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CLIMATE
high summer temperatures; others shun low win-
ter temperatures; others shun the dryness which of-
ten accompanies high summer temperatures. These
plants inhabit latitudes between 22° and 45° N. or
40° S., so long as the moisture continues sufficiënt.
There is a cool season—coldest month below 64.5°
(18° C.)—and a hot summer—warmest month over
72° (22° C.),—or a mild winter—coldest month over
48° (6° C.)—or both. The classic Mediterranean
climate is found in this belt. The mesotherm belt
jcontains the tea, maté, rice, cotton, magnolia, hickoiy,
arbor vit®, hemlock, wheat, com, olive, fig, grape,
heath, cinchona, etc.
D. Mikrotherm.8: need less heat, lower mean an-
nual temperature, cooler and shorter summers, and
colder winters. The warmest month is at least 50°
(10° C.) and not over 72° (22° C.); the coldest is be-
low 48° (6° C.), with at least an occasional snow-
cover in winter and sufficiënt rainfall in the warmer
season. Evergreen and deciduous forests, grains,
and, in the warmer portions, fruit and corn are found.
E. Heki8totherm8: plants of the Arctic zone, be-
yond the limits of tree growth and of the zone of
scrubby Antarctic vegetation. These need the least
heat. Mosses, lichens, and similar lowly forms are
typical.
A simple scheme of distribution of these five groups
of plants may first be developed with reference to an
ideal continent, stretching from pole to pole, with
oceans on both sides and without mountains (Fig.
CLASSIFICATION OF CLIMATES
63
11). Here a a is the western and b b the eastern
coast. The approximate latitudes are given at the
margins. The groups of de Candolle’s System are
arranged as shown, if the xerophytes are limited to
Fig. ii. General Distribution op
Plant Zones
the deserts and steppes, and if those woody plants
of the megatherm and mesotherm zones which are
adapted to a dry climate are included within these
zones. The typical zonal arrangement is interrupted
in latitudes 20° to 50° by the fact that the arid dis-
64
CLIMATE
trict of the xerophytes (B) is wedged in on the west
coast between A and C. Farther east, zone B broad-
ens poleward, cuts through the middle of the meso-
therm zone, and usually ends without reaching the
east coast.
The five principal types are further subdivided un-
til the whole number of climates reaches twenty-four.
The special conditions which characterise each cli-
mate are carefully determined, and each sub-climate
is named after one of its characteristic plants or ani-
CLASSIFICATION OF CLIMATES
66
mals; or af ter some distinctive meteorological pheno-
menon; or, again, af ter the general character of its
vegetation. Fig 12 gives the limits of the different
Fig. 13. Na mes of Climates
at Sea-Level
sub-climates, and also the characteristic conditions of
temperature and precipitation.1 Fig. 18 gives the
1 Figures are degrees Fahr. C = coldest month. W = warmest
month. 4 M = 4 months. dr. 1.2 in. = driest month rainfall 1.2 in-
ches. D. 18° and D 36° = difference between extreme months
18° and 36°. q = quotiënt obtained by dividing the amount of
rainfall in the wettest month (in mm.) by the maximum vapour'
tension (in mm.) at the mean temperature of the same month, an
5
66
CLIMATE
scheme of the sub-climates for the lowlands, with their
names. Four climates which do not occur at sea-
leve! are here lacking (C7, E8, E4, F)The verti-
Alti- tude m. 4000 Vertical Diatrfb Continental lype Alti- tude m.
9000 4000
9000 ic*-« e 9000
1000 I 9000
ÏOOO
Fïo. 14. Vertical Distribution op Climates
cal distribution of these climates, much simplified, is
shown in Fig 14. The descent of the climatic strata
from equator to higher latitudes is shown on the right
for the Continental, and on the left for the marine type,
as far as about latitude 57°. Climates Cl to C4,
and Dl and D2, have large temperature ranges, and
are therefore lacking at the equator and on the ocean;
while C5 to C7, and D3, have small ranges, and are
not found on the continents in higher latitudes. The
general control of pressure, winds, and ocean currents
over the climatic types is shown in the two following
ideal diagrams, in which the two vertical lines indi-
cate the west and east coasts of the ideal continent,
and the area included reaches to the middle of the ad- * 1
expression which combines the effect of rainfall and evaporating
power. r=rain probability of rainiest month.
1C7, High savanna climate; E3, Yak, or Pamir climate; E4,
Chamois or high alpine climate; F, perpetual frost, without life.
CLASSIFICATION OF CLIMATES
67
jacent ideal oceans. The line 0°-0° is the equator
(Figs. 15 and 16). The short arrows give the wind
direction 500-1000 metres above the surface; calms
are represented by the sign o ; the long broken arrows
Fig. 15. Prbssurb and Winds in January
indicate the prevailing surface ocean currents. At a a
there is a rise of cold water from beneath the surface
of the ocean. The curving lines are sea-level isobars;
the lower pressures are shaded. The letters and
68
CLIMATE
boundaries, drawn in short, slanting lines in Fig. 16
indicate the climatic districts of Fig. 11. Fig. 15 is
similar to Fig. 16, as far as these climatic districts are
Fig. 16. Pressure and Winds in July
concemed. Therefore the letters and boundaries
are omitted. Fig. 17 shows the geographical distri-
bution of the climatic types and sub-types.
Ravenstein’s Hygrothermal Types. Recognising
69
Fig. 17. Köppen’s Classification of Climates in Relation to Vegetation
70
CLIMATE
the importance of relative humidity as a climatic
factor in its influence upon life, upon agriculture and
upon industry, and basing his grouping of climates
upon certain relations between temperature and rela-
tive humidity, Ravenstein proposes a subdivision of
the earth’s surface into sixteen hygrothermal climatic
types. The general characteristics and examples of
these types are as foiïows:
1. Hot (73° and over) and very damp (humidity
81% or more): Batavia, Cameroons, Mombasa.
2. Hot and moderately damp (66-80%): Ha-
vana, Calcutta.
8. Hot and dry (51-65%): Bagdad, Lahore,
Khartum.
4. Hot and very dry (50% or less): Disa, Wadi
Halfa, Kuka.
5. Warm (58° to 72°) and very damp: Walfish
Bay, Arica.
6. Warm and moderately damp: Lisbon, Rome,
Damascus, Tokio, New Orleans.
7. Warm and dry: Cairo, Algiers, Kimberley.
8. Warm and very dry: Mexico, Teheran.
9. Cool (88° to 57°) and very damp: Greenwich,
Cochabamba.
10. Coól and moderately damp: Vienna, Mel-
boume, Toronto, Chicago.
