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