsoons, or in front
of cyclonic stormt, are wetter than interiors, but drier
than west coasts. Winter is the season of maximum
rainfall over oceans, islands, and west coasts, for the
westerlies are then most active, cyclonic storms are
then most numerous and best developed, and the cold
lands chili the inflowing damp air. At this season,
however, the low temperatures, high pressures, and
tendency to outflowing winds over the continents are
unfavourable to rainfall, and the interior land areas,
as a rule, then have their minimum. The warmer
months bring the maximum rainfall over the conti-
nents. Then conditions are favourable for inflowing
damp winds from the adjacent oceans; there is the
best opportunity for convection; thunder-showers
readily develop on the hot aftemoons; the capacity of
the air for water vapour is greatest. Continents,
from equator to higher latitudes, thus have a tend-
  CHARACTERISTICS OF ZONES—TEMPERATE 115

ency to maximum rainfall in the warm season; sum-
mer rains, as a whole, predominate over the lands.
The marine type of rainfall, with a winter maximum,
extends in over the western borders of the continents,
and is also found in the winter rainfall of the sub-
tropical beits. These winter rains are in some respects
like the winter rains on windward coasts in the trades.
Coastal lands reached by them are well watered, and
droughts need not be feared. Rainfalls are heaviest
along the tracks of most frequent cyclonic storms.

For Continental stations, the typical daily march of
rainfall is shown in the accompanying data for Berlin
and New York.

DAILY MARCH OF RAINFALL (THOIJSANDTHS OF THB DAILY

MEAN).

I. Continental Type.

Hours.   Berlin. New York.

12 p.m.—2 a.m...........................76

2-4    83

4-6      74

6-8    69

8-10 ..........................62

10-Noon ........................68

Noon—2 p.m.............................85

2-4    105

4-6    104

6-8    113

8-10    83

10-12 p.m.......................78

79

85

79

80
74
81
83
95
91
90
85
78

The chief maximum is in the afternoon, and the
secondary maximum comes in the night or early
moming. The chief minimum comes between 10 a.m.
and 2 p.m. Coast stations generally have a night
  116

CLIMATE

maximum, and a minimum between 10 a.m. and 4
p.m., as illustrated in the following data for Valentia.

DAILY MARCH OF RAINFALL AT YALENTIA (THOUSANDTHS OF
DAILY MEAN)

II. Marine Type.

12 p.m—2 a.m..............................88

2-4    93

4-6    93

6-8    90

8-10    84

10-Noon ............................76

Noon—2 p.m................................74

2-4    75

4-6    80

6-8 ...............................82

8-10 ..............................82

10-12 p.m...........................83

Humidity and Cloudiness. Arrhenius gives the
mean cloudiness for different latitudes as follows:

70® N. 60° 50° 40° 30° 20° 10° Eq. 10° 20° 30° 40° 50° 60° S.
59   61 48 49 42 40 50 58 57 48 46 50 66 75

The higher latitudes of the temperate zones thus
have a mean cloudiness which equals and even exceects
that of the equatorial belt. The amounts over the
oceans and coasts are greater than inland. The beits
of minimum cloudiness are at about lat. 30° N. and
S. Over the Continental interiors, the cloudiest sea-
son is summer, but the amount is never very large.
Otherwise, winter is generally the cloudiest season,
with a fairly high mean annual amount.

The absolute humidity, as a whole, decreases as the
  CHARACTERISTICS OF ZONES—TEMPERATE 117

temperature falls. The relative humidity averages
ninety per cent., more or less, over the oceans, and is
high under the clouds and rain of cyclonic storms, but
depends, on land, upon the wind direction; winds
from an ocean or from a lower latitude being damper,
and those from a continent or from a colder latitude
being drier.

Seaaons: Their Effects on Man. Seasons in the
temperate zones are classified according to tempera-
ture—not, as in the tropics, by rainfall. The four
seasons are important characteristics of these zones,
especially of the middle latitudes of the north tem-
perate zone. Here spring and autumn intervene as
transition seasons between the colder winter with
snow, and warmer summer with more or less rain.
Towards the equatorial margins of the zones, the dif-
ference in temperature between summer and winter
becomes smaller, and the transition seasons weaken
and even disappear. At the polar margins, the
change from winter to summer, and vice versa, is so
sudden that there also the transition seasons dis-
appear. These seasonal changes are of the greatest
importance in the life of man.

Weather. An extreme changeableness of the
weather, depending on the succession of cyclones and
anticyclones, is another characteristic. For most of
the year and most of the zone, settled weather is un-
known. The changes are most rapid in the northern
portion of the north temperate zone, especially on
the continents, where the cyclones travel fastest. The
  118

CLIMATE

nature of these changes depends on the degree of de-
velopment, the velocity of progression, the track, and
other conditions of the disturbance which produces
them. The changes may be sudden and marked, or
faint and slow; the wind may back or veer; the pre-
cipitation may be heavy or light; the wind velocity
may be light, or of hurricane force; anticyclones may
be clear, or may have clouds, and not infrequently,
precipitation. There is an almost endless variety of
such examples. The detailed study of these varying
phases of cyclonic and anticyclonic weather Controls
belongs to meteorology. It suffices here to say that
the particular weather types resulting from this con-
trol give the climates their distinctive character, and
that the study of climate through these types is the
only method of appreciating the actual conditions.
Annual and monthly averages of the different cli-
matic elements alone are misleading, and give but a
lifeless picture. The cyclonic unit, although its period
is irregular and of varying length, is an essential
basis of computation and comparison.

The weather types vary with the season and with
the geographical position. They result from a com-
bination, more or less irregular, of periodic, diurnal
elements, under the regular control of the sun, and
of non-periodic cyclonic and anticyclonic élements.
In summer, on land, when the cyclonic element is
weakest and the solar control is the strongest, the
dominant types are associated with the regular
changes from day to night. Daytime cumulus
  CHARACTERISTICS OF ZONES—TEMPERATE 119

clouds; diurnal variation in wind velocity; afternoon
thunderstorms, with considerable regularity, char-
acterise the warmest months over the continents and
present an analogy with tropical conditions. Cy-
clonic and anticyclonic spells of hotter or cooler,
rainy or dry, weather, with varying winds differing
in the temperatures and the moisture which they
bring, serve to break the regularity of the diurnal
types. On the oceans, the diurnal characteristics are
much less marked.

In winter, the non-periodic, cyclonic control is
strongest. Local conditions of heat and cold become
subordinate to the general control by the cydone,
which imports weather from a distance. The irregu-
lar changes from clear to cloudy, from warmer to
colder, from dry air to snow or rain, extend over
large areas and show little diurnal control. Spring
and autumn are transition seasons and have transition
weather types. In spring, the growing diurnal
quality is marked by the increasing importance of
local Controls; the appearance of convectional pheno-
mena such as spring rains; the struggle between the
cyclonic and the solar Controls of temperature, now
one and now the other being paramount, but the lat-
ter gaining and the former losing. Cold spells, with
cyclonic winds and clouds, recall winter. Warm
spells, with marked diurnal temperature range, pre-
sage summer. In autumn, the decreasing frequency
and importance of diurnal phenomena, such as thun-
der-showers, high afternoon temperatures, and the
  120

[Prometheus]

CLIMATE

like; the active radiation and cooling during the
longer nights, with resulting fogs; and the increas-
ing control by the cyclone, point to winter’s coming.

Weather types thus differ with the seasons. They
differ also in Continental and marine climates. They
differ according to topography and cyclonic and anti-
cyclonic tracks. The oceans in the south temperate
zone have a constancy of non-periodic cyclonic
weather changes through the year which resembles
only faintly that over the oceans of the northem hemi-
sphere. Winter types differ little from summer
types. The diurnal control is never very strong.
Stormy weather prevails throughout the year, al-
though the weather changes are more frequent and
stronger in the colder months.

Climatic Subdivisions. From whatever point of
view the temperate zones be considered, it is clear that
there are fundamental differences between the north
and south temperate. The latter is sufficiently in-
dividual to be given a place by itself. The marginal
sub-tropical beits must also be considered as a separ-
ate group by themselves. The north temperate zone
as a whole includes large areas of land, stretching over
many degrees of latitude, as well as of water. Hence
it embraces so remarkable a diversity of climates that
no single district can be taken as typical of the whole.
lts climate has been called “ a crazy quilt of patches.”
It is a zone of marked seasonal variations and of
great extremes, annual, diurnal, cyclonic. The
simplest and most rational scheme for a classification
  CHARACTERISTICS OF ZONES—TEMPERATE 121

of these climates is based on the fundamental differ-
ences which depend upon land and water, upon the
prevailing winds, and upon altitude. Thus there are
the ocean areas and the land areas. The latter are
then subdivided into western (windward) and eastern
(leeward) coasts, and interiors. Mountain climates
remain as a separate group.

South Temperate Zone. If the climate of the
north temperate zone is “ a crazy quilt of patches,”
that of the south temperate is a piece of fairly uni-
form texture and appearance throughout. This is
the effect of the large ocean surface. The whole
meteorological régime is more uniform than in the
northem zone. Although the solar climate of the
southem hemisphere is more severe than that of the
northem, the physical climate is very much less ex-
treme. It has been pointed out that this zone may
properly be called “temperate”; that its tempera-
ture changes are small; its prevailing winds are
stronger and steadier than in the northern hemisphere;
its seasons more uniform; its weather prevailingly
stormier, more changeable, and more under cyclonic
control. The uniformity of the climatic conditions
over the far southem oceans is monotonously unat-
tractive. The Continental areas are small, and de-
velop to a limited degree only the more marked
seasonal and diurnal changes which are characteristic
of lands in general. The summers are less stormy
than the winters, but even the summer temperatures
are not high. Such an area as that of New Zealand,
  122

CLIMATE

with its mild climate and fairly regular rains, is really
at the margins of the zone, and has, much more
favourable conditions than do the islands farther
south. These islands, in the heart of this zone, have
dull, cheerless, and inhospitable climates, with snow
sometimes in midsummer. The zone enjoys a good
reputation for healthfulness, which fact has been
ascribed chiefly to the strong and active air move-
ment, the relatively drier air than in corresponding
northern latitudes, and the cool summers. It must
be remembered, also, that the lands are mostly in the
sub-tropical belt, which possesses peculiar climate ad-
vantages, as will be seen. The northem oceans
repeat, in a much modified fora», many of the charac-
teristics of the south temperate oceans. Except to
leeward of the broad lands, the northern oceans have
the conservative features typical of ^ marine climates
the world over.

Sub-tropical Belts: Mediterranean Climates. At
the tropical margins of the temperate zones, in the
latitudes of the tropical high pressure areas, are
the so-called sub-tropical beits. Far enough from the
equator to be free from continued high temperatures,
and near enough to it to be spared the extreme cold
of higher latitudes, these transition beits are among
the most favoured of the world. Their rainfall ré-
gime is altemately that of the westerlies and of the
trades. They are thus associated, now with the tem-
perate, and now with the tropical zones. In winter,
the equatorward migration of the great pressure and
  CHABACTERISTICS OF ZONES—TEMPERATE 123

wind systems brings these latitudes under the control
of the westerlies, whose frequent irregular storms
give a moderate winter precipitation. These winter
rains recall the winter trade rains of the tropics,
although their origin is different. They are not
steady and continuous, but are separated by spells of
fine, sunny weather. The amounts vary greatly.1

In summer, when the trades are extended pole-
wards by the outflowing equatorward winds on the
eastern side of the ocean highs, mild, dry, and nearly
continuous fair weather prevails, with general north-
erly winds.

The sub-tropical beits of winter rains and dry
summers are not very clearly defined. They do not
extend continuously around the world. They are
mainly limited to the western coasts of the continents,
and to the islands off these coasts, in latitudes between
about 28° and 40°. Their degree of development
and their importance vary in different longitudes.
The sub-tropical belt is exceptionally wide in the east-
em hemisphere, and reaches far inland there, em-
bracing the countries bordering on the Mediterranean
in Southern Europe and northern Africa, including
the Azores and the famous Riviera, and then extend-
ing eastward across the Dalmatian coast and the
southem part of the Balkan peninsula into Syria,
Mesopotamia, Arabia north of the tropic. Persia, and
the adjacent lands. In the great eastward extension

1 In round numbers, Lisbon has 28.60 inches; Madrid, 16.60;
Algiers, 28.15; Nice, 33.00; Rome, 29.90; Ragusa, 63.90.
  124

CLIMATE

of the winter rains in this area, the development of
secondary lows over the Mediterranean Sea is an im-
portant factor. The fact that the Mediterranean
countries are so generally included in this belt has led
to the use of the name “ Mediterranean climates.”
Owing to the great irregularity of topography and
outline, the Mediterranean province embraces many
varieties of climate, but the dominant characteristics
are the mild temperatures, except on the higher ele-
vations, and the sub-tropical rains.

On the west coasts of the two Americas, the sub- '
tropical belt of winter rains is clearly seen in Cali-
fomia and in northem Chile, on the west of the coast
mountain ranges. Between the region which has rain
throughout the year from the stormy westerlies, and
the districts which are permanently arid under the
trades, there is an indefinite belt over which rains fall
in winter. In Southern Africa, which is controlled
by the high pressure areas of the South Atlantic and
South Indian oceans, the south-westem Coastal belt
has winter rains, decreasing to the north, while the
east coast and adjoining interior have summer rains,
from the south-east trade. There is sub-tropical veg-
etation on the south-east coast, and a cool, dry climate
on the south-west coast. Southern Australia is cli-
matically similar to south Africa. In summer, the
trades give rainfall on the eastern coast, which de-
creases inland. In winter, the westerlies give mod-
erate rains, chiefly on the south-westem coast.
Northern Chile, California, south-westem Australia,
  CHARACTERISTICS OF ZONES—TEMPERATE 125

and the Cape province of Africa are thus all in the
sub-tropical belt.