11. Cool and dry: Tashkent, Simla, Cheyenne.
12. Cool and very dry: Yarkand, Denver.
18. Cold (32° or less) and very damp: Ben Nevis,
Sagastyr, Godthaab.
CLASSIFICATION OF CLIMATES
71
14. Cold and moderately damp: Tomsk, Pike’s
Peak, Polaris House.
15. Cold and dry: (No example given).
16. Cold and very dry: Pamir.
Classification of Rainfall Systems.—The seasonal
occurrence of rainfall has suggested a classification
of the rainfall systems of the world into types. While
these schemes are useful in climatological study, they
are hardly to be considered as classifications of cli-
Fig. 18. Herbkrtson’s Major Natural Regions
mate. Mühry1 2 gave a rigid scheme of rainfall types
in six beits for each hemisphere, these beits being
divided by latitude lines; and Koppen has prepared
a useful map of the hyetal regions of the world, based
on the seasonal distribution of rainfall types.3
1A. Mühry: Klimatographische Uebersicht der Erde, Leipzig
and Hfeidelberg, 1862, 741-744. Also: AUgemeine geographische
Meteorologie, 1860, 145, and note 28, 199. Containing chart, as
well as the scheme of rainfall types.
2 See Atlas of Meteorology, Plate 19.
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CLIMATE
Herbert8orÏ8 Natural Geographical Region*.—A
scheme of “ natural geographical regions ” has been
suggested by Herbertson,1 the basis of classifica-
tion being a certain unity of temperature, rainfall
seasons, configuration and vegetation (Fig. 18).
The different types of natural regions recur in
fairly systematic order on the different continents,
being chiefly controlled by marine and Continental in-
fiuences, and each type, wherever found, has certain
similar general relations to human life and develop-
ment, as well as to animals and plants. The types
are as follows:
1. Polar. (a) Lowlands (Tundra type); (b)
Highlands (Ice-cap type).
2. The cool temperate regions. (a) Western
margin (West European type); (b) Eastern mar-
gin (Quebec type); (c) Interior lowlands (Siberian
type); (d) Interior mountain area (Altai type).
8. The warm temperate regions. (o) Western
margin with winter rains (Mediterranean type);
(b). The eastern margin, with summer rains (China
type); (c) The interior lowlands (Turan type); (d)
Plateau (Iran type).
4. (a) The west tropical deserts (Sahara type);
(b) East tropical lands (Monsoon type); (c) Inter-
tropical table-lands (Sudan type).
1 A. J. Herbertson: “ The Major Natural Regions: An Essay in
Systematic Geography.” Geogr. Joum. xxv., 1965, 300-309. A
revised chart has been published in Herbertson’s The Senior
Geography, Oxford, 1907. (The Oxford Geographies, Vol. III.)
CLASSIFICATION OF CLIMATES
73
5. Lofty tropical or sub-tropical mountains (Tib-
etan type).
6. Equatorial lowlands (Amazon type).
Summary and Conchmom. The broad classifica-
tion of climates into the three general groups of
marine, Continental, and mountain, with the subor-
dinate divisions of desert, littoral, and monsoon, is
convenient for purposes of summarising the interac-
tion of the climatic elements under the Controls of
land, water, and altitude. But in any detailed study,
some scheme of classification is needed in which simi-
lar climates in different parts of the world are
grouped together, and in which their geographic dis-
tribution receives particular consideration. It is ob-
vious from the preceding paragraphs that an almost
infinite number of classifications might be proposed;
for we may take as the basis of subdivision either the
special conditions of one climatic element, as, for ex-
ample, the same mean annual temperature, or mean
annual range of temperature, or the same rainfall, or
rainy seasons, or humidity, and so on. Or again,
similar conditions of the combination of two or more
elements of climate may be made the basis of classifi-
cation. Or we may take a botanical, or a zoölogical
basis. Of the classifications which have been pro-
posed, special reference is here made to those of
Supan, Koppen, and Herbertson. That of Supan,
taken as a whole, gives a rational, simple, and satis-
factory scheme of grouping, whose frequent use in
climatic descriptions would tend toward system, sim-
74
CLIMATE
plicity, and facility of comparison. It emphasises
the essentials of each climate, and serves to impress
these essentials upon the mind by means of the com-
pact, well-considered summary which is given in the
case of each province described. Obviously, no clas-
sification of climates which is at all complete can ap-
proach the simplicity of the ordinary classification of
the zones.
Köppen’s admirable scheme of subdividing climates,
with the emphasis on the botanical side, is perhaps
better adapted to the use of students of plant geogra-
phy than of general climatology. But it has the
great merit of recognising the existing differences of
climate between east and west coasts, and between
coasts and interiors. The co-ordination of districts
of vegetation and of climate, which this scheme so
strikingly emphasises, is a noteworthy fact in clima-
tology. The subdivision could obviously be continued
almost indefinitely.
Herbertson’s classification of the natural geo-
graphical regions is, on the whole, not very unlike that
adopted in Supan’s climatic provinces, but is less com-
plete. It is obvious that no scheme of subdivision
of this kind can be regarded as being rigid or as sat-
isfying all students of questions of distribution.
Nevertheless, some general grouping of climatic re-
gions with reference to similar features of tempera-
ture and rainfall and configuration, is a distinct help
in most geographical studies. The larger types
naturally recur on the several continents, in a fairly
CLASSIFICATION OF CLIMATES
75
systematic fashion. It results from this fact that
there is a recurrence, in a large way, of somewhat
similar conditions of life. This is a particularly help-
ful consideration in investigations of the economie
and political history of mankind. The chief pecul-
iarities of the important types can be readily leamed;
the special variations in individual areas may be in-
vestigated for each case by itself.
Ravenstein’s hygrothermal types rest upon unsatis-
factory data, and regions of very different climatic
conditions are grouped together because they happen
to have the same mean annual temperature and rela-
tive humidity.
CHAPTER IV
THE CHABACTERISTICS OF THE ZONES: I.—THE
TROPICS
General: Climate and Weather—Temperature—The Seasons—
Physiological Effects of Heat and Humidity—Pressure—Winds
and Rainfall—Land and Sea Breezes—Thunderstorms—Cloudi-
ness—Intensity of Skylight and Twilight—Climatic Subdivi-
sions:—I. The Equatorial Belt.—II. Trade Wind Beits.—III.
Monsoon Beits.—IV. Mountain Climate.