Fig. 23. Monthly Distribution of Rainfall: Sur-tropical
Winter Rains

M: Malta. W.A: Western Australia

The sub-tropical climates follow the tropical high
pressure beits across the oceans, but they do not re-
tain their distinctive character far inland from the
west coasts of the continents (except in the Mediter-
ranean case), nor on the east coasts. On the latter,
summer monsoons and the occurrence of general sum-
mer rains interfere, as in eastern Asia and in Florida,
and to some extent in South America east of the
Andes.

Strictly winter rains, with a maximum in Decem-
ber or in June, according to the hemisphere, are typi-
  126

CLIMATE

cal of the coasts and islands of this belt. The more
Continental areas have a tendency to spring and
autumn rains. The rainy and dry seasons are most
marked at the equatorward margins of the belt, and
thus recall the tropical characteristic of dry and wet,
rather than cold and hot seasons. With increasing
latitude, the rain is more evenly distributed through
the year, the summer becoming more and more rainy
until, in the Continental interiors of the higher lati-
tudes, the summer becomes the season of maximum
rainfall. The monthly distribution of rainfall in two
sub-tropical regions is shown in the accompanying
data and curves (see Fig. 28).

ANNUAL MABCH OF RAINFALL! SUB-TROPICAL TYPE (in thoUS-

andths of the annual mean).

Southern Italy
Western   Sicily

Australia   Malta

Latitude............................. 32.3° S. About 38° N.

January............................... 14   130

February.............................. 18   93

March................................. 30   98

April................................. 64   75

May...................................150   35

June..................................183   23

July..................................168   8

August................................166   28

September............................. 93   73

October............................... 59   133

November.............................. 32   144

December.............................. 23   160

The following table (from Supan), giving the sea-
sonal distribution of rainfall in Southern Europe, in
  CHARACTERISTICS OF ZONES—TEMPERATE 127

percentages of the annual mean, shows very clearly
the change in the rainfall season in going from north
to south. In the northem Tyrol, the normal type of
central Europe prevails. In Sicily, the summer is
almost rainless: the sub-tropical type is fully
developed.

SEASONAL DISTRIBUTION OF RAINFALL IN CENTRAL AND SOUTH-
ERN europe (in percentages of the annual mean).

   Winter   Spring   Summer   Autumn
Northern Tyrol   16   24   37   23
Southem Tyrol   14   26   28   32
Po Valley   20   26   24   30
Central Italy   25   24   17   34
Southem Italy   31   25   11   33
Sicily   39   22   3   36
Malta   48   14   2   36

In Alexandria the dry season lasts nearly eight
months; in Palestine, from six to seven months; in
Greece, about four months.

The sub-tropical rains are peculiarly well developed
on the eastern coast of the Atlantic Ocean, and are
clearly illustrated in the accompanying diagram,
after Supan (see Fig. 24).

The different types of rainfall are as follows:

I.   North of lat. 40° N. Rain throughout the year.

II.   Lats. 40°_27° N. Dry in summer (sub-tropical
rains).

III.   Lats. 27°-19° N. Always deficiënt in rainfall.

IV.   Lats. 19°-7° N. Dry in winter (tropical

rains).

V.   Lats. 7°-l° N. Always rainy (equatorial belt).
  128

CLIMATE

VI.   Lats. 1° N.-17° S. Dry in winter (tropical rains).

VII.   Lats. 17°-30° 8. Always dry.

VIII.   Beyond lat. 30° S. Dry in summer (sub-tropical
rains).

(IX. Always rainy on the oceans. The African west
coast does not extend into this zone.)

50W

40

30

ZO

10

0

m

20

30

Winter Summer

I

II

m
w

v

vi

m
| m

Summer Winter

Fig. 24. Rainy and Rainless Zones on
Eastern Atlantic Coast

The winter rains which migrate equatorward are
separated by the Sahara from the equatorial rains
which migrate poleward. An unusually extended
migration of either of these rain beits may bring them
  CHARACTERISTICS OF ZONES—TEMPERATE 129

close together, leaving but a small part, if any, of the
intervening desert actually rainless. The Arabian
desert occupies a somewhat similar position. Large
variations in the annual rainfall, and droughts, may
be expected towards the equatorial margins of the
sub-tropical beits. Irrigation is practised in many
places.

TABLB OF MEAN MONTHLY TEMPERATURES FOR SELECTED SUB-
TROPICAL STATIONS.1

   Continental      Insular   
   Bagdad   Cordoba   Bermuda   Auckland
Lat.   33°19' N.   31°25' S.   32°20' N.   36°50' N.
Long.   44°26' E.   64°12' W.   64°43' W.   174°51' E.
Altitude   39 ft.   1440 ft.   148 ft.   276 ft.
January   50.9°   73.4°   62.4°   67.1°
February   53.1   72.3   61.9   67.5
March   62.1   68.4   61.7   65.5
April   69.3   61.0   64.4   61.5
May   82.0   54.5   69.6   56.7
June   89.6   49.1   74.8   53.2
July   92.8   50.0   78.8   51.8
August   92.7   54.3   80.1   52.2
September   85.6   58.6   78.1   54.5
October   76.5   63.5   73.4   57.4
November   62.1   68.5   67.6   60.3
December   52.5   71.8   63.7   64.8
Mean   72.4   62.1   69.7   59.4
Range   41.9   24.3   18.4   15.7

The main features of the sub-tropical rains east of
the Atlantic are repeated on the Pacific coasts of the

1 Given to nearest tenth of a degree Fahr.

9
  m

CLIMATE

two Americas. In North America, the rainfall de-
creases from Alaska, Washington, and northern Ore-
gon southwards to Lower California, and the length
of the summer dry season increases. The mean an-
nual rainfall (1871-1901) at Neah Bay, Wash., is
112.40 inches; at San Francisco, Cal., 22.88 inches,
and at San Diego, Cal., 9.40 inches. At San Diego,
six months (May-October) have each less than five
per cent. of the annual precipitation, and four of
these have one per cent. The southem extremity of
Chile, from about latitude 88° S. southward, has
heavy rainfall throughout the year from the wester-
lies, with a winter maximum. Northem Chile is per-
sistently dry. In the intermediate area there are
winter rains and dry summers. Neither Africa nor
Australia extends far enough south to show the dif-
ferent members of this system well. New Zealand
is almost wholly in the prevailing westerly belt.
Northem India is unique in having summer monsoon
rains, and also winter rains from weak cyclonic
storms, which correspond to the sub-tropical winter
rains.

From the position of the sub-tropical beits to lee-
ward of the oceans, and at the equatorial margins of
the temperate zones, it follows that their temperatures
are not extreme. Further, the protection afforded
by mountain ranges, as by the Alps in Europe and
the Siërra Nevada in the United States, is an im-
portant factor in keeping out extremes of winter cold.
The annual march, and ranges, of temperature de-
  CHARACTERISTICS OF ZONES—TEMPERATE 131

pend upon position with reference to Continental or
marine influences. This is seen in the accompanying
data and curves for Bagdad, Cordoba, Bermuda, and
Auckland (see Fig. 25).

Fig. 25. Annual March of Temperature for
Selected Sub-tropical Stations
Bd: Bagdad. Ba: Bermuda. A: Auckland.
C: Cordoba

Autumn is, as a rule, a good deal warmer than
spring, as in all the eastern Mediterranean basin, the
Canaries, and Madeira. This basin is particularly
  132

CLIMATE

favoured in winter, not only in the protection against
cold afforded by the mountains, but also in the high
temperature of the sea itself. The Southern Alpine
valleys and the Riviera are well situated, having good
protection and a Southern exposure. The coldest
month usually has a mean temperature well above
82°. Mean minimum temperatures of about, and
somewhat below, freezing occur in the northem por-
tion of the district,1 and in the more Continental lo-
calities minima a good deal lower have been observed.
(At San Diego, Cal., the absolute minimum is 82°;
at San Francisco, 29°.) Mean maximum tempera-
tures of about 95° occur in northem Italy, and of
still higher degrees in the southem portions. Some-
what similar conditions exist in the sub-tropical
district of North America. Under the control of
passing cyclonic storm areas, hot or cold winds, which
often owe some of their special characteristics to the
topography, bring into the sub-tropical beits, from
higher or lower latitudes, unseasonably low or high
temperatures. These winds have been given special
names (mistral, sirocco, bora, chamsin, leste, leveche,
pampero, southerly burster, etc.)

These beits enjoy abundant sunshine, being among
the least cloudy districts in the world. The accom-
panying data and curve, giving an average for ten
stations, show the small annual amount of cloud, the
winter maximum and the marked summer minimum,
in a typical sub-tropical climate. (Fig. 26).
i Nice, 30.4°; Rome, 25.7°; Palermo, 32°; Athens, 28.8°.
  CHARACTERISTICS OF ZONES—TEMPERATE 133

MONTHLY DISTRIBUTION OP CLOUDINESS IN A SUB-TROPICAL
CLIMATE (EASTERN MEDITERRANEAN, LAT. 33.8° N.)

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year
4.9 4.6 3.8 3.7 2.8 1.3 1.1 1.3 1.8 2.5 4.0 4.7   3.0

The winter rains do not bring continuously over-
cast skies; and it has been well said that the problem
of securing a maximum rainfall with a maximum
number of clear days has been solved on the southem
Alpine slopes. A summer month with a mean

10

9

8

7

6

5

4

3

2

I

0

Fig. 26. Annual March of Cloudiness in a Sub-tropical Climate
(Eastern Mediterranean)

cloudiness of 0.1 is not exceptional in the drier parts
of the sub-tropics. The winter cloudiness in northem
Italy is 5.0 to 6.0; in summer, 8.0 to 4.0. Cairo
has an annual mean of 1.9, and in June it has
0.8. Biskra, on the northem margin of the Sahara,
has 264 clear days. In the central valley of Cali-
  134

CLIMATE

fornia, the number of clear days is similarly very
large.

With prevailingly fair skies, even temperatures, and
moderate rainfall, the sub-tropical beits possess many
climatic advantages which fit them for health resorts.
The long list of well-known resorts on the Mediter-
ranean coast, and the shorter list for Califomia, bear
witness to this fact.

North Temperate Zone: Weet Coaete. Marine
climatic types are carried by the prevailing wester-
lies on to the western coasts of the continents, giving
them mild winters and cool summers, abundant rain-
fall and a high degree of cloudiness and relative
humidity. North-western Europe is particularly
favoured because of the remarkably high tempera-
tures of the North Atlantic Ocean, and because of the
influence of the winds controlled by the low pressure
area off Iceland. In January, north-western Europe
has temperatures from 20° to 40° in excess of the
normal for the latitude. The north-western coast of
North America has temperatures more than 10° too
warm for the latitude. January means of 40° to 50°
in the British Isles and on the northem French coast
occur in the same latitudes as those of 0° and 10° in
the far interior of Asia. In July, means of 60° to
70° in the former contrast with 70° and 80° in the lat-
ter districts. The conditions are somewhat similar
in North America. Along the western coasts of
North America and of Europe the mean annual
ranges are under 25°,—actually no greater than some
  Fig. 27. annual March op
Temperature for Selected
Stations in the Temperate

Zones

[Prometheus]

S. I: Scilly Isles. P: Prague. C:
Charcow. S: Semipalatinsk. K;
Kiakhta. B: Blagoweschtschensk.
Sa: Sakhaliu. T: Thorshavn.
Y: Yakutsk

135
  13$

CLIMATE

of those within the tropics.. Irregular cyclonic tem-
perature changes are, however, marked in the tem-
perate zone, while absent in the tropics. The data
and curves for the Scilly Isles and for Thorshavn,
Faroe Islands, illustrate the insular type of temper-
ature on the west coasts (see Fig. 27). In the
Faroes the mean maximum is 65.1°, and the mean
minimum 16°. It will be noted that the poleward
decrease in the mean annual and the mean winter
temperatures is very slow between latitudes 50° and
62° N. on the west coast of Europe.

TABLB OF MBAN MONTHLY TBMPEBATUBBg FOB 8BLBCTBD STA-
TIONS IN THE TEMPERATE ZONES.

   Along lat. 60° N.                     At lat. 62°N.   
   West  Coast   Continental               Bast  Coast   Insular   Conti-  nental
   Scilly  Isles   Prague   Charcow   Semi-  pala-  tinsk   Kiakh-  ta   Blago-  wescht-  schensk   Sakha-  lin   Thors-  havn,  Faroe  Isles   Yakutsk,  B.Si-  beria
Lat.   49°55'   50°5/   SO*^   50°24'   50°21'   50° 15'   50°50'   62°2'   62°1'
Long.   6°20' W.   14°26' E.   36°11'   80°13'   106°31'   127°38'   142°7' E.   6°44' W.   129°43'E
Alt. (ft.)   98.4   662.7   413.4   593.8   2526   360.9   180.4   29.5   328.1
Jan.   45.7°   29.8°   16.5°   0.5°   -15.9°   -13.9°   -0.4°   37.8°   -45.2°
Peb.   45.7   32.0   22.1   1.8   -5.4   -3.3   5.0   38.1   -35.0
March   46.0   37.8   29.3   14.4   16.9   14.4   15.8   37.8   -10.7
April   48.7   47.3   44.8   38.3   34.3   34.7   31.1   41.9   15.1
May   52.5   55.9   58.8   57.2   48.7   49.6   41.4   45.0   40.3
June   57.9   63.3   65.1   68.0   63.1   63.7   50.7   49.5   58.5
July   60.8   66.7   69.6   72.0   66.4   70.5   60.3   51.4   65.8
Aug.   61.2   65.3   66.4   67.3   61.7   65.8   62.2   51.3   59.7
Sept.   58.6   58.8   56.1   54.9   48.0   53.2   53.6   48.7   42.3
Oct.   54.0   48.7   45.5   38.1   32.0   34.2   39.6   43.9   15.8
Nov.   49.6   37.6   33.8   20.1   11.8   9.7   22.5   40.6   -21.3
Dec.   47.3   31.3   23.2   6.1   -2.7   -9.2   7.3   38.1   -41.1
Year   52.3   47.9   44.3   36.5   29.9   30.8   32.4   43.7   12.0
Range   15.5   36.9   53.1   71.5   82.3   84.4   62.6   13.6   111.0
  CHARACTERISTICS OF ZONES—TEMPERATE 137

MONTHLY DISTRIBUTION OF RAINFALL (IN THOUSANDTHS OF
THE ANNUAL MEAN). TEMPERATE ZONE.

   Continental Summer Rains      Coast Rains   
   Moderate   Very heavy   Uniform  Distribution   Fall and Winter Rains
   Central Europe North of Alps   Northern  Asia   Atlantic  Coast,  North  America   North-west  Europe
Lat.   About 50° N.   About 55° N.   About 40° N.   About 60° N.
January   57   20   84   100
February   56   17   77   80
March   68   18   85   72
April   71   35   70   56
May   92   75   80   58
June   115   235   81   64
July   121   215   96   * 70
August   117   122   87   80
September   82   133   84   102
October*   75   58   91   110
November   74   40   86   102
December   72   32   79   106

The monthly distribution of rainfall, with the
marked maximum in the fall and winter which is
characteristic of the marine régime, is illustrated in
the last column of the table above, for north-western
Europe, and in the corresponding curve (see Fig.
28).

On the northem Pacific coast of North America,
the distribution is similar. Thus at Olympia, Wash-
ington, there is a distinct cold season maximum, as
appears in the following data:
  138

CLIMATE

MONTHLY DISTBIBUTION OF RAINFALL AT OLYMPIA, WASHING-
TON (IN THOUSANDTHS OF THE) ANNUAL MEAN).

January .......................................159

February ......................................135

March ......................................... 95

April ..........................................65

May ........................................... 44

June ...........................................31

July .......................................... 13

August ........................................ 13

September ..................................... 53

October ....................................... 86

November ......................................124

December ......................................182

In the southem hemisphere, the western coasts of
southem South America, Tasmania, and New Zea-
land show the same type.

The cloudiness and relative humidity average high
on western coasts, with the maximum in the colder
season. The difference in general rainfall conditions
between the west coast, typified by the exaggerated
case of Valentia, in south-westem Ireland, and the
Mediterranean, is seen in the number of rainy days
in each district. Valentia has nearly 250. In the
Mediterranean, they may be set down as about 100,
in round numbers.

The west coasts, therefore, including the important
climatic province of western Europe, and the coast
provinces of north-western North America, New
Zealand, and Southern Chile, have, as a whole, mild
winters, equable temperatures, small ranges, and
  CHARACTERISTICS OF ZONES—TEMPERATE 139

abundant rainfall, fairly well distributed through the
year. The summers are relatively cool, especially on
the Chilean coast.

Fig. 28. Annual March of Rainfall: Temperate Zones
C. E: Central Europe. A: Northern Asia. N. A: Atlantic Coast of
North America, N. W.E: Northwest Europe

Continental Interiors. The equable climate of the
western coasts changes, gradually or suddenly, into
the more extreme climates of the interiors. In
Europe, where no high mountain ranges intervene,
  140

CLIMATE

the transition is gradual, and broad stretches of coun-
try have the benefits of the tempering influence of the
Atlantic. In North America, the change is abrupt,
and comes on Crossing the lofty western mountain
barrier. The data in the table on page 186, and the
corresponding curves in Fig. 27, illustrate well the
gradually increasing severity of the climate with in-
creasing distance inland in Eurasia. Central Europe
is seen to lie between the modified marine climate of
the west coast and the Continental conditions of
Russia and Siberia. Its mean temperatures do not
differ very much from those on the coast, but the
seasons are more sharply contrasted.

The Continental interiors of the north temperate
zone have the greatest extremes in the world. To-
wards the Arctic circle, the winters are extremely se-
vere, and January mean temperatures of —10° and
—20° occur over considerable areas. At the cold pole
of northem Siberia a January mean of —60° is found.
Mean minimum temperatures of —40° occur in the
area from eastern Russia, over Siberia and down to
about latitude 50° N. At Verkhoyansk, an im-
portant town just beyond the Arctic circle, the ab-
solute minimum is below —90°. Over no small part
of Siberia minimum temperatures below —70° may
be looked for every winter. Thorshavn and Yak-
utsk (see table on page 186) are excellent examples
of the temperature differences along the same lati-
tude line. The winter in this interior region is domi-
nated by a marked high pressure. The weather is
  CHARACTERISTICS OF ZONES—TEMPERATE UI

prevailingly clear and calm. The ground below a
slight depth is frozen the year around, over wide
areas. The moderate snowfall is sufficiënt, with the
continued cold, to make sleighing possible for im-
mense stretches all over the country. The frozen
rivers can be crossed without bridges. This unifying
influence, of easy winter communication, has been
most important in Russian history, as Leroy-Beau-
lieu has pointed out. The extremely low tempera-
tures are not disagreeable except when the steppes
are swept by icy storm winds (buran, purga),
carrying loose snow, and often resulting in loss of
life.

In the North American interior, the winter cold is
somewhat less severe. The lowest January mean
temperatures are —80°, in the extreme northem por-
tion of the continent. Mean annual minima of —40°
occur down into the northern interior portion of the
United States. The lowest is about —60°, near Great
Bear Lake, with an absolute minimum of about —72°.
North American winter weather in middle latitudes
is often interrupted by cyclones, which, under the
steep poleward temperature gradiënt then prevail-
ing, cause frequent, marked, and sudden changes in
wind direction and temperature over the central and
eastern United States. Cold waves and warm waves
are common, and blizzards resemble the buran or
purga of Russia and Siberia. With cold northerly
winds, temperatures below freezing are carried far
south towards the tropic. The January mean tem-
  142

CLIMATE

peratures in the southem portions of the Continental
interiors average about 50° or 60°.

In summer, the northern Continental interiors are
warm, with July means of 60° and thereabout.
These temperatures are not much higher than those
on the west coasts, but as the northem interior win-
ters are much colder than those on the coasts, the in-
terior ranges are very large. The mean annual
extreme ranges exceed 150° in northem North
America and 170° in Siberia. Mean maximum tem-
peratures of 85° occur beyond the Arctic circle in
north-eastern Siberia, and beyond latitude 60° in
North America. In spite of the extreme winter cold,
agriculture extends remarkably far north in these
regions, because of the warm, though short, summers,
with favourable rainfall distribution. The July
isotherm of 50° is about the northem limit of tree
growth. Beyond a zone of stunted tree growth,
comes the tundra. The summer heat is sufficiënt to
thaw the upper surface of the frozen ground, and
vegetation prospers for its short season. At this
time, great stretches of flat surface become swamps.
The southem interiors have torrid beat in summer,
temperatures of over 90° being recorded in the south-
western United States and in southem Asia. In
these districts the diurnal ranges of temperature are
very large, often exceeding 40°, and the mean
maxima exceed 110°.

In South America, the interior of Argentina has
moderate mean annual ranges (20°-80°); the mean
  CHARACTERISTICS OF ZONES—TEMPERATE 143

maxima reach 95°—100° and even higher, and the
mean minima fall below 23°. The west coast has
smaller ranges (less than 20°); lower mean maxima
(77°-86°), and higher mean minima (32°-23°).

The winter maximum rainfall of the west coasts
becomes a summer maximum in the interiors. The
change is gradual in Europe, as is the change in tem-
perature, but more sudden in North America. The
curves for central Europe and for northem Asia (see
Fig. 28) illustrate these Continental summer rains.
The summer maximum becomes more marked with
the increasing Continental character of the climate.
Thus, while June to August in central Europe supply
about thirty-five per cent. of the annual precipita-
tion, in northem Asia, excluding the coast, they give
nearly sixty per cent. The rains of Asia are actually
comparable, in relative intensity, at their maximum,
with the rains of the tropics. In Bengal, e. g., June
to August give only fifty-seven per cent. of the
annual rainfall. The winter dry season of Asia is,
however, very different from a tropical dry season, be-
cause of the difference in conditions of vegetation and
of snow cover. In North America, Nebraska, a state
which is typical of a considerable district of summer
rains, receives about sixty per cent. of the annual
rainfall in the months of April, May, June, and
July.

The change in rainfall season with increasing dis-
tance from the Atlantic Ocean in Eurasia is well
brought out by Supan in the following table:
  144

CLIMATE

TABLE SHOWING SEASONAL DISTRIBUTION OF RAINFALL IN

EURA8IA (IN PERCENTAGES OF THE ANNUAL MEAN).

   Winter   Spring   Summer   Autumn
Ireland   28   21   24   27
Western England   28   19   24   29
Eastern England   23   19   28   30
North-western Germany   23   22   31   24
Central Germany   20   23   34   23
Eastern Germany   19   22   37   22
Western Russia   16   21   39   24
Central Russia   16   22   37   25
Western Siberia   13   13   42   32
Eastern Siberia   9   12   58   21

There is also a well-marked decrease in the amount
of rainfall inland. In western Europe, the rainfall
averages 20-30 inches, with much larger amounts
(reaching 80-100 inches and even more) on the
bold west coasts, as in the British Isles and Scan-
dinavia, where the moist Atlantic winds are deflected
upwards, and also locally on mountain ranges, as on
the Alps. There are small rainfalls (below 20
inches) in eastern Scandinavia and on the Iberian
peninsula. Eastern Europe has generally less than
20 inches; western Siberia about 15 inches, and east-
em Siberia about 10 inches. In the Southern
part of the great overgrown continent of Asia, an ex-
tended region of steppes and deserts, too far from
the sea to receive sufficiënt precipitation, shut in by
mountains, and controlled in summer by drying
northerly winds, receives less than 10 inches a year,
and in places less than 5 inches. In this interior
district of Asia, population is inevitably small, and
suffers under a condition of hopeless aridity.
  CHARACTERISTICS OF ZONES—TEMPERATE 145

The North American interior has more favourable
rainfall conditions than Asia, because the former
continent is narrower. The heavy rainfalls on the
western slopes of the Pacific coast mountains
correspond, in a general way, to those on the west
coast of Europe, although they are heavier (over 100
inches at a maximum). The close proximity of the
mountains to the Pacific, however, involves a much
more rapid decrease of rainfall inland than is the case
in Europe, as may be seen by comparing the isohyetal
lines in the two cases. The rain-shadow influence of
the Pacific coast Cordilleras extends about half-way
across the continent. A considerable interior region
is left with deficiënt rainfall (less than 10 inches) in
the south-west. The eastern portion of the continent
is freely open to the Atlantic and the Gulf of Mexico,
so that moist cyclonic winds have free access, and
rainfalls of over 20 inches are found everywhere east
of the lOOth meridian. These conditions are much
more favourable than those in eastern Asia. The
greater part of the interior of North America has the
usual warm-season rains. In the interior basin, be-
twèen the Rocky and Siërra Nevada mountains, the
higher plateaus and mountains receive much more
rain than the desert lowlands. Forests grow on the
higher elevations, while irrigation is necessary for
agriculture on the lowlands. The rainfall here comes
chiefly from thunder-storms.

In southem South America, the narrow Pacific
slope has heavy rainfall (over 80 inches). East of

io
  146

CLIMATE

the Andes the plains are dry (mostly less than 10
inches). The Southern part of the continent is very
narrow, and is ópen to the east. It is also more open
to the west than is the country farther north, owing
to the decreasing height of the mountains southward.
Hence the rainfall increases somewhat to the south,
coming in connection with passing cyclones. Tas-
mania and New Zealand have most rain on their
western slopes.

In a typical Continental climate, the winter, except
for radiation fogs, is very clear, and the summer is
the cloudiest season, as is well shown in the follow-
ing data and curve for eastern Asia. In a more
moderate Continental climate, such as that of central
Europe, and much of the United States, the winter
is the cloudiest season (see Fig. 29).

MONTHLY DISTRIBUTION OF CLOUDINE88 IN CONTINENTAL

CLIMATES.

I. Eastern Asia. 10 stations. Lat. 56.5° N. Long. 115° E.
Jan. Feb.   Mar. Apr.   May June July   Aug.   Sept. Oct.   Nov.   Dec.   Year.

3.1 3.4   3.9 4.7   5.7 5.6 6.2   6.0   5.5 5.4   4.8   4.2   4.9

II. Central Europe. Hnngarian Plain. Lat 47° N.
Jan. Feb.   Mar. Apr.   May June July   Aug.   Sept. Oct. Nov. Dec.   Year.

6.5 5.9   5.7 5.6   5.4 5.3 4.4   4.2   4.6 5.8   6.6   6.9   5.6

In the first case, the mean cloudiness is small; in
the second, there is a good deal of cloud all the year
around.

The vast Continental interiors, whose climatic
features have here been outlined, can obviously be
  CHARACTERISTICS OF ZONES—TEMPERATE 147

subdivided into smaller climatic provinces almost
indefinitely, as pointed out in Chapter III.