General: Climate and Weather. The so-called
“ torrid zone ” has been variously bounded. lts
limits have been set at the tropics (lat. 23^°); at the
mean annual isotherms of 68°, which also correspond
closely with the poleward extension of palms; and at
the polar margins of the trade winds. The dominant
characteristic of this great belt, embracing but a little
less than one-half of the earth’s surface, is the re-
markable simplicity and unifórmity of its climatic
features. This simplicity is reflected in the striking
regularity in the recurrence of the ordinary weather
phenomena. The tropics lack the proverbial uncer-
tainty and changeableness which characterise the
weather of the higher latitudes. In the torrid zone,
weather and climate are essentially synonymous
terms. Periodic phenomena, depending upon the
daily and annual march of the sun, are dominant.
Non-periodic weather changes are wholly subordi-
76
CHARAVTERISTICS OF ZONES—TROPICS 77
nate. The succession of daily weather changes is even
more regular, and the distribution of the climatic ele-
ments is even more uniform over the tropical oceans
than over the lands. In special regions only, and at
special seasons, is the regular sequence of weather
temporarily interrupted by an occasional tropical
cyclone. These cyclones, although comparatively in-
frequent, are notable features of the climate of the
areas in which they occur. Generally bringing very
heavy rains, and thus locally increasing the total an-
nual precipitation by a considerable amount, they yet
cause no marked temperature changes such as those
which are the common accompaniments of extra-
tropical cyclones. The devastation produced by one
of these storms often affects the economie condition
of the people in the district of its occurrence for many
years.
Temperature. The sun is always well up in the
sky. The length of day and night varies little.
Hence the mean temperature is high, it is very uni-
form over the whole zone, and there is little variation
during the year. The mean annual isotherm of 68°
is a rational limit at the polar margins of the zone,
and the mean annual isotherm of 80° encloses the
greater portion of the land areas, as well as much of
the tropicaK oceans. The isotherms are thus far
apart. The warmest latitude circle for the year is
not the equator, but latitude 10° north. The highest
mean annual temperatures, shown by the isotherm
of 85°, are in central Africa, in India, the north of
78
CLIMATE
Australia and Central America, but, with the excep-
tion of the first, these areas are small. Massowah, on
the Red Sea, has an annual mean of over 86°. The
temperatures average highest where there is little
rain, and not in the belt of heavy equatorial rains,
where the clouds afford some protection from the
sun’s rays. In June, July, and August there are
large districts in the south of Asia, and in northern
Africa, with temperatures over 90°. Winds blowing
out from these heated deserts are uncomfortably hot
and dusty.
Over nearly all of the zone the mean range of tem-
perature is less than 10°, and over much of it, especi-
ally the oceans, it is less than 5°. At Equatorville,
in the interior of Africa, on the Congo, the mean
annual range is only a little over 2°; at Iquitos (lat.
8.7° S.), in Peru, it is 4.8°* Even near the margins
of the zone, where the seasonal di ff erences are great-
est, the ranges are less than 25°, as at Calcutta, Hong
Kong, Rio de Janeiro and Khartum. The mean
daily range is usually larger than the mean annual.
Thus at Equatorville the former is about 14.5°. It
has been well said that “night is the winter of the
tropics.” The differences between the maximum and
minimum temperatures of the year near the equator
are not much greater than the daily range. Over an
area covering parts of the Pacific and Indian Oceans,
from Arabia to the Caroline Islands and from Zan-
zibar to New Guinea, as well as on the Guiana coast,
CHABACTERISTICS OF ZONES—TROPICS 79
the minimum temperatures do not normally fall be-
low 68°,. and over much of the torrid zone as a whole
they do not fall below 59°. Towards the margins of
the zone, however, the minima on the continents fall
to, or even below, 82°. Maxima of 115°, and even over
120° (122°), occur over the deserts of northern
Africa. A district where the mean maxima exceed
118° extends from the western Sahara to northwest-
ern India, and over central Australia. Near the
equator the maxima are therefore not as high as those
in many so-called “ temperate ” climates. The
greater portion of the torrid zone is a water surface,
and marine conditions are therefore typical for most
of it. These tropical oceans show remarkably small
variations in temperature. The ChaUenger re-
sults showed a daily range of hardly 0.7° in the sur-
face water temperature on the equator, and Schott de-
termined the annual range as 4.1° on the equator; 4.3°
at latitude 10°, and 6.5° at latitude 20°. It has been
clearly pointed out by Hann that the uniform dis-
tribution of temperature throughout the year—the
dominant feature of the tropics—results not only
from (1) the small variation in insolation and in the
length of the day; but also (2) from the great extent
of the zone, which makes it impossible for cold winds
from higher latitudes to penetrate into the lower lati-
tudes; (8) the oblique course of the trades, which are
well warmed on their indirect road towards the equa-
tor; (4) the slight noctumal cooling, where the air
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Prometheus:
CLIMATE
is damp and vapour is readily condensed; and (5) the
great extent of the tropical oceans, which gives so
much of the zone a marine climate.
The Seasons. In a true tropical climate, seasons,
in the temperate zone sense, do not exist. The varia-
tions in temperature throughout the yeai' are so slight
that the seasons are not classified according to tem-
perature, but depend on rainfall and the prevailing
winds. The life of animals and plants in the tropics,
and of man himself, is regulated very largely, in some
cases almost wholly, by rainfall. Agriculture pros-
pers, or fails, according to the sufficiency and punct-
ual appearance of the rains. After a long dry season,
when the rains come, there is an extraordinarily
sudden awakening of the parched and dusty vegeta-
tion. Where, on the other hand, there is abundant
moisture throughout the year, a tree may at the same
time be carrying buds, blossoms, and ripe fruit.
Vegetation under these conditions has been well called
non-periodic. Although the tropical rainy season is
characteristically associated with a vertical sun (i. e.,
summer), that season is not necessarily the hottest
time of the year. The temperature is usually some-
what lower under the clouds. The rainy season often
goes by the name of winter for this reason, and also
because the weather is dull. The time of the maxi-
mum temperature is also controlled by the rainy sea-
son. Towards the margins of the zone, with increas-
ing annual ranges of temperature, seasons in the ex-
tra-tropical sense gradually appear.
CHARACTERISTICS OF ZONES—TROPICS 81
Physiological Effects of .Heat and Humidity.
Tropical monotony of heat is associated with high
relative humidity, except over deserts and in dry sea-
sons. The air is therefore muggy and oppressive.
The high temperatures are disagreeable and hard to
bear. The “ hot-house air ” has an enervating effect.
Energetic physical and mental action are often diffi-
cult, or even impossible. The tonic effect of a cold
winter is lacking. The most humid districts in the
tropics are the least desirable for persons coming from
higher latitudes; the driest are the healthiest. The
most energetic natives are the desert-dwellers. The
monotonously enervating heat of the humid tropics
weakens, so that man becomes sensitive to slight tem-
perature changes. A fall of temperature to within
a few degrees of 70° seems to some tropical natives
almost unbearably cold, and certain African tribes
sleep on clay banks heated inside by fires, although
the mean temperature of the coldest month is over
70°. In drier climates such changes are more easily
home. The intensity of direct insolation, as well as
of radiation from the earth’s surface, may produce
sunstroke and heat prostration. “Beware of the
sun ” is a good rule in the tropics.