Fig. 29. Annual March op Cloudiness in Continental and
Mountain Climates: Temperate Zones.

£ : Central Europe. A : Eastern Asia. M : Mountain

East Coasts. The prevailing winds carry the ma-
rine climate of the oceans on to the western coasts of
the temperate zone lands. They also carry the Con-
tinental climates of the interiors off over the eastern
coasts of these same lands, and even for some distance
on to the adjacent oceans. The east coasts, therefore,
have Continental climates, with modifications result-
ing from the presence of the oceans to leeward, and
are necessarily separated from the west coasts, with
which they have little in common. On the west coasts
of the north temperate lands the isotherms are far
apart. On the east coasts, they are crowded together.
The east coasts share with the interiors large annual
  148

CLIMATE

and cyclonic ranges of temperature. At latitude 55°
N., for example, the east coast of Asia has a mean
annual range which is four times as large as that of
the west coast. A glance at the isothermal maps of
the world will show at once how favoured, because
of its position to leeward of the warm North Atlantic
waters, is western Europe as compared with eastern
North America. A similar contrast, less marked, is
seen in eastern Asia and western North America.
In eastern Asia, there is some protection, by the coast
mountains, against the extreme cold of the interior,
but in North America there is no such barrier, and
severe cold winds sweep across the Atlantic coast
States, even far to the south. Owing to the prevail-
ing offshore winds, the oceans to leeward have rela-
tively little effect. In the north-east, the cold water
is effective in giving cooler summers than farther
south.

As already noted, the rainfall increases from the
interiors towards the east coasts. In North America,
the distribution through the year is very uniform,
with some tendency to a . summer maximum, as in the
interior (see Fig. 28).

In eastern Asia the winters are relatively dry and
clear, under the influence of the cold offshore mon-
soon, and the summers are warm and rainy, with the
northward extension of the south-east monsoon, which
reaches as far as lat. 60° N. The summer maximum
of rainfall on this coast is clearly shown in the follow-
ing data (Trabert):
  CHARACTERISTICS OF ZOU ES—TEMPERATE 149

MONTHLY DISTRIBUTION OF RAINFALL. EAST COAST OF ASIA.

(IN PERCENTAGES OF ANNUAL mean).

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year.

2 2   3   6   8 10 12 21 16 11 6   3 19.3 ins.

[Prometheus]

Rainfalls of 40 inches are found on the east
coasts of Korea, Kamchatka, and Japan, while in
North America, which is more open, they reach
farther inland. Japan, although occupying an in-
sular position, has a modified Continental, rather than
a marine climate. The winter monsoon, after Cross-
ing the water, gives abundant rain on the western
coast, while the winter is relatively dry on the lee of
the mountains, on the east. Japan has smaller tem-
perature ranges than the mainland.

Mountain Climates. The mountain climates of
the temperate zone have the usual characteristics
which are associated with altitude everywhere. If
the altitude is sufficiently great, the decreased tem-
perature gives mountains a polar climate, with the
difference that the summers are relatively cool, while
the winters are mild, owing to inversions of tempera-
ture in anticyclonic weather. Hence the annual
ranges are smaller than over lowlands. At such
times of inversion, the mountain tops often appear
as local areas of higher temperatures in a general
region of colder air over the valleys and lowlands.
The increased intensity of insolation aloft is an im-
portant factor in giving certain mountain resorts
their deserved popularity in winter {e. g., Davos and
  150

CLIMATE

Meran). Of Meran it has been well said that from
December to March the nights are winter, but the
days are mild spring. Mountains provoke rainfall,
even in arid Continental interiors, and thus we have
well-forested plateaus and mountain slopes rising
above desert lowlands. The diurnal ascending air
currents of summer usually give mountains their
maximum cloudiness and highest relative humidity
in the warmer months, while winter is the drier and
clearer season. This is shown in the data below (see
Fig. 29).

MONTHLY DISTRIBUTION OP CLOUDINESS. MOUNTAIN CLIMATE. ’

(CENTRAL EUROPE. ALPINE SU M MITS. 8500 FT. LAT. 47° N.
BEVEN STATIONS.)

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year.
5.0 5.3 6.0 6.5 7.0 6.7 6.1 5.8 6.0   6.0 5.5 5.1   5.0

The clouds of winter are low; those of summer
are higher. Hence the annual march of cloudiness
on mountains is usually the opposite of that on
lowlands.
  CHAPTER VI

THE CHABACTERISTICS OF THE ZONES: III.—THE
POLAR ZONES

General: Relation to Man, Animale, and Plants—Temperature—
Pressure and Winds—Rain and Snow—Humidity, Cloudiness,
and Fog—Cyclones and Weather—Twilight and Optical
Phenomena—Physiological Effects.

General: Relation to Man, Animale, and Plant».
The temperate zones merge into the polar zones at
the Arctic and Antarctic circles or, if temperature
is used as the basis of classification, at the isotherms
of 50° for the warmest month, as suggested by Su-
pan. The frequent use of maps on the Mercator
projection tends to give us an exaggerated idea of
the size of the polar zones. When limited by the polar
circles, these zones occupy but 0.08 of the surface of
each hemisphere, the whole area being 1.00. As-
tronomically they are distinguished by the fact that
at all places within them the sun is above the horizon
at least one full twenty-four hours each year, and be-
low it the same length of time. This longer or shorter
absence of the sun gives the climate a peculiar char-
acter, not found elsewhere. At the poles, the day
  152

CLIMATE

and the year are alike. These zones obviously have
the most oblique insolation.

For but a very small part of the polar zones have
we any knowledge, by observation, of the climate.
The fragmentary records of the earlier expeditions
gave scattering information about the weather. The
longer and more complete records of recent expedi-
tions give much more accurate and satisfactory re-
sults. It is now becoming possible to see more clearly
what the climatic conditions really are. But as yet
no scientific presentation of polar climatology is pos-
sible. We are still dealing with the meteorology of
the polar zones, rather than with their climates. More
is known of the Arctic than of the Antarctic. From
the latter there are already several excellent con-
tributions, but up to this time no record both as long
and as complete as that of the Nansen Expedition
has been obtained. The admirable report, by Dr.
Mohn, on the results of this expedition, embracing
three years’ observations, discussed with great care,
and well illustrated by curves and charts, is a monu-
mental piece of work. Under the able directorship of
Mr. Walter G. Davis, however, the Argentine Me-
teorological Service is steadily accumulating observa-
tions at its far Southern stations, on the South Orkney
and South Shetland Islands, and although these sta-
tions are not within the Antarctic zone, they will fur-
nish valuable information concerning the great ocean
area surrounding this zone.

Beyond the isotherm of 50° for the warmest month,
  CHARACTERISTICS OF ZONES—POLAR 153

forest trees and cereals do not grow. In the northern
hemisphere this line is well north of the Arctic circle
in the Continental climate of Asia, and north of it in
north-western North America. It is north of it also
in northem Scandinavia, but falls well south in east-
em British America, Labrador, and Greenland, and
also in the North Pacific Ocean. In the southem
hemisphere this isotherm crosses the southem extrem-
ity of South America, and runs nearly east and west
around the globe.

In the Arctic climate, vegetation must make rapid
growth in the short, cool summer. In the highest
latitudes the summer temperatures are not high
enough to melt snow on a level. Exposure is there-
fore of the greatest importance. Arctic plants grow
and blossom with great rapidity and luxuriance where
the exposure is favourable, and where the water from
the melting snow can run off. The soil then dries
quickly, and can be efïectively warmed. On the
other hand, when the water stands, it may freeze again
and again, and the soil undemeath has no oppor-
tunity to warm. Of Novaya Zemlya^Baer has re-
ported that the level surfaces are polar deserts, while
the slopes at the foot of the mountains, unless covered
with boulders, are like gardens in summer. Protec-
tion against cold winds is another important factor in
the growth of this vegetation. Over great stretches
of the northern plains the surface only is thawed out
in the warmer months, and swamps, mosses, and
lichens are found above etemally frozen ground.
  154

CLIMATE

Trees often grow in favourable conditions along
streams when the intervening plains are typical
tundras. Direct insolation is very effective in high
latitudes. Where the exposure is favourable, snow
melts in the sun even when the temperature of the air
in the shade is far below freezing. It has been re-
ported that at Assistance Bay (lat. 74^>° N.), in
March, when the air temperature was about —25°,
snow near stones and other dark objects melted in the
sun. Even the mean daily temperature of the snow
surface may he higher than the air temperature. The
injurious effect of polar climate upon vegetation,
especially upon trees, has been attributed by Kihl-
mann to an insufficiënt water-supply furnished hy
the roots deep in the cold ground. From the upper
parts of the tree, exposed to sunshine and wind, evap-
oration proceeds rapidly, and the tree dries up.
Protective devices against excessive evaporation, not
unlike those of desert plants, are found.

Arctic and Antarctic zones differ a good deal in
the distribution and arrangement of land and water
around and in them. The southem zone is sur-
rounded by a wide belt of open sea; the northem, by
land areas. The northem is therefore much affected
by the conditions of adjacent Continental masses.
Nevertheless, the general characteristics are appar-
ently much the same over both, so far as is now
known, the Antarctic differing from the Arctic
chiefly in having colder summers, and in the regularity
of its pressure and winds. The cold Antarctic sum-
  CHARACTERISTICS OF ZONES—POLAR 155

mers are the chief cause of the poverty of the Antarc-
tic flora. Both zones have the lowest mean annual
temperatures in their respective hemispheres, and
hence may properly be called the cold zones.

Temperature. At the solstices, the two poles re-
ceive the largest amounts of insolation which any part
of the earth’s surface ever receives. It would seem,
therefore, that the polar temperatures should then be
the highest in the world, but as a matter of fact they
  156

CLIMATE

are nearly or quite the lowest. Temperatures do not
follow insolation in this case because much of the lat-
ter never reaches the earth’s surface; because most

Fig. 31. July North Polar Isotherms

of the energy which does reach the surface is ex-
pended in melting the snow and ice of the polar areas;
and also because the water areas are large, and the
duration of insolation is short. Hence the mean
annual temperatures at both poles are nearly, or
quite, the lowest in the world.
  CHARACTERISTICS OF ZONES—POLAR 157

A set of monthly isothermal charts of the north
polar area, based on all available observations, was
prepared by Mohn and published in the volume on
Meteorology of the Nansen Expedition. These
charts give the most authentic information now at
hand regarding Arctic temperatures. In the winter
months there are three cold poles, in Siberia, in
Greenland, and at the pole itself. In January, the
mean temperatures at these .three cold poles are —49°,
—40°, and —40° respectively.

The Siberian cold pole becomes a maximum of tem-
perature during the summer, but the Greenland and
polar minima remain throughout the year. In April
the lowest isotherm, — 22°, is in Greenland, and the
north pole is then within the area enclosed by —18.4°.
In July the temperature distribution shows consider-
able uniformity; the gradients are relatively weak.

A large area in the interior of Greenland, and one
of about equal extent around the pole, are within the
isotherm of 82°. Hence the statement frequently
made, that no places in the northem hemisphere have
mean temperatures below freezing in July, is not cor-
rect. In October the interior of Greenland is en-
closed by —18°, and at the pole we find —11.2°. For
the year a large area around the pole is enclosed by
the isotherm of —4°, with an isotherm of the same
value in the interior of Greenland, but a local area
of — 7.6° is noted in Greenland, and one of —11.2° is
centred at lat. 85° N. and long. 170° E. It 'will be
seen that the temperatures are relatively lower to-
  158

CLIMATE

wards the eastern sides of the great continents. The
ordinary mean annual isothermal chart shows, within
the Arctic circle, temperatures of 40° off the Nor-
wegian coast and — 5° beyond lat. 75° N., north of

Fig. 32. Mean Annual North Polar Isotherms

Asia and North America. The January chart shows
80° off the Norwegian coast, and — 60° at Verkho-
yansk, in Siberia. The July chart shows 60° over the
continents, to 40° in extreme north-eastem Asia.
  CHARACTERISTICS OF ZONES—POLAR 159

The north polar chart of annual range of tempera-
ture shows a maximum range of about 120° in Siberia;
of 80° in North America; of 75.6° at the north pole,
and of 72° in Greenland. The north pole obviously
has a Continental climate. The minimum ranges are
on the Atlantic and Pacific oceans. The mean an-
nual isanomalies show that the line of zero anomaly
passes through the pole (as it must do). The in-
terior of Greenland has a negative anomaly in all
months. The Norwegian sea area is 45° too warm in
January and February. Siberia has +10.8° in sum-
mer, and — 45° in January. Between Bering Strait
and the pole there is a negative anomaly in all months.
The influence of the Gulf Stream drift is clearly seen
on this chart, as it is also on that of mean annual
ranges.

The mean temperatures of the higher northern
latitudes in January, July, and for the year have been
determined by Mohn with the following result:

MEAN TEMPERATURES OF THE HIGHER NORTHERN LATITUDES.

   1 60°   65°   70°   75°   80°   85°
Jan.   + 3.0°   -9.4°   -15.3°   -20.2°   -26.0°   -36.6°
July   57.4   54.3   45.1   38.1   35.6   32.5
Year   30.0   21.6   12.7   5.5   -0.6   -6.2

For the north pole itself, Mohn gives the follow-
ing results, obtained by graphic methods:

MEAN TEMPERATURES OF THE NORTH POLE.