Pressure. The uniform temperature distribution
in the tropics involves uniform pressure distribution.
Pressure gradients are weak. The annual fluctuations
are slight, even on the continents. The diurnal varia-
tion of the barometer is so regular and so marked
that, as von Humboldt said, the time of day can be
82
CLIMATE
told withui about fifteen minutes if the reading of the
barometer be known. Even severe thunderstorms do
not overcome the regular diurnal march of the press-
ure, but the approach of tropical cyclones can be
foretold by the pressure changes.
Winds and Rainfall. Within the tropics, there are
both heavy rains and large districts of very deficiënt
precipitation. Along the barometric equator, where
the pressure gradients are weakest, is the equatorial
belt of calms, variable winds and rains—the dol-
drums. This belt, with its actively ascending, damp,
hot air, offers exceptionally favourable conditions for
abundant rainfall, and belongs among the rainiest
regions of the world, averaging probably about one
hundred inches. The rainfall is so heavy that the
salinity of the surface waters of the oceans is actu-
ally less than in the latitudes of the trades. The
sky is prevailingly cloudy, especially in the early
afternoon hours; the air is hot and oppressive; heavy
showers and thunderstorms are frequent, chiefly in
the afternoon and evening—conditions not very un-
like those which exist during certain spells of sum-
mer weather in the north temperate zone. There are
the dense tropical forests of the Amazon and of equa-
torial Africa. There frost and drought need not be
feared. This belt of calms and rains, of variable
width and rather indefinite limits, shifts north and
south of the equator after the sun. It is dreaded by
seamen because sailing vessels are apt to have long
delays in Crossing it. The calm belt is generally
CHABACTERISTICS OF ZONES—TROPICS 83
somewhat narrower than the belt of rains, the warm
ascending air being carried north and south, and giv-
ing precipitation beyond the limits of the calm zone.
In striking contrast are the easterly trade winds,
blowing between the tropical high pressure beits and
the equatorial belt of low pressure, and supplying to
the doldrum belt a constant flow of warm air which
already contains a large amount of water vapour,
evaporated from the oceans by the trades, and needs
only a moderate cooling in order to give abundant
rainfall. Of great regularity, embracing about one-
half of the earth’s surface, and adding greatly to the
uniformity of tropical climates, the trades have long
been favourite sailing routes because of the steadi-
ness of their winds, the infrequency of storms, the
brightness of their skies, and the freshness of the air,
all of which are in pleasing contrast with the muggy
and oppressive calms of the doldrums. The most de-
sirable house-sites in the tropics are very commonly
on the top of some elevation, exposed to the trade
wind. The trades are subject to many variations.
Their northern and southem margins shift north and
south af ter the sun; at certain seasons they are
interrupted, often over wide areas near their equator-
ward margins, by the migrating belt of equa-
torial rains and by monsoons; near lands, they
are often interfered with by land and sea breezes;
in certain regions, they are invaded by violent
cyclonic storms. The trades, except where they blow
onto windward coasts, or over mountains, are natu-
84
CLIMATE
rally drying winds, for they blow from higher to lower
latitudes. Some facts seem to show that there is a
descending component in the trades. They cause the
deserts of northern and Southern Africa, eastern Asia,
Australia, and Southern South America. Over the
oceans, the only rains in the trade wind beits are in
the form of passing showers.
The monsoons on the southem and eastern coasts
of Asia are the best known winds of their class. In
the northern summer, the south-west monsoon, warm
and sultry, blows over the latitudes from about 10°
north to and beyond the northem tropic, between
Africa and the Philippines, giving rains over India,
the East Indian Archipelago, and the east coasts of
China. These winds reach a storm velocity over the
Arabian sea. In winter, the south-east monsoon, the
normal, cold-season, Continental outflow from Asia,
combined with the north-east trade, generally cool and
dry, covers the same district, extending as far north
as latitude 30°. Crossing the equator, these winds
reach northern Australia, and the western islands of
the South Pacific, as a north-west rainy monsoon,
while this region in the opposite season has the normal
south-east trade. Other monsoons are found
in the Gulf of Guinea and in equatorial Africa.
Wherever they occur, they control the seasonal
changes.
The most important climatic phenomenon of the
year in the tropics is the rainy season. Tropical
rains are, in the main, summer rains, i. e., they follow,
CHARACTERI8T1C8 OF Z0NE8—TROPICS 85
as a general rule, soon after the “ vertical sun,” 1 the
rainy season coming when the normal trade gives way
to the equatorial belt of rains, or when the summer
monsoon sets in. There are, however, many cases
of a rainy season when the sun is low, especially on
windward coasts in the trades. Tropical rains come
usually in the form of heavy downpours and with
a well-marked diurnal period, the maximum varying
with the locality between noon and midnight. The
conditions at Calcutta, as shown in the accompany-
ing data, are fairly typical.2
DIURNAL DISTRIBUTION OF RAINFALL AT CALCUTTA.
12 P.M. 2 A.M. 50 12 M.—2 P.M. 111
2-4 A.M. 71 2-4 P.M 116
4-6 A.M. 65 4-6 P.M. 120
6-8 A.M. 71 6-8 p.m. 128
£-10 A.M. 58 8-10 p.m. 73
10 A.M. 12 M. 92 10 P.M. 12 P.M. 45
Local influences are, however, very important, and
in many places night rainfall maxima are found.
The tropical rainy season is therefore not to be
thought of as a period of continuous rains, falling
steadily day and night for week after week. The
momings are often fine, with clean air, well washed
by the rains of the preceding afternoon or night.
Woeikof’s detailed studies of tropical rainfalls, as a
whole, lead him to the conclusion that (1) the inten-
sity of tropical rains averages higher than in middle
1 It will be remembered that at all places within the tropics the
sun is vertical twice in the year.
2 Seven year record; expressed in thousandths of the daily mean.
86
CLIMATE
latitudes, but the difference is not great; (2) the
heaviest short downpours have, so far as observation
now goes, occurred in middle latitudes; (8) general,
moderate rainfalls lasting continuously for many
hours, which are common in the temperate zones, are
known in many parts of the tropics and have even
been given special names; (4) the heaviest daily rain-
falls have been noted outside the tropics, as at Cherra-
punji, for example; and (5) it is likely that the most
intense rains in the tropics fall during large tropical
cyclones.
Land and Sea Breezes. The sea breeze is an im-
portant climatic feature on many tropical coasts.
With its regular occurrence, and its cool, clean air,
it serves to make many districts habitable for white
settlers, and has deservedly won the name of “ the
doctor.” On not a few coasts, the sea breeze is a
true prevailing wind. The location of dwellings is
often determined by the exposure of a site to the sea
breeze. For this reason, many native villages are put
as near the sea as possible. The houses of well-to-do
foreigners often occupy the healthiest and most de-
sirable locations, where the sea breeze has a free en-
trance, while the poorer native classes live in the lower,
less exposed and less desirable places. A social
stratification is thus determined by the sea breeze.
Thunderstorms. Local thunderstorms are fre-
quent in the humid portions of the tropics. They
have a marked diurnal periodicity; find their best
opportunity in the equatorial belt of weak pressure
CHARACTERISTICS OF ZONES—TROPICS 87
gradients and high temperature, and- are commonly
associated with the rainy season, being most common
at the beginning and end of the regular rains. In
many places, thunderstorms occur daily throughout
tljeir season, with extraordinary regularity and great
intensity. Lightning is, however, reported as very
seldom doing any damage. Attention has been
called to the fact that the frequent electrical dis-
charges cause the rain water to be relatively rich in
nitric acid. This condition, together with the carbon
dioxide in the rain water and the high temperature
of the same, promotes active and deep rock decom-
position. In higher latitudes, where the ground may
be frozen part of the year, and where the decompos-
ing action of rain water is less, there is less of this
effect. In northem India, hail-storms of great
violence occur, and persons have been killed by
them.
Cloudiness. Taken as a whole, the tropics are not
favoured with such clear skies as is often supposed.
Cloudiness varies about as does the rainfall. The
maximum is in the equatorial belt of calms and rains,
where the sky is always more or less cloudy. The
minimum is in the trade latitudes, where fair skies as
a whole prevail.1 The equatorial cloud belt moves
north and south after the sun. Wholly clear days
are very rare in the tropics generally, especially near
1 Supan, Grundzüge der Physischen ErdJcunde, 3d ed., 1903, Fig.
18, page 53, gives a diagram showing the distribution of rainfall
and cloudiness (also of other elements) according to latitude.
88
CLIMATE
the equator, and during the rainy season heavy clouds
usually cover the sky.
Tropical clouds and rainfall, as a whole, repeat, in
an exaggerated form, the summer conditions of much
of the north temperate zone. Broken skies; cumulus
and cumulo-nimbus clouds; heavy showers or thun-
derstorms—these usually characterise the rainy sea-
son. Skies clear, or flecked with scattered small
cumuli, are typical of the dry season. Wholly over-
cast, dull days, such as are common in the winter of
the temperate zone, occur frequently only on tropical
coasts in the vicinity of cold ocean currents, as in
Peru and on parts of the west coast of Africa. In
these same regions ocean fogs are common.
Intensity of Skylight and Twilight. The inten-
sity of the light from tropical skies by day is trying,
even almost unbearable, to newcomers. The intense
insolation, together with the reflection from the
ground, increases the general dazzling glare under a
tropical sun, necessitating protection of some sort.
The far-famed deep blue of the tropical sky is much
exaggerated. During much of the time, smoke from
forest and prairie fires (in the dry season); dust (in
deserts), and water vapour give the sky a pale, whit-
ish appearance. In the heart of the trade wind beits
at sea, the sky is much more of a deep blue. The
beauties of tropical sunrise and sunset, and of the
tropical night, have, however, not been overrated.
Twilight within the tropics is shorter than in higher
latitudes, but the coming on of night is less sudden
CHABACTERISTICS OF ZONES—TROPICS 89
than is generally assumed. Pechuel-Lösche and
others have shown that it is possible, on the Loango
coast, to read ordinary print twenty to thirty minutes
after sunset.
Climatic Subdivision*. The rational basis for a
classification of the larger climatic provinces of the
torrid zone is found in the general wind systems and
in their control over rainfall. Following this scheme
there are these subdivisions: I. The equatorial belt;
II. The trade wind beits; III. The monsoon beits.
In each of these subdivisions there are modifications,
due to ocean and Continental influences. In general
both seasonal and diurnal phenomena and changes
are more marked in Continental interiors than on the
oceans, islands, and windward coasts. Further, the
effect of altitude is so important that another subdi-
vision should be added to indude IV. Mountain
climates.
I. The Equatorial Belt. Within a few degrees
of the equator, and when not interfered with by other
Controls, the annual curve of temperature has two
maxima following the two zenithal positions of the
sun, and two minima at about the time of the solstices.
This, which is known as the equatorial type of annual
march of temperature, is illustrated in the data
and curves for the interior of Africa, Batavia, and
Jaluit. (Fig. 19).
The greatest range is shown in the curve for the in-
terior of Africa; the curve for Batavia illustrates in-
sular conditions with less range; and the oceanic type,
90
CLIMATE
for Jaluit, Marshall Islands, gives the least range.
At Jaluit, the daily maxima for the entire year are
between 88° and 91.5° and the daily minima between
75° and 77°. This doublé maximum is not a
universal phenomenon, there being many cases where
but a single maximum occurs, as will be seen
later.
TAB LH OF MEAN MONTHLY TEMPERATURES FOR SELECTED
TROPICAL STATIONS1
I. Equatorial Type II. Tropical Type
Conti- nental Insular Marine Continental Monsoon Insular
Africa interior Batavia Jaluit, Marshall Islands Wadi Halfa Alice Springs Nagpur Hono- lulu James- town
Lat. 8.1° N. 8° 11' S. 5° 65' N. 21° 63' N. 23° 38' S. 21°9' N. 21° 18' N. 15° 66' S.
Loog. 23.6° E. 106° 50' E. 169° 40' E. 31° 20' E. 133° 37'E. 79° 11' E. 157° 60'N. 5° 43'W.
Altitude 1837 ft. 23 ft. 10 ft. 426 ft. 1926 ft. 1093 ft. 49 ft. 39 ft.