Jan.   Feb. Mar. Apr.   May   June   July

-41.8°   -41.8°   -31.0°   -18.4°   8.6°   28.4°   30.2°

Aug.   Sept. Oct. Nov. Dec.   Year

26.6°   8.6°   -11.2°   -27.4°   -36.4°   -8.9*
  160

CLIMATE

It appears that the region about the north pole is
the coldest place in the northern hemisphere for the
mean of the year, and that the interior ice desert of
Greenland, together with the inner polar area, are to-
gether the coldest parts of the northem hemisphere in
July. In January, however, Verkhoyansk, in north-
eastem Siberia, just within the Arctic circle, has a
mean temperature of about — 60°, while the inner
polar area and the northem interior of Greenland
have only —40°. Future exploration in the im-
mediate vicinity of the north pole may show
a lower January mean temperature there than at
present appears. Such exploration will, moreover,
certainly necessitate readjustment of the isothermal
lines as now drawn for this polar area. It may be
noted that the isotherm of 32° in January crosses the
Arctic circle in the north-eastem Atlantic. Else-
where it is south of this line. By December all land
within the Arctic circle is below the freezing point.
Thus far no minima as low as those of north-eastem
Siberia have been recorded in the Arctic, and the
Arctic maxima are much lower than those of Siberia.
During the last Peary expedition, the winter of
1905-06 was distinguished by “ comparatively high ”
temperatures.

For the Antarctic our knowledge is still very frag-
mentary, and relates chiefly to the summer months,
but the numerous well-equipped expeditions of the
last ten years have brought back very valuable re-
sults, extending in a few cases over all parts of the
  CHARACTERISTICS OF ZONES—POLAR 161

year. On the February south polar isothermal chart
published in 1898 (after Buchan), the isotherm of
80° was shown essentially coincident with the Ant-
arctic circle, while a part of the isotherm of 25° was
drawn inside of the circle. Using the newest data
available, Hann has determined the mean tempera-
tures of the higher southem latitudes as follows:1

MEAN TEMPERATURES OF HIGH SOUTHERN LATITUDES.

S. Lat.   50°   60°   70°   80°
Mean Annual   41.9°   28.4°   11.3°   -3.6°
January   46.9   37.8   30.6   20.3
July   37.2   18.3   —8.0   -24.7

These temperatures can be compared with those
given on page 159, for northern latitudes. From
lat. 70° S. polewards, Hann finds that the southem
hemisphere is colder than the northern. Antarctic
summers are decidedly cold. The mean temperature
of the warmest month over the whole Antarctic zone
is below the freezing point,2 while within the Arctic
circle mean temperatures above 82° are generally
found, except in the interior of Greenland and
around the pole. The low temperatures of the
south polar summer, which are probably due to the
great Continental mass of ice around the south pole,
are responsible for much of the difficulty and disa-

1   Nature, Jan. 5, 1905, p. 221.

2   At Cape Adare, a mean January temperature of slightly over
32° was obtained on one expedition, under abnormally favourable
conditions.

SS
  162

CLIMATE

greeable character of Antarctic exploration. They
prevent much melting of snow and ice, and are
monotonous and depressing. The mean annual tem-
peratures experienced have been in the vicinity of
10°-15°, and the minima of an ordinary Antarctic
winter go down to —40° and below, but so far no
minima of the severest Siberian intensity have been
noted. The British expedition on the Discovery re-
corded — 67.7° at Cape Armitage at noon, May 16,
1903, and also noted — 40° in midsummer. The
maxima have varied between about 85° and 50°.

The temperatures at the south pole itself fumish
an interesting subject for speculation. It is likely
that near the south pole will prove to be the coldest
point on the earth’s surface for the year, as the dis-
tribution of insolation would imply, and as the
conditions of land and ice and snow there would
suggest. There is, however, room for doubt whether
the lowest mean annual temperature will be at the
mathematical pole. It is almost certain that the low-
est winter and summer temperatures in the southem
hemisphere will be found in the immediate vicinity of
the pole. One attempt to draw isotherms for the
Antarctic zone, on the basis of the recent data, is that
of Passerat, who has charted the mean winter and
mean summer temperatures. Obviously these charts
are based on extremely incomplete data, and can only
be regarded at best as tentative in the highest degree.
On the mean winter chart, the isotherm of —4°
(—20° C.) is mostly within the Antarctic circle, and
  CHABACTERISTICS OF ZONES—POLAR 163

in places well within it, while —13° (—25° C.) ap-
pears between lats. 70° and 80° on Graham Land
and Victoria Land. On the mean summer chart the
isotherm of 82° (0° C.) is about half within and half
without the Antarctic circle, and partly within lat.
70°. The isotherm of 23° (—5° C.) appears on
Victoria Land, between lats. 70° and 80°. Krebs
has attempted to draw isotherms for the far Southern
latitudes, using the data collected during the years
1901-1904. The isotherms on the mean annual
isothermal chart of the world given in Hann’s
Lehrbuch der Meteorologie have been extended to
include latitudes up to 80° S. The lowest tempera-
ture shown is indicated by portions of the isotherm of
— 4° at about latitude 80° S.

It must not be supposed that the isotherms in the
Antarctic region run parallel with the latitude lines.
They bend polewards and equatorwards at different
meridians, although much less than in the Arctic.

The annual march of temperature in the north polar
zone, for which we have the best comparable data, is
peculiar in having a much-retarded minimum, in
February or even in March—the result of the long,
cold winter. The temperature rises rapidly towards
summer, and reaches a maximum in July. Autumn
is warmer than spring. Winter comes on gradually,
the summer slowly “ falling asleep.” The polar type
of annual march of temperature is illustrated in the
accompanying curves (See Fig. 33.).

The continents do not penetrate far enough into
  Fig. 33. Annual March of
Temperature : Polar Type

N. Z., Novaya Zemlya. F. J.t
Franz Joseph's Land. G. L.
Grinnell Land.

164

[Prometheus]

  CHABACTERISTICS OF ZONES—POLAR 165

the Arctic zone to develop a pure Continental climate
in the highest latitudes. Verkhoyansk, in lat. 67° 6'
N., almost on the Arctic circle, furnishes an excellent
example of an exaggerated Continental type for the
margin of the zone, with an annual range of 120°.
One-third as large a range is found on Novaya
Zemlya, The diurnal period of temperature is noted
during the time when the sun is visible, but is hardly,
or not at all, perceptible during the dark season.
During the latter, according to the Frarn observa-
tions, the day hours are usually colder than the night
hours. This appears to be an effect of winds, for
colder, northerly winds prevailed during the hours of
daytime, and milder, southerly winds by night. Polar
climate as a whole has large annual and small
diurnal ranges, but sudden changes of wind may
cause marked irregular temperature changes within
twenty-four hours, especially in winter. The small
ranges are associated with greater cloudiness, and
vice versa. The mean diurnal variability is very
small in summer, and reaches its maximum in win-
ter, about 7° in February, according to Mohn.

Pressure and Winds. Owing to the more sym-
metrical distribution of land and water in the South-
ern than in the northern polar area, the pressures and
winds have a simpler arrangement in the former, and
may be first considered. Recent Antarctic explora-
tion has considerably modified some of the views
which have been held regarding the general winds of
the south polar area, and their controlling pressures.
  166

CLIMATE

The rapid southward decrease of pressure, which is
so marked a feature of the higher latitudes of the
southem hemisphere on the isobaric charts of the
world, does not continue all the way to the south pole.
Nor do the prevailing westerly winds, constituting
the “ circumpolar whirl,” which are so well developed
over the southem portions of the Southern hemi-
sphere oceans, blow all the way home to the south
pole. The steep poleward pressure gradients of these
southem oceans end in a trough of low pressure,
girdling the earth at about the Antarctic circle.
From here the pressure increases again towards the
south pole, where a permanent inner polar anticy-
clonic area is found, with outflowing winds deflected
by the earth’s rotation into easterly and south-easterly
directions. A chart of the south polar isobars for
February (after Sir John Murray and Dr. Buchan),
published in 1898, showed a pressure of 29.00 inches
in the low pressure girdle, and the isobar of 29.50
inches around the inner polar area. These easterly
winds have been observed by the recent expeditions
which have penetrated far enough south to cross the
low pressure trough. The limits between the pre-
vailing westerlies and the Outflowing winds from the
pole (“ easterlies ”) vary with the longitude and mi-
grate with the seasons. The change in passing from
one wind system to the other is easily observed. The
Belgica, for example, in lats. 69^°-711/2° S., and
longs. 81°-95° W., was carried towards the west by
the easterly winds in summer, and in winter was
  CHARACTERISTICS OF ZONES—POLAR 167

driven east by the westerlies, and then again to the
west. The Belgica thus lay in winter on the equa-
torial, and in summer on the polar side of the trough
of low pressure. The seasonal change in wind direc-
tion was very marked, being almost monsoon-like in
character. On the other hand, the English expedi-
tion at lat. 77° 50' S. was persistently on the polar
side of the trough, with dominant S., S.E., and E.
winds. The smoke from Mt. Erebus, however,
showed prevailing south-westerly currents. The
German expedition on the Gauss was also under the
régime of the easterly winds during its stay in winter
quarters. The Belgica had fewer calms than some
stations nearer the pole. The south polar anticy-
clone, with its surrounding low pressure girdle, mi-
grates with the season, the centre apparently shifting
polewards in summer and towards the eastern hemi-
sphere in winter. The cloudier winds are poleward;
the clearer winds blow out from the pole. The out-
flowing winds from the polar anticyclone sweep down
across the inland ice and are usually cold. Under
certain topographic conditions, descending across
mountain ranges, as in the case of the Admiralty
Range in Victoria Land, these winds may develop
high velocity and take on typical foehn character-
istics, raising the temperature to an unusually high
degree. From the fact that certain warm, southerly
winds have been reported from the mountainous
eastern coast of Victoria Land as being damp and
snow-laden, Sir Clements Markham has suggested
  168

CLIMATE

that they come from an open ocean, beyond the south
pole. Foehn winds have been noted in the South
Orkneys, from W.N.W. They are also known on
both coasts of Greenland, when a passing cyclonic
depression draws the air down from the icy interior.
These Greenland foehn winds are important climatic
elements, for they blow down warm and dry, raising
the temperature even 80° or 40° above the winter
mean, and melting the snow.

In the Arctic area the wind systems are less clearly
defined, and the pressure distribution is much less
regular, on account of the irregular distribution of
land and water. The isobaric charts published in the
report of the Nansen Expedition show that the North
Atlantic low pressure area is more or less well de-
veloped in all months. Except in June, when it lies
over southem Greenland, this tongue-shaped trough
of low pressure lies in Davis Strait, to the south-west
or west of Iceland, and over the Norwegian Sea. In
winter it greatly extends its limits farther east into
the inner Arctic Ocean, to the north of Russia and
Siberia. Between May and August it is much less
well developed. The Pacific minimum of pressure is
found south of Bering Strait and in Alaska. Be-
tween these two regions of lower pressure, the divide
extends from North America to eastern Siberia. This
divide has been called by Supan the “ Arktüche
Windschdde." High pressures are found in North
America and in Siberia from September to March,
the maxima being in Asia. The belt of somewhat
  CHABACTERISTICS OF ZONES—POLAR   169

lower pressure connecting these two maxima is situ-
ated between Bering Strait and the north pole. In
July and August the maximum pressure is between
Greenland and somewhat east of Spitzbergen. The
pressure gradients are steepest in winter. At the
pole itself, pressure seems to be highest (about 30.079
ins.) in April, and lowest (29.882 ins.^ from June to
September. The annual range is therefore only
about 0.20 in.

The prevailing westerlies, which in the high south-
em latitudes are so symmetrically developed, are in-
terfered with' to such an extent by the varying
pressure Controls over the northem continents and
oceans, in summer and winter, that they are often
hardly recognisable cn the wind maps. The isobaric
and wind charts prepared by Buchan show that on
the whole the winds blow out from the inner polar
basin, especially in winter and spring. During his
last expedition, in the winter of 1905-06, Peary
reports “every few days we had violent winds from
the south—sometimes in the shape of squalls of a few
hours’ duration, sometimes continuing as furious
gales for two or three days.” During a westerly
gale of six days’ duration (lat. 85° 12' N.) Peary and
members of his party drifted some seventy miles to
the eastward on the ice. .

In the European and North American polar areas
the annual march of pressure gives a spring maxi- •
mum, in April and May, and a minimum in January
or February. The daily fluctuations in pressure in
  170

CLIMATE

these circumpolar latitudes are about twice as large
in winter as in summer.

Rain and Snow. Rainfall on the whole decreases
steadily from equator to poles. The amount of pre-
cipitation must of necessity be comparatively slight
in the polar zones (15-10 ins., and less), chiefly be-
cause of the small capacity of the air for water va-
pour at the low temperatures there prevailing; partly
also because of the decrease, or absence, of local con-
vectional storms and thunder-showers.1 Even cy-
clonic storms cannot yield much precipitation. The
polar zones, therefore, have a permanent defidency
of precipitation. Their deserts of snow and ice are
climatic deserts in more senses than one. These ex-
tended snow and ice fields naturally tend to give an
exaggerated idea of the actual amount of precipita-
tion. It must be remembered, however, that evap-
oration is slow at low temperatures, and melting is
not excessive. Hence the polar store of fallen snow
is well preserved; interior snow fields, ice sheets, and
glaciers are produced. Nansen is of the opinion that
the amount of condensed vapour, much of it being in
the form of “ frozen fog ” (hoar frost) and not readily
measurable, exceeds evaporation in the polar districts.