Jan. 73.4° 77.5° 80.8° 81.3° 85.6° 68.2° 70.0° 74.7°
Feb. 77.2° 77.7° 81.0° 68.6° 83.3° 73 8° 70.3° 75.9°
Mar. 83.8° 78.4° 80.6° 73.0° 77.9° 83 7° 70.9° 73.8°
April May 85.3° 79.3° 80.4° 81.0° 68.5° 90.3° 72.9° 75.0°
83.7° 79.5° 80.4° 87.1® 80.6° 94.3° 74.3® 88.9°
Tune 81.5° 78.8° 80.2° 91.4° 54.0° 85.6° 76.1° 70.5°
July 78.4° 78.3° 80.2° 93.4° 51.8° 80.1° 77.2° 71.8°
Aug. 75.7° 78.6° 80.4° 91.6° 59.4° 80.2° 77.5° 69.4°
Sept. 77.7° 79.3° 80.4° 87.1° 66.6° 80.4° 77.2° 87.6°
Oct. 78.1° 79.5° 80.8° 83.1° 73.4° 78.6° 76.5° 86.7°
Nov. 75.7° 79.0° 80.8° 71.4° 79.7® 72.3° 73.8° 67.8°
Dec. 72.0° 78.1° 80.6° 84.8° 82.8° 66.7° 71.4° 71.8°
Mean 78.6° 78.8° 80.6° 79.3° 70.3° 79.5° - 73.9° 71.1°
Range 12.4° 2° 0.8° 32.1° 33.8° 27.6° 7.5 10.2°
As the belt of rains swings back and forth across
the equator after the sun, there should be two rainy
seasons with the sun vertical, and two dry seasons
when the sun is farthest from the zenith, and while
1 Given to nearest tenth of a degree Fahr.
CBARACTERISTIC8 OF ZONES—TROPICS 91
the trades blow. These conditions prevail on the
equator, and as far north and south of the equator
(about 10°-12°) as sufficiënt time elapses between
the two zenithal positions of the sun for the two rainy
Fig. 19. Annual March of Temperature: Equatorial Type.
A: Africa, interior. B: Batavia. J: Jaluit, Marshall Islands.
seasons to be distinguished from one another. In
this belt, under normal conditions, there is, therefore,
no dry season of any considerable duration. The
doublé rainy season is clearly seen in equatorial
Fig. 20. Annual March of Rainfall
in the Tropics
S. A: South Africa. Q: Quito. S. P:
Sao Paulo. M: Mexico. H: Hilo.
P. D: Port Darwin.
92
CHABACTERISTICS OF ZONES—TROPICS 93
Africa and in parts of equatorial South America.
The maxima lag somewhat behind the vertical sun,
coming in April and November, and are unsymmetri-
cally developed, the first maximum being the principal
one. The minima are also unsymmetrically devel-
oped, and the so-called “ dry seasons ” are seldom
wholly rainless. In this equatorial belt, the annual
range of rainfall is generally below 20%; in the west-
ern portion of the Malay Archipelagd and on the
upper Amazon, it is below 10%. In these latitudes,
therefore, the distribution of rainfall is not unlike that
in extra-tropical latitudes which are under the marine
regime of rainfall, there being precipitation at all
seasons.
This rainfall type with doublé maxima and minima
has been called the equatorial type, and is illustrated
in the following data and in the curves for south
Africa and Quito. (Fig. 20).
The mean annual rainfall at Quito is 42.12 inches.
These doublé rainy and dry seasons are easily modi-
fied by other conditions, as by the monsoons of the
Indo-Australiaii area, for example, so there is no
rigid belt of equatorial rains extending around the
world. In South America, east of the Andes, the
distinction between rainy and dry seasons is often
much confused. In this equatorial belt, the cloudi-
ness is high throughout the year, averaging .7 to .8,
with a relativeljj small annual period. The data and
curve following are fairly typical, but the annual
period varies greatly under local Controls. (Fig. 21).
94
Prometheus:
CLIMATE
TABLB SHOWING MONTHLY DISTRIBUTION OP RAINPALL POR
SELECTED TROPICAL STATIONS1
Tropics
Doublé Rainy Single Rainy Season
Season Equatorial Margin of Tropics Trade Rains Monsoon . Rains
Southem Northem
Southern Africa Quito S&o Paulo Mexico Hilo Port Darwin
Latitude 6° S. Equator 23.5° S. 19.4° N. 19.7° N. 12.5° S.
Jan. 86 77 195 7 79 241
Feb. 80 92 156 9 94 215
March 123 115 103 26 86 166
April May 195 165 58 26 94 61
91 109 60 85 66 23
June 10 35 46 174 55 1
July 7 25 19 180 82 0
Aug. 17 52 31 207 81 2
Sept. 37 60 60 179 73 5
Oct. 61 91 82 79 88 38
Nov. 188 94 74 20 95 72
Dec. 105 85 116 8 107 176
TABLB SHOWING MONTHLY DI8TRIBUTION OP CLOUDINESS IN AN
BQUATORIAL CLIMATE. (CAMEROONS; GABOON. LAT. 3°
N., WEST AFRICA.)
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov Dec. Year
5.4 6.3 7.0 7.2 7.4 7.7 8.9 8.6 8.4 8.0 7.4 6.6 7.4
At greater distances from the equator than about
10° or 12°, the sun is still vertical twice a yearwithin
the tropics, but the interval between these two dates
is so short that the two rainy seasons merge into one,
in summer, and there is also but one dry season, in
1 The figures in this table are thousandths of the mean annual
rainfall. In the first column of the table, the average of a con-
siderable number of stations is given.
CHARACTERISTICS OF ZONES—TROPICS 95
winter. This is the so-called tropical type of rain-
fall,1 and is found where the trade beits are encroached
upon by the equatorial rains during the migration of
these rains into each hemisphere. It is illustrated in
E: Equatorial type. M: Monsoon type
the data and curves for Sao Paulo, Brazil, and for the
city of Mexico (see rainfall table above and Fig. 20).
The mean annual rainfall at Sao Paulo is 54.18
inches, and at Mexico 22.99 inches.
The districts of tropical rains of this type lie along
the equatorial margins of the torrid zone, outside of
the latitudes of the equatorial type of rainfall. The
rainy season becomes shorter with increasing distance
from the equator. The weather of the opposite sea-
sons is strongly contrasted. The single dry season
lasts longer than either dry season in the equatorial
1 Supan calla it the margimU type of the tropics.
96
CLIMATE
belt, reaching eight months in typical cases, with the
wet season lasting four months. The lowlands often
become dry and parched during the long, dry trade
wind season (winter), and vegetation withers away,
while grass and flowers grow in great abundance
and all life takes on new activity during the time
when the equatorial rainy belt, with its calms, variable
winds, and heavy rains, is over them (summer). The
Sudan lies between the Sahara and the equatorial
forests of Africa. It receives rains, and its vegeta-
tion grows actively, when the doldrum belt is north
of the equator (May-August). But when the trades
blow (December-Match), the ground is parched and
dusty. The Venezuelan llanos have a dry season in
the northem winter, when the trade blows. The
rains come in May-October. The campos of Brazil,
south of the equator, have their rains in October-
April, and are dry the remainder of the year. The
Nile overflow results from the rainfall on the mount-
ains of Abyssinia during the northward migration
of the belt of equatorial rains.