The commonest form of precipitation is naturally
snow, the summer limit of which, in the northem
hemisphere, is near the Arctic circle, with the excep-
tion of Norway. In lat. 70° N., at Boothia Felix,

1 Locally, under exceptional conditions, as in the case of the
western coast of Norway, the rainfall is a good deal heavier.
  CHARACTERISTICS OF ZONES—POLAR 171

40 per cent. of the precipitation from June to August
comes in the form of snow. So far as exploration has
yet gone into the highest latitudes, rain falls in sum-
mer, and it is doubtful whether there are places near
sea-level where all the precipitation falls as snow.
It is also uncertain whether any mountains reach a
height where nothing but snow falls. Von Drygalski
believes that the inland ice-cap of Greenland, over
2600 feet above sea-level, meets these conditions.
Perhaps the interior of the south polar continent
never has rain. The snow of the polar regions
is characteristically fine and dry. Schwatka has
pointed out that the snow huts of the Eskimos could
not be built with the kind of snow that falls in the
United States. At low polar temperatures flakes
of snow are not found, but precipitation is in the form
of ice spicules. The £nest glittering ice needles
(“diamond dust”) often fill the air, even on clear
days and in calm weather, and, gradually descending
to the surface, slowly add to the depth of snow on the
ground. Dry snow is also blown up from the snow-
fields on windy days, interfering with the transpar-
ency of the air. Snowfalls at temperatures of — 40°,
and even below, have been reported from eastern
Siberian and Arctic stations. It is probable that
under these conditions the air is watmer aloft.

Humidity, Cloudiness, and Fog. The absolute
humidity must be low in polar latitudes, especially in
winter, on account of the low temperatures. Rela-
tive humidity varies greatly, and very low readings
  172

CLIMATE

have often been recorded. Cloudiness seems to de-
crease somewhat towards the inner polar areas, after
passing the belt of high cloudiness in the higher lati-
tudes of the temperate zones (see table, p. 116). In
the marine climates of high latitudes, the summer,
which is the calmest season, has the maximum cloudi-
ness; the winter, with more active wind movement, is
clearer. The data and curve given below illustrate
these conditions (see Fig. 34). The summer maxi-
mum is largely due to fogs, which are produced where
warm, damp air is chilled by coming in contact with
ice. They are formed over open waters, as among
the Faroe Islands, for example, and open water
spaces, in the midst of an ice-covered sea, are com-
monly detected at a distance by means of the “ steam
fog” which rises from them. Fogs are uncommon
in winter, when they occur as radiation fogs, of no
great thickness. The small winter cloudiness, which
is reported also from the Antarctic zone, corresponds
with the low absolute humidity and small precipita-
tion. The coasts and islands bathed by the warm

ANNUAL MARCH OF CLOUDINESS IN POLAR LATITUDE8. MARINE

type. (Beven stations. Lat. 70° N.)

Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year.
7.1 6.8 6.4 7.0 7.7   8.3 8.5 8.2   8.0   8.0 6.8 6.6   7.5

waters of the Gulf Stream drift usually have a higher
cloudiness in winter than in summer. The place of
fog is in winter taken by the fine snow crystals, which
often darken the air like fog when strong winds raise
the dry snow from the surfaces on which it is lying.
  CHARACTERISTICS OF ZONES—POLAR 173

As yet there is little detailed information concern-
ing the cloud forms and movements in the polar
zones, and the reports are rather confusing. The rec-
ords of the Nansen Expedition show a greater cloudi-

10

9

8

7

6

5

4

3

2

I

0

Fig. 34. Annual March of Cloudiness in the North Polar
Zone: Marine Type

ness by day, and with stronger winds. Cumulus forms
are rare, even in summer, and it is doubtful whether
this cloud occurs at all in its most typical develop-
ment. Clearly defined cloud forms have been re-
ported by some observers to be very rare indeed in
the Arctic, especially in the winter sky. On the other
hand, Lieutenant Royds, of the Discovery, reports
that he never saw such striking and beautiful ex-
amples of every kind of cloud as within the Antarctic
circle. At Griffith Island, in the north polar zone,
two months passed without clouds. And “ day after
day, with glorious clear skies and continuous sun-
shine ” is reported by the Discovery in the Antarctic.
  174

CLIMATE

Stratus is probably the commonest cloud of high lati-
tudes, often covering the sky for days without a
break. In place of well-developed cloud forms, the
air is filled with fog in summer, which often grows
into poorly defined stratus clouds. Cirrus cloud
forms probably decrease polewards. At the South
Orkneys, cirrus was observed at altitudes of 6000
to 8000 feet. Nansen’s results give an average cloud
movement from W.N.W. and N.W. In the Ant-
arctic, Nordenskjöld reports cirrus from W. to
W.S.W., and the Belgica expedition noted cirrus
from the east in summer only.

Cyclones and Weather. • The prevailing westerlies
continue up into the margins of the polar zones.
Many of their cyclonic storms—the weather Controls
of temperate latitudes—also continue on to the polar
zones, giving sudden and irregular pressure and
weather changes. The inner polar areas seem to be
beyond the reach of frequent and violent cyclonic dis-
turbance. Calms are more common; the weather is
quieter and fairer; precipitation is less. Most of the
observations thus far obtained from the Antarctic
come from this marginal zone of great cyclonic ac-
tivity, violent winds, and wet, disagreeahle, inhospit-
able weather, and therefore do not show the features
of the actual south polar climate.

The most thorough study of cyclonic movements in
the highest latitudes is that in connection with the
Nansen expedition in the F ram. During the three
vears of her drift, depressions passed on all sides of
 

CHARACTERISTICS OF ZONES—POLAR 175

her, with a preponderance on the west. The direc-
tion of progression averaged nearly due east, and
the hourly velocity twenty-seven to thirty-four miles,
which is ahout that in the United States. The rainy
winds were usually S. and S.E., while N.E. and N.W.
were least likely to bring rain or snow. For the
higher latitudes, most of the cyclones must pass hy
on the equatorial side of the observer, giving “ back-
ing ” winds in the northem hemisphere. The main
cyclonic tracks are such that the wind characteristi-
cally backs in Iceland, and still more so in Jan Mayen
and on the eastern coast of Greenland, these districts
lying on the north and west of the path of progres-
sion. Frightful winter storms occasionally occur
along the east coast of Greenland and off Spitzber-
gen. During the drift of the Fram the southerly
winds were the warmest in winter and the northerly
the coldest, showing that, at the 82d parallel of lati-
tude, the Siberian cold pole ceases to have much
influence.

For much of the year in the polar zones the diurnal
control is weak or absent. The successive spells of
stormy or of fine weather are wholly cyclonically
controlled. Extraordinary records of storm and
gale have been brought back from the far south and
the far north. The Swedish Antarctic expedition,
for example, under Nordenskjöld, in 1902-08, ex-
perienced for five months, beginning in May, a period
of storms with short intermissions never exceeding
three days, and during all of this period the average
  176

CLIMATE

wind velocity was twenty-three miles an hour, and
for a fortnight it averaged forty-five miles. The Dis-
covery reported a gale on July 19,1902, which lasted
ten hours with a velocity of eighty-five miles an hour.
Wind direction and temperature vary in relation to
the position of the cyclone. During the long, dreary
winter night the temperature falls to very low read-
ings. Snowstorms and gales alternate at irregular
short intervals with calmer spells of more extreme
cold and clear sides. The periods of greatest cold
in winter are calm. A wind from any direction will
bring a rise in temperature. This probably results
from the fact that the cold is the result of local radia-
tion, and a wind interferes with these conditions by
importing higher temperatures, or by mixing upper
and lower strata. During a northern polar winter
the average thickness of ice formed over the oceans,
where no storms or strong tides interfere, reaches six
feet and more. Nansen found a thickness of over
eight feet in one year. During the long summer
days the temperature rises well above the winter
mean, and under- favourable conditions certain
phenomena, such as the diurnal variation in wind ve-
locity, for example, give evidence of the diurnal con-
trol. But the irregular cyclonic weather changes
continue, in a modified form. There is no really
warm season. Snow still falls frequently. The
summer is essentially only a modified winter, from
the point of view of temperate zone man, especially
in the Antarctic, where accounts of low temperatures,
  CHARACTERISTICS OF ZONES—POLAR 177

high winds, frequent fogs, and much cloud do not
give a very cheerful picture of weather conditions. In
summer, clear spells are relatively warm, and winds
bring lower temperatures. In spite of its lack of
high temperatures, the northem polar summer, near
the margins of the zone, has many attractive qualities
in its clean, pure, crisp, dry air, free from dust and
impurities; its strong insolation; its slight precipita-
tion. In certain places, as on the interior f jords of
Greenland and on the tundras of Asia and North
America, the summer brings swarms of gnats and
flies, which are an extreme annoyance, and the pre-
valent summer fogs are a serious disadvantage.

Twilight and Optica! Phenomena. The monotony
and darkness of the polar night are decreased a good
deal by the long twilight, due to the high degree of
refraction at low temperatures. The sun actually
appears and disappears some days before and after
the times which are geometrically set. Light from
moon and stars, and from the aurora, also relieves the
darkness. Optical phenomena of great variety,
beauty, and complexity are common. Solar and
lunar haloes and coronas, and mock suns and moons
are often seen. Auroras seem to be less common and
less brilliant in the Antarctic than in the Arctic.
Sunset and sunrise colours within the polar zones are
described as being extraordinarily brilliant and
impressive.

[Prometheus]

  CHAPTER VII

THE HYQIENE OF THE ZONES

Introduction: Some General Relations of Climate and Health—
A Complex Subject—Climate, Micro-organisms and Disease—
Geographical Distribution of Disease—Tropics: General Physi-
ological Effects—Tropical Death-rates—Hygiene in the
Tropics—Tropical Diseases—Malaria—Yellow Fever—Dys-
entery—Diarrhoeal Disorders—Tropical Abscess of the Liver
—Cholera—Plague—Sunstroke and Related Conditions—
Dengue—Beri-beri—Other Minor Diseases—General Con-
clusions: Tropics—Temperate Zones: General—Winter and
Summer   Diseases—Tuberculosis—Pneumonia—Diphtheria—

Influenza—Bronchitis—Rheumatism—Measles and Scarlet
Fever—Typhoid Fever—Whooping Cough—Cholera Infantum
—Hay Fever—Polar Zones: General—Scurvy—Climate and
Health: General Conclusions.

'Introduction: Some General Relation» of Climate
and Health. From earliest times people have sought
in atmospheric conditions an explanation of the oc-
currence of disease, and have often found in statistics
of mortality and of weather a more or less striking
parallelism. Many fairly ohvious facts naturally
point to some relation of cause and effect in this
matter. Some diseases are found principally in
the warmer climates; others seem to prefer the colder.

178
  THE HYGIENE OF THE ZONES

179

Some are usually more active in the warmer, or the
drier, months; others have shown the contrary rela-
tion. High altitudes are free from some diseases
which prevail near sea-level, and have certain favour-
able climatic characteristics long recognised in the
treatment of disease. The pure air, increased res-
piration, and deeper breathing are stimulating and
health-giving; they are beneficial in many affections
of the lungs, although occasionally over-stimulating
in nervous and cardiac troubles. In the case of other
diseases, again, altitude has no effect. Dry climates,
especially deserts, whose air is usually exceptionally
pure and aseptic, are generally healthful, and are
beneficial in many cases where mountain climates are
too stimulating. The climates within forested areas
have proved especially favourable in cases of phthisis.
Ocean air, pure and dust-free, with its saline con-
stituents and equability of temperature, is beneficial
to most persons as a moderate tonic and as a restora-
tive in many illnesses. Winds are important agents
in promoting health. The cool, refreshing sea-breeze
of the tropics brings in pure air from the sea, and is
one of the most important desiderata in hot climates.
Winds are active ventilating and purifying agents
where population is congested. Fogs and clouds, by
cutting off sunlight, weaken one of the best agents in
promoting health, for the germicidal action of sun-
light has been proved by many investigators. Stern-
berg has called it “ one of the most potent and one
of the cheapest agents for the destruction of patho-
  180

CLIMATE

genie bacteria,” and says “ its use for this purpose is
to be recommended in making practical hygienic
recommendations.” In London, a higher death-rate
after a long fog may, however, result from the lower
temperature during the fog, and not from any direct
effect of the fog itself.

A Complex Subject. Facts like the foregoing
naturally prejudice one in favour of a causal connec-
tion between atmospheric conditions and disease.
Nevertheless, such studies have often led to very con-
tradictory conclusions. Diseases usually character-
istic of one zone are known to spread widely over
other zones. Diseases which usually prefer the
warmer months sometimes occur in the coldest.
Rules, previously determined as the result of careful
investigation, often break down in a most perplex-
ing way. Some of the difficulty in this lack of agree-
ment results from untrustworthy statistica, often
collected under very varying conditions and really
not comparable. Curves are smoothed to such an ex-
tent that they can be made to show anything. Conclu-
sions are drawn in individual cases which are neither
of general application, nor do they even apply locally
on any other occasion than the special one in question.
Most of this disagreement comes from the fact
that not only may the different weather elements
themselves, temperature, moisture, wind, sunshine,
and so on, each have some effect in the produc-
tion of a disease, which it is impossible to determine,
but so many other factors are concemed in the mat-
  THE HYGIENE OF THE ZONEB

181

ter that confusion and contradiction in the conclusions
reached are inevitable. Sanitation, food, water, habits,
altitude, character and moisture of the soil, race,
traffic, and other Controls serve to complicate the
problem still further. In most studies of climate
and health some, or even many, of these factors have
not received attention. Hence the results have
usually been incomplete. Local, peculiar, and tem-
porary conditions may play a large part in the pre-
valence of disease. Overcrowding under unhygienic
conditions, especially indoors during cold weather,
and traffic by rail, steam, caravan, or on foot, are often
more important than climate. The frequent escape
of mountain, of desert, and of polar peoples from epi-
demics is to be attributed in most cases to the smaller
chance of importing disease because of little inter-
course with the outside world, and of spreading it,
when imported, because of the scattered population.
It may be noted, however, that the crowding indoors
and the sparseness of population in these two cases
are more or less directly climatically controlled.