Simple tropical rainfalls, as shown in the above
curves, are typical of large areas, but they are not in-
frequently complicated by association with trade or
monsoon rains, as in the West Indies, Central Amer-
ica, and India. The true doldrum rains may come
along the polar margin of the equatorial low-pressure
belt, when this belt is moving equatorward, followed
by the trades.
The so-called tropical type of temperature variation,
Fig. 22. Annual March of Tempera- .
ture: Tropical Type
W: Wadi Halfa. N: Nagpur. A: Alice
Springs. H: Honolulu. J: Jamestown,
St. Helena
7
97
98
CLIMATE
with one maximum and one minimum, is illustrated
in the data given in the table on page 90, and
in the accompanying curves for Wadi Halfa, in Up-
per Egypt; Alice Springs, Australia; Nagpur, India;
Honolulu, Hawaii, and Jamestown, St. Helena.
The effect of the rainy season is often shown in a
displacement of the time of maximum temperature to
an earlier month than the usual one. During the
rains, the temperature is apt to remain constant, as
in the case of Nagpur, and of other stations in
India, Mexico, and the interior of Senegambia. This
type of temperature curve is characteristic of most
of the tropics outside of the latitudes reached by the
equatorial belt.
II. Trade Wind Beits. The trade beits near
sea-level are characterised by fair weather, steady
winds, infrequent light rains or. even an almost com-
plete absence of rain; very regular, although slight, *
annual and diurnal ranges of temperature, and a
constancy and regularity of weather which is more
truly “ temperate ” than that of most of the so-called
temperate zone. The climate of the ocean areas in
the trade wind beits is indeed the simplest and most
equable in the world, the greatest extremes—and even
these are moderate—being found to leeward of the
larger lands, where the Continental conditions are
carried offshore by the prevailing winds. On the
lowlands swept over by the trades, beyond the polar
limits of the equatorial rain belt (roughly between
lats. 20° and 80°), are most of the great deserts of the
CHARACTERI8TICS OF ZONES—TROPICS 9Ö
world. These deserts extend directly to the water’s
edge on the leeward, western coasts of Australia,
south Africa, and South America. In the two lat-
ter regions, the desert conditions are further helped
by the presence of cold ocean currents offshore. Be-
cause of their great extent, these trade wind deserts
constitute one of the most important climatic districts
in the world.
The ranges and extremes of temperature are much
greater over the deserts, especially the Continental in-
teriors, than over the oceans of the trade wind beits.
Minima of 32° or less occur during clear, quiet nights,
and daily ranges of over 50° are common. The mid-
summer mean temperature rises above 90°, with noon
maxima of 110°, or more, in the non-cloudy, dry air
of a desert day. The days, with high, dry winds,
carrying dust and sand, with extreme heat, accent-
uated by the absence of vegetation, are disagreeable
or even dangerous to life; but the calmer nights, with
active radiation under clear skies, are much more
comfortable. The nocturnal temperatures are even
not seldom too low for comfort in the cooler season,
when thin sheets of ice may form. Under the strong
insolation by day and the quick cooling by night,
rocks in the deserts split and break up. On the whole,
however, man is less susceptible to the larger tem-
perature ranges in tropical deserts than to the smaller
ones in the equatorial belt, because of the lower rela-
tive humidity in the former case. In the trade wind
deserts, as in other arid regions, man is nomadic.
100
CLIMATE
While the trades are drying winds as long as they
blow strong over the oceans, or over lowlands, they
contain a large amount of water vapour, and readily
beoome rainy if they are cooled during an ascent
over a mountain or highland. Hence the windward
(eastern) sides of mountains or bold coasts in the
trade wind beits are well watered, while the leeward
sides, or interiors, are dry. Mountainous islands in
the trades, like the Hawaiian Islands, many of the
East and West Indies, the Philippines, Bomeo, Cey-
lon, Madagascar, Teneriffe, etc., show marked diver-
gences of this sort. The eastern coasts of Guiana,
Central America, south-eastern Brazil, south-eastem
Africa, and eastern Australia are well watered, while
the interiors are very dry in the two last-named coun-
tries. The eastern highland of Australia constitutes
a more effective barrier than that in south Africa;
hence the Australian interior has a more extended
desert. South America in the south-east trade belt is
not well enclosed on the east, and the most and por-
tion is an interior district near the eastern base of
the Andes, where the land is low. Even far inland,
the Andes again provoke precipitation along their
eastern base, and the narrow Pacific Coastal strip, to
leeward of the Andes, is a very pronounced desert
from the equator to about lat. 30° S. The cold ocean
waters, with prevailing southerly (drying) winds
alongshore, are additional factors in causing this
aridity. The Peruvian climatic province is abnorm-
ally cool. Highlands in the trade beits are therefore
CE ARA CTERISTICS OF ZONES—TROPICS 101
moist on their windward slopes—even in deserts,
mountains provoke local rainfall, tree growth, and
local streams—and becomes oases of luxuriant plant
growth, while close at hand, on the leeward sides, dry
savannas or deserts may be found. The damp, rainy
and forested windward (N.E. trade) side of Central
America was, from the earliest days of European oc-
cupation, left to the natives, while the centre of civili-
sation was naturally established on the more open and
sunny south-western side.
The rainfall associated with the conditions just de-
scribed is known as the trade type. These rains have
a' maximum in winter, when the trades are most ac-
tive, this being a departure from the general rule of
summer rains in the tropics. In cases where the trade
blows steadily throughout the year against mount-
ains or bold coasts, as on the Atlantic coast of Cen-
tral America, there is no really dry season. The data
and curve for Hilo (mean annual rainfall 145.24
inches), on the windward side of the Hawaiian
Islands, show typical conditions (see Fig. 20). The
tropical rains are convectional, and therefore prefer
the warm season; the trade rains are orographic, and
have a winter maximum.
The trade type of rainfall is often much compli-
cated by the combination with it of the tropical type
and of the monsoon type (see next paragraph).
Zanzibar, for example, has its principal maximum of
rainfall in April, which is pure tropical, and has a
secondary maximum in December, which is trade.
102
CLIMATE
Again, on the lee of highlands which receive a winter
maximum on their windward slopes, summer rains
may occur at the time when the trade is weakest, and
the otherwise long dry season is interrupted by scat-
tering showers. In the Malay archipelago, there are
complications of equatorial and trade rains; likewise
in the West Indies. Trade rains often have a tend-
ency to give precipitation both day and night, while
torrid zone rains generally prefer the day.
III. Monsoon Beits. In a typical monsoon re-
gion, such as that of India, eastern Asia, and the ad-
jacent islands, the rains follow the vertical sun, and
therefore have a simple annual period much like that
of the tropical type above described, the dry season
coming when the sun is lowest (winter). This mon-
soon type of rainfall is well illustrated in the data
and curve for Port Darwin (mean annual rainfall
62.72 inches), in Australia. This summer monsoon
rainfall results from the inflow of a large body of
warm, moist air from the sea on to a land area; a con-
sequent retardation of the velocity of the air currents,
as the result of friction, and an ascent of the air, the
rainfall being particularly heavy where the winds.
have to climb over high lands. Thus, in India, the
precipitation is heaviest at the head of the Bay of
Bengal, where Cherrapunji, at the height of 4455
feet in the Khasi Hills, has a mean annual rainfall of
between 400 and 500 inches; along the southem base
of the Himalayas (60 to 160 inches); on the bold
western coast of the peninsula (80 to 120 inches and
CHARACTERISTICS OF ZONES—TROPICS 103
over), and on the mountains of Burma (up to 160
inches). In the rain-shadow of the Western Ghats,
the Deccan often suffers from drought and famine
unless the monsoon rains are abundant and well dis-
tributed, and the decreasing rainfall up the Ganges
valley leaves the Indus plain with a deficiency (less
than five inches). The prevailing direction of the
rainy monsoon wind in India is south-west; on the
Pacific coast of Asia, south-east. This monsoon dis-
trict is very large, including the Indian Ocean, Ara-
bian Sea, Bay of Bengal, and adjoining Continental
areas; the Pacific coast of China, the Yellow and
Japan seas, and numerous islands from Bomeo to
Sakhalin on the north and to the Ladrone Islands on
the east. Where the seasons are clearly defined in
India, they are three in number: a cool, dry season
(winter) when northerly trade winds prevail, and
when there is little or no rainfall except in the north-
em provinces, where moderate cyclonic storms oc-
casionally occur; a wet season, sultry and oppressive,
with the inflowing south-west monsoon of summer;
and a hot, dry season before the beginning of the
rains. The beginning of the monsoon rains usually
comes suddenly (“ burst ”), with heavy thunder-
storms. A typical temperature curve for a monsoon
district is that for Nagpur, in the Indian Deccan
(see Fig. 22), and a typical cloudiness curve is given
in Fig. 21, the maximum coming near the time of the
vertical sun, in the rainy season, and the minimum in
the dry season.
104
CLIMATE
TABLE SHOWING MONTHLY DISTRIBUTION OP CLOUDINESS IN A
MONSOON CLIMATE (BENGAL, LAT. 23.5° N.).1
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year
1.9 1.8 2.6 3.0 4.5 7.5 8.5 8.4 7.5 4.3 2.5 1.8 4.5
In the Australian monsoon region, which reaches
across New Guinea and the Sunda Islands, and west
of Australia, in the Indian Ocean, over latitudes
0°-10° S., the monsoon rains come with north-west
winds in the period between November and March or
April. The northern portion of Australia is thus
watered by zenithal summer rains, and the Southern
portions of Sumatra, Borneo, and Java are also un-
der the influence of this north-west monsoon. The
essential features of the whole Indo-Australian mon-
soon belt, therefore, are a fairly uniform distribution
and small annual range of temperature; and well-
marked periodic rains, coming with north-west or
south-west winds according to the hemisphere.
The general rule that eastern coasts in the tropics
are the rainiest finds exceptions in the case of the rainy
western coasts in India and other districts with simi-
lar rains. On the coast of the Gulf of Guinea, for
example, there is a small rainy monsoon area during
the summer; heavy rains fall on the seaward slopes of
the Cameroon Mts. Not far inland, Baliburg shows
a doublé maximum of the equatorial type. Gorée,
lat. 15° N., on the coast of Senegambia, gives a fine
example of a rainy (summer) and a dry (winter)
monsoon. A case of a special kind is the Somali
1 Five stations.
CHABACTERISTICS OF ZONES—TROPICS 105
coast, which trends N.E-S.W., and is therefore
parallel with both monsoons. Hence at no season
can it become very rainy, and mean annual rainfalls
of 40 inches , are not recorded until the coast takes a
turn to the south, at Mombasa.
Numerous combinations of equatorial, trade and
monsoon rainfalls are found, often creating great
complexity. In the case of Port Darwin, the station
is near enough to the equator to have two rainy sea-
sons (equatorial type) when the sun is vertical, as is
frequently the case in the West Indies and Central
America in the same latitude. The rainiest month,
however, is January, between the two times of verti-
cal sun, but during the height of the monsoon, there
being a rainy season of four months and a dry season
of eight months. The monsoon thus interferes with
the typical occurrence of equatorial rains. It is also
true that the dry season in monsoon districts is drier
than the two dry seasons of the equatorial type.
Batavia, on the island of Java, has simple monsoon
rains. Buitenzorg, on the same island, has a mon-
soon maximum in January, two months bef ore the
sun is vertical for the first time, and it has a regular
tropical maximum of rainfall in October, following
the second zenithal position of the sun. The north
coast of Ceram, in the Moluccas, has north-west sum-
mer monsoon rains, with a maximum in February,
while the south coast has winter rains, with the south-
east trade. The rainy and dry seasons thus come
under different conditions and at opposite times on
106
CLIMATE
the two coasts. These two sets of conditions are
often very close together, with a divide between them.
On the island of Hawaii, Hilo, on the east side, is
exposed to the,north-east trade and has a winter maxi-
mum of rainfall. Kailua, on the lee side, has about
one-third as much rainfall, with a summer maximum.
The islands of the East Indian archipelago furnish
many examples of such curious complications. The
eastern coast of Madagascar has south-east trade
winds fairly uniformly through the year, while the
interior and west coast have a summer maximum—
the normal tropical rainfall season.
IV. Mountain Climate. Within the tropics, alti-
tude is chiefly important because of its effect in tem-
pering the heat of the lowlands, especially at night.
If tropical mountains are high enough, they carry
snow the year around, even on the equator, and the
zones of vegetation may range from the densest
tropical forest at their base to the snow on their sum-
mits. The highlands and mountains within the
tropics are thus often sharply contrasted with the
lowlands, and offer more agreeable and more healthful
conditions for white settlement. They are therefore
often sought out by residents from colder latitudes
as the most attractive resorts. In India, the hill sta-
tions are crowded during the hot months by civilian
and military officials, and it has been well said that
India is ruled from 7,000 feet above sea-level. The
climate of many tropical plateaus and mountains
has the reputation of being a “ perpetual spring.”
CHARACTERISTICS OF Z0XE8—TROPICS 107
Thus, on the interior plateau of the tropical Cordil-
leras of