Climate, Micro-organisms and Disease. The
cause of disease Ls now no longer sought directly
in meteorological conditions, but in the effects, more
or less direct, of these conditions upon the micro-
organisms which are the specific cause of the disease.
Atmospheric conditions may help or may retard the
development of the micro-organism, and may
strengthen or weaken the individual’s power of
resistance against the attacks of the germ, as well as
  182

CLIMATE

affect his susceptibility. Thus new views have re-
placed the old. Winds used to be regarded as the
chief agents in spreading epidemics: now it is known
that disease cannot be carried far by winds, for the
micro-organisms do not long maintain their power in
the free air and under the sun. Rain has been sup-
posed directly to control the distribution of diseases:
now we believe that precipitation acts only indirectly,
through drinking water, or through its control of
the dust in the air. Dust from dry soil may
contain the geruis of infectious diseases, and aggra-
vates affections of the respiratory organs. Harm-
ful exhalations are no longer believed to be given off
by the soil, but the condition of the soil as to moisture
and temperature may affect the development and
diffusion of certain micro-organisms. Some parallel-
ism has been discovered between the prevalenoe of
certain diseases, such as diarrhoea and typhoid fever,
and soil temperatures or the ground-water level.

Geographical Distribution of Disease. The scheme
of classifying disease geographically, on a broad
climatic basis, is attractive, but not very satisfactory.
For, on the one hand, many diseases are practically
universal in extent, showing great independence of
climate, and on the other, the history of many dis-
eases is still in the making. In the distribution of
disease too many factors are concemed to make any
simple and accurate treatment possible as yet. In
spite of this complexity, however, certain broad gen-
eral statements may be made, useful in enabling the
  THE HYGIENE OF THE ZONES

183

layman properly to co-ordinate his ideas on the sub-
ject, and fairly accurate within reasonable limits.

Tropics: General Physiological Effects. The uni-
formly high temperatures of the tropics, especially
when combined with high humidity and the character-
istically small diurnal variability of temperature,
have certain fairly well established physiological
effects. Among these the following are commonly
noted: increased respiration; decreased pulse action;
profuse perspiration; lessened activity of stomach
and intestines, and tendency to digestive disorders; a
depression of bodily and mental activity, enervation,
indifference, disinclination to exertion,—in fact, a
general, ill-defined condition of debility; increased
activity of the liver; surexcitation of the kidneys. In
damp, hot air, evaporation from lungs is slight; the
blood becomes more diluted; there is a deficiency in
the number of red corpuscles in consequence of the
diminished proportion of oxygen in the air. There
is less power to do work; greater fatigue from work;
lowered vitality. All this renders the body less able
to resist disease. An anaemic condition in the moist
tropics is widespread.

Tropical Death-rates. As compared with the
death-rates in colder latitudes, tropical death-rates
average high. They range from the appalling rate
of 483 per 1000 among European troops on the Gold
Coas{ in 1829-1886, through 121 per 1000 for
European troops in Jamaica in 1820-1886, down to
so low a rate as 14.84 per 1000 for British troops in
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CLIMATE

India in 1896. These death-rates, however, repre-
sent such very diverse conditions of season, climate,
race, occupation, soil, mode of life, food, dwelling,
etc., that they cannot legitimately be compared with
one another. The prevalence of some special dis-
ease in exceptionally virulent or widespread develop-
ment will raise the death-rate of any year far
beyond its usual figure. Again, the presence of some
insect which causes loss of crops, and the resulting
lowered vitality of the people in consequence of in-
sufficiënt food, may easily swell the death-rate. Nor
can these tropical death-rates properly be compared
with the death-rates noted under different conditions
in other latitudes. (A recent attempt to compare the
death-rate among American troops in the Philippines
with the general death-rate in certain American cities
is an excellent example of the danger of comparing
two totally different things). So various and so
complex are the controlling factors that critical com-
parative study is not worth while. Tropical death-
rates are certainly high, but this fact should not be
attributed solely to the dangers of the climate. Bad
sanitary conditions and lack of medical attendance
account for many, if not most, of the high tropical
death-rates among the natives; and an irrational mode
of life explains many deaths among persons coming
from cooler climates. Tropical death-rates are be-
ing reduced with remarkable rapidity in all coun-
tries which are wholly or partly under white control,
and especially among European troops in the
  THE HYGIENE OF THE ZONES

185

tropics. This is the result of experience with tropical
conditions, and of the increased precautions which
are now taken in selecting and caring for the men.

Hygiene in the Tropics. Under the special condi-
tions of tropical climates, the resident who comes
from a cooler latitude needs to take special precau-
tions regarding his mode of life and personal hy-
giene. A rational, temperate mode of life, especially
the avoidance of alcoholic excess; regular exercise;
non-fat-producing food; clothing suited to the cli-
mate, such as duck or linen for outside garments dur-
ing the day, and light woollen for the cool of the
evening and night; careful attention to the site and
construction of dwellings; all possible sanitary pre-
cautions; keeping cool during the warmest hours and
season by the use of fans or punkahs, by frequent
baths, and by abstaining from hard work; protection
against mosquitoes by means of sereens; frequent
change of climate by retuming to cooler latitudes,—
all these are important. It seems like a contradic-
tion, but it is a fact, that the danger of taking cold
in the tropics is very great, and must be carefully
guarded against. General Wolseley is reported to
have said of the tropics, “ not to get cold is to avoid
almost certainly all the causes of disease,” and a re-
cent writer has well said that these words should be
inscribed on the walls of all barracks in the tropics.
The situation may be summed up in the rule: “ Re-
spect the sun, and rain, and wind; clothe with a view
to avoiding chili, and live temperately.” The dan-
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CLIMATE

ger of becoming chilled is greatest during the cooler
hours of evening and night, during rains, or when
cool winds blow. The skin does not react well in the
tropics, hence chills are frequent with even slight tem-
perature changes, especially when there is wind. As
to the best style of dwelling for the tropics, there is
no absolute agreement. The material can best be
determined by the local conditions in each case.
Wood, stone, and thatch are employed successfully.
Of whatever construction, houses should be roomy
and airy, and protected against direct sunshine dur-
ing the hottest hours of the day.

Tropical Diseases. In addition to the physiologi-
cal effects just considered, certain diseases are so
much at home in the tropics that they have come to
be known as tropical diseases. This designation,
however, as Sir Patrick Manson uses it in the title of
his famous work, does not mean diseases confined
to the tropics, but is employed in a meteorological
sense for diseases associated with, but not solely or
even directly due to, high temperatures. Tropical
climatic conditions, per se, piobably do not injuri-
ously affect the natives of the tropics any more than
do the conditions of extra-tropical climates affect
those who live in them.

Sir Patrick Manson has made the fact very dear
that the difference between the diseases of tropical
and extra-tropical latitudes lies in the specific cause
of these diseases. For the development of certain
disease germs, certain temperatures are required.
  THE HYGIENE OF THE ZONES

187

Sometimes the temperature is too high; sometimes
too low. Again, certain media are necessary in
propagating certain diseases, as e. g., a third organ-
ism, other than the disease germ itself, and man, who
has the disease. The third organism may be a tropi-
cal species, as in the case of the tsetse fly; if so, the
disease is a tropical disease. The opportunity for
contracting the disease is best, or exists solely, in the
tropics. Again, some diseases are the result of
toxins generated by germs living in an external med-
ium. One condition of development of these germs
may be a certain high temperature. Thus the dis-
ease is a tropical disease, e.g., beri-beri. On the
other hand, when everything seems favourable, nat-
ural enemies of the germs themselves, or of the
organism which subtends the germs, may destroy
them. Dr. Manson’s conclusion, which is the result
of careful study, may well be accepted as an authori-
tative statement. “ The more we learn about these
diseases, the less important in its hearing on their
geographic distribution, and as a direct pathogenic
agency, becomes the róle of temperature per se, and
the more the influence of the tropical fauna.”

Besides the more or less direct effects of exposure
to tropical sun and heat, such as sunstroke, heat ex-
haustion, and the like, there are malaria, in varied
forms, and dysentery, the two worst enemies of white
residents in the tropics; dengue; ulcers; yaws; tropi-
cal ahscess of the liver, a common and dreaded dis-
ease; diseases like yellow fever, cholera, and plague,

V
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CLIMATE

which are more or less limited to certain localities, and
are being hemmed in more and more by modern sani-
tary measures; many other infectious diseases which
are common to colder as well as warmer latitudes; and
beri-beri, elephantiasis, and other diseases which at-
tack the coloured race chiefly, and are therefore of a
medical rather than of a practical interest to white
people. The fact that plague, and leprosy, and to
some extent cholera as well, are practically limited
to the tropics, is the result of modern sanitary precau-
tions in the extra-tropics. The unsanitary condi-
tions among tropical peoples favour the spread of
these and similar diseases, and not the climate per
se. Nevertheless it is as clear as day, in the words of
Dr. Manson, that these very unsanitary conditions
are “ more or less an indirect outcome of tropical cli-
mate.” There is a greater variety in tropical than
in extra-tropical diseases, but then many diseases
common in cooler latitudes prevail also near the
equator, and many diseases prevail near the equator
which have practically been banished from higher
latitudes. Tropical climate is not the sole, or even
in many cases the determining factor. Most
tropical diseases attack both natives and whites;
sometimes the former suffer most; sometimes the
latter. There is no rigid rule; but the racial element
is often very potent.

Malaria. Malaria, next to tuberculosis one of the
most important of diseases, was formerly considered
a poisonous, gaseous emanation from the soil. It is
  THE HYGIENE OF THE ZONES

189

now known to be a germ disease. In 1880, Laveran,
a French army surgeon in Algiers, discovered a para-
site in the blood of malarious persons. Manson later
suspected mosquitoes as thê means of propagating
the malarial parasite. (Dr. A. F. A. King, of
Washington, ,D. C., had advanced a similar sugges-
tion in 1841.) Ross, at Manson’s suggestion in
1894, followed up the clue in India, and established
the fact. His work, and that of Grassi, Koch, and
others, has shown that the insect here concemed is a
mosquito of the single genus Anopheles, and that
malaria is due mainly, if not solely, to the injection
of the parasites into the blood of human beings by
the bite of mosquitoes previously infected by stinging
some human heing suffering from malaria.

Malaria is very widely distributed, from the polar
circles to the equator, but the endemic foei, Manson
points out, tend to become more numerous towards
the equator. There is, on the whole, a fairly regular
decrease in frequency and in severity from equator
poleward. In certain parts of the tropics, as, for ex-
ample, the Gold Coast, the mouths of the Congo and
Zamhesi, New Guinea, etc., malaria is so prevalent
and so severe that the question of residence there for
the white race has been practically controlled thereby.
The disease is commonly associated with swamps,
and moist low-lying districts, while uplands and
well-drained areas are usually less afïected. This
relation, however, seems to be somewhat less appar-
ent in the tropics than in higher latitudes. Malaria
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[Prometheus]

CLIMATE

is perennial in the tropics, with a general tendency to
a maximum in the wanner or rainy season. In the
temperate zone the maximum is in late summer or
early autumn.

It is clear, with the mosquito theory so well estab-
lished that Koch can say, of tropical Africa, “ where
there are mosquitoes there is malaria, and where there
are no mosquitoes there is no malaria,” that the older
views regarding the relation of climate and soil to
malaria must have undergone some change. Never-
theless, there is still a fairly definite relation of cause
and effect in this matter. For the development of
the malarial parasite in the body of the mosquito a
certain degree of heat is necessary, probably a mean
temperature of at least 60° F. Hirsch pointed out,
some years ago, that 60° F. is the limit at which ma-
larial fevers can occur. Hence it happend that the
same mosquito may be harmless at low temperatures
and dangerous at higher. Rainfall is important be-
cause the malaria-bearing mosquito passes part of its
life in water. Hence lakes, and especially marshes,
pools, and swamps are critical Controls as breeding-
places of the mosquitoes. Rain thus differs in its
effects according to the amount of precipitation, and
according to the conditions present where the rain
falls. A rain which in one place floods and scours
out mosquito-breeding pools, in another may just
suffice to fill hollows and low-lying places where
mosquitoes may then breed. Digging up the soil,
whether for the first time or not, may result in hol-
  THE HYGIENE OF THE ZONES

191

lows where puddles and pools may collect, and thus
give rise to malaria. The ground-water level, by
afïecting soil-moisture, also plays a part, but decom-
posing vegetable matter is no longer believed to be
an essential. Many occurrences or non-occurrences
of malaria, unexplained on any meteorological
grounds, may be ascribed to the presencé or absence
of the malaria-bearing mosquito.

The best prevention of malaria is to screen persons
who have the disease, so that they cannot infect mos-
quitoes, and to screen all doors and Windows so that
healthy individuals may not be bitten by infected
mosquitoes. Wholesale protection of this kind has
recently been attempted in Havana, on the Isthmus
of Panama, in West Africa, and elsewhere. The
danger of being bitten by the Anopheles, whose habits
are chiefly nocturnal, is greatest at night, but resid-
ence in tropical malarial districts for white persons
is always safest away from native huts and villages.
The draining and filling up of swamps, pools, and
puddles; levelling of the surface of the ground; culti-
vation of the soil by planting trees or other forms of
vegetation; destruction of the larvae by pouring oil
on the standing waters; location of dwellings on
high, dry sites; having these dwellings properly
screened,—all these precautions should be taken.
Further, a rational and scientific use of quinine, and
a change of climate to a higher latitude, are both very
important measures in case of the contraction of the
disease. Residence at an altitude of a few thousand
  192

CLIMATE

feet, where the temperature is lower than at sea-
level, is usually a sure preventive, but the mountain
climates may be injurious to persons suffering from
heart or lung troubles, or from rheumatism.

Relapses are very common after a malarial attack,
and an ansmic condition may continue for a long
time. According to Koch, these relapsing cases in-
fect the new mosquitoes each spring, but the same
authority believes it possible to destroy all the para-
sites in such cases, before the spring comes, by the
use of quinine.

Malaria is one of the greatest obstacles in the way
of white occupation of many tropical countries.
Ross spoke well when he said that the success of im-
perialism depends largely on success with the micro-
scope. The hope for the future lies in the determined
effort to destroy the malaria-bearing mosquitoes, and
to protect individuals from infection by these mos-
quitoes. Preventable, to a large extent, malaria cer-
tainly is, but it is beyond the range of human power
to eradicate the disease, certainly within any time
which is of present political interest. In the light
of the new discoveries, however, white residents in
the tropics are now in far less danger from malarial
infection than they were a few years ago.

TelUm Fever. Yellow fever is endemic only on
the eastern coast of the Americas, and on the western
coast of Africa, chiefly within the tropics, although it
frequently extends beyond them, as an epidemie, even
to latitudes between 40° and 50°. It frequents
  THE HYGIENE OF THE ZONES

193

especially the squalid quarters of seacoast towns and
the shores of large navigable rivers, readily follow-
ing railways, canals, and other highways of travel.
The opening of the Panama Canal and the establish-
ment of new steamship lines between Central Amer-
ica and the Hawaiian and Samoan Islands, where
no yellow fever has occurred, may easily be fol-
lowed by the introduction of the disease into those
islands. Within the tropics the rainy season brings
the maximum prevalence of the disease; in extra-
tropical latitudes, the summer and autumn. Hirsch
asserts that it has not gained a foothold at tempera-
tures below 68° F. Manson states that a tempera-
ture over 75° F. is needed for its development in
epidemie form. Yellow fever weakens as cold
weather approaches, and epidemics disappear when
the temperature reaches 32° F., although the vitality
of the germ may not be extinguished by frost
(Manson). Stations more than a few hundred, or
thousand, feet above sea-level are free from the dis-
ease, probably because of their lower temperatures.
The altitude of this zone varies, but at the maximum,
yellow fever has only very rarely occurred as high as
4000 feet above sea-level.

The actual cause of yellow fever is still unknown.
The brilliant work of Reed, Carroll, Agramonte, and
Guiteras has shown that the intermediate host, and
the diffusing agent of the yellow fever parasite is a
mosquito of the genus Stegomyia fasdata, which has
previously been infected by biting a person suffering
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CLIMATE

from yellow fever. The disease is non-contagious
where S. fasdata is not present, as at Petropolis,
near Rio de Janeiro. Vigorous campaigns against
the mosquito have recently produced a remarkable
decrease of the disease at Havana, on the Isthmus of
Panama, and at New Orleans in 1905. The endemic
character of yellow fever in Rio is believed by
Manson to be kept up by the continual arrival of
foreigners who are susceptible to the disease. New-
comers are chiefly attacked. After one attack, im-
munity is usually secured. Persons who have lived
for some time in endemic areas without having the
fever are more or less exempt, or may have the dis-
ease in mild form. The immunity of natives who
leave their home decreases with the length of their
absence. Negroes enjoy comparative immunity; the
yellow race is more, and the white race most, suscep-
tible. Of the white race, northerners are more sus-
ceptible than southerners.

Dysentery: Diarrhceal Disorders. Dysentery oc-
curs epidemically in all latitudes, but has its home in
the warmer climates, as a whole increasing in severity
and frequency with approach to the equator. Some
form of dysentery is almost always present in lower
latitudes, where this disease is next in importance to
malaria in causing high death-rates and in its lasting
effects. High temperatures are clearly necessary
for the development of the disease germ, but numer-
ous other Controls are also needed. The maximum
is usually in the hottest, or wettest, months; cooler
  THE HYGIENE OF THE ZONES

195

weather checks the disease. In India, the latter half
of the rainy season shows the maximum. Altitude
cannot be relied on to give relief from dysentery;
residents on mountains often suffer more than those
at lower levels. Lack of sanitary precautions, im-
pure water, overcrowding, poor food, excesses of all
kinds, are predisposing causes. The best preventive
is a rational, temperate mode of life; protection of the
more susceptible parts of the body against chills, and
a proper regulation of the whole system. Epidem-
ics of dysentery seem independent of the effects of
wind, rain, and atmospheric humidity. Immunity
is not secured after one attack, several attacks being
common.

In extra-tropical latitudes, diarrhoeal disorders
show a similar dependence on temperature, for they
are most frequent in summer and early autumn.
Usually the hotter the summer, the greater the pre-
valence and the severity of these complaints, and the
higher the death-rate from them. Other factors are,
however, concemed in this matter, so that “all at-
tempts to express the diarrhoeal mortality of a given
place as a function of the temperature only have
failed.” Soil temperature is one factor between
which and the death-rate from diarrhoeal disorders
some relation has been made out.

i

Tropical Abscess of the IAver. Rare in temper-
ate and cool climates, tropical abscess of the liver, as
the name implies, is mainly a disease of warmer lati-
tudes and usually accompanies or follows dysentery.
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CLIMATE

Among the predisposing causes the most potent are
injudicious and intemperate habits, especially over-
eating and over-indulgence in alcoholic beverages;
insufficiënt exercise; exposure; chills, and in general
the “ congestive and degenerative conditions inciden-
tal to tropical life.” Heat, malaria, and dysentery
are active precursors of liver abscess, in that they
lower the vitality. The disease is most common dur-
ing the colder or rainier season, when chills are most
frequent, but temperature is not the sole control.
The physiological adjustment of a person from a
colder latitude to tropical conditions of climate throws
a considerable strain upon the liver. The result,
especially if intemperate living is indulged in, is
likely to be liver abscess. Chiefly because of their
disregard of proper hygiene, white men and women
are generally more liable to have the disease than na-
tives; the death-rate among white troops in the tropics
is much higher than among native troops in the case
of this disease. Tropical liver abscess is most, but
by no means solely, to be expected in the earlier years
of residence in the tropics. Persons suffering from
the disease should, if possible, be sent to a temperate
climate, although there are many cases of recovery
even in the tropics.

Cholera. Cholera is due to the specific microbe,
the comma bacillus, discovered by Koch in 1883.
From its home in India, it has spread in great waves
as an epidemie over most of the globe, the last ad-
vance reaching its maximum extension early in the
  THE HYGIENE OF THE ZONEB

197

decade 1890-1900, in northern Europe. Cholera
has gone as far north as Bergen in Norway, and in
Siberia up to about latitude 60° N. No general re-
lations can be established between the occurrence of
cholera and climatic or weather conditions. Local
conditions exercise an important control. In higher
latitudes, however, cholera seems most frequent to-
wards autumn, decreasing with falling temperatures.
Cholera is chiefly prevalent in low-lying places, on
river banks, and where human beings are over-
crowded under unsanitary conditions. The at-
mosphere is clearly not the agent for carrying the
bacillus, for the latter does not keep its morbific char-
acter long in the free air. The principal agent in
spreading the disease is traffic; but drinking-water
certainly also plays a part. As a whole, cholera is
rarer and milder in the higher latitudes, and has de-
creased in Europe in cold weather, coming up again
in summer. It has, however, also been active at low
temperatures. With many exceptions, there may be
said to be a decrease with altitude, and soil moisture
may also play a part.

Plague. The specific cause of plague is a bacillus
discovered by Kitasato, a pupil of Koch, in 1894, and
also independently by Yersin. Formerly very wide-
spread, plague is now confined to the sub-tropical dis-
tricts of Southern Asia and of the Mediterranean.
It has become a disease of warm climates, because
it depends upon the unsanitary conditions in which
tropical natives live, and it attacks the poorer part of
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CLIMATE

the population. Filth, famine, social misery, anJ.
overcrowding are predisposing causes. The con-
clusions regarding the relation of plague to weather
and climate are almost as numerous as are those who
have investigated this subject, but it is clear that
plague is not limited by isotherms, and that meteoro-
logical conditions do not spread it, or solely control
it. The Indian Plague Commission concludes that
there is no direct connection between plague and
climate; Hirsch had previously stated that the rela-
tion is unsettled. In the tropics, however, the dis-
ease has, on the whole, had a cool season, and in
higher latitudes a warm season, maximum. As to
altitude, plague has occurred at high levels in cold,
dry climates, and at low levels where the climate is
warm and moist. It has prevailed when the tem-
peratures were so low that people suffered with the
cold (Roumelia, 1787-8), and at temperatures so
high that sunstrokes occurred (Smyrna, 1785).
On the whole, plague has chiefly prevailed under
moderately high temperature and moisture condi-
tions, and where the soil is damp and the ground low.
These facts do not, however, necessarily point to
cause and effect.

The best preventives of plague are pure air and
modern sanitation. In India, Haffkine has been
very successful with inoculation. Plague travels by
trade routes. Persons sick with, or incubating
plague, and infected clothing and personal effects,
carry the infection.
  THE HYGIENE OF THE ZONES

•199

1Simstroke and Related Conditions. Several dis-
agreeable and some fatal results of heat and hu-
midity, not to be classed as diseases, are common in
the tropics, and to a considerable extent also in extra-
tropical latitudes, even as far as latitude 50° to 60°
N. Sunstroke and heat prostration are most com-
mon in the tropics when the air is damp during calms,
and in temperate latitudes during the hottest spells
of summer, when the weather conditions are tropical
in character. The germ origin of sunstroke has
been maintained by Sambon, but the cause is to be
found in the effects of insolation, direct and reflected;
the air temperatures, and the undue heating of the
body. The skin of white persons when exposed to
the sun in the tropics often becomes burned and
blistered, and travellers commonly suffer because of
lack of protection of neck or limbs under sunshine.
Exposure to the sun does not always explain sun-
stroke, for at sea the tropical sun is less fatal than on
land,1 and places with apparently similar conditions
of insolation differ much as regards the prevalence of
sunstroke. A great deal doubtless depends on oc-
cupation. Many forms of heat exhaustion are in-
duced by exposure to high temperatures, but greatly
aggravated by unsuitable clothing, impaired physical
condition, and intemperance.

A study of the sunstroke weather of August,
1896, in the United States, led Dr. W. F. R. Phillips

1 Stokers and firemen suffer from prostration on steamers in the
tropics, but here artificial heat is partly responsible.
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CLIMATE

to conclude that the number of sunstrokes followed
the excess of the temperature above the normal more
closely than it did any other meteorological element;
that there was no definite relation to the relative or
absolute humidity; and that the liability to sunstroke
increased in proportion as the mean temperature of
any day approached the normal maximum tempera-
ture for that day.

Sunstroke is most common among those who are
exposed to the sun, and at hard work under condi-
tions which retard or check the cooling of the body
by radiation or conduction. The best protection
against simstroke and heat prostration in general,
and especially in the tropics, is to be found in the use
of suitable light and loose clothing; loose, wide-
brimmed, and well-ventilated headgear; avoidance of
exposure to sun and to high temperatures in general;
the use of a white umbrella; avoidance of alcohol and
of an excess of heating foods, and in a temperate life
in all respects. Poor health, fatigue, and violent ex-
ercise are all predisposing causes. Tropical camps
should be located in cool and well-ventilated places,
and tents should have doublé roofs.

Dengue. Dengue is a “highly infectious, febrile
disease, characterised by severe rheumatoid pains in
joints and limbs, and in some cases by a cutaneous
eruption of varying character and duration.” It is
distinctly a disease of warm climates, although it has
occurred as far north as latitude 40° in Europe and
in North America, and as far south as the southem
  THE HYGIENE OF THE ZONES

201

tropic. It comes mostly in the hottest months, and
is almost always checked by cold weather. Moisture
has a subordinate influence. Dengue resembles yel-
low fever in its prevailing preference for coasts,
deltas, and large river valleys; in its relation to over-
crowding and unsanitary conditions, and in its ad-
vance along routes of travel. Dengue attacks any
race, and immunity is not secured by one attack.
There is often a recurrence.

Beri-beri. A dropsical affection, combined with a
disturbance of motion and sensation, and of heart
action, beri-beri is found prindpally in or near
the tropics, being especially common in the Malay
Peninsula, and the adjacent archipelago, where it is
often a scourge. It is especially liable to break out
among gangs of labourers. Beri-beri epidemics are
most common during the rainy season. High tem-
perature and dampness are controlling factors, as are
poor health, fatigue, privation, chili, overcrowding,
etc. Damp years are apt to be marked by the sever-
ity and prevalence of beri-beri.

Other Minor Diseases. Among the minor tropical
diseases may be named sleeping sickness, limited to
tropical Africa and almost wholly confined to the
negro; and yaws, also distinctly tropical in distribu-
tion, requiring high temperature and moisture, found
chiefly in some of the larger island groups, and prin-
cipally affecting the negro.

General Conclusions: Tropics. All parts of the
equatorial zone are not equally disagreeable or hostile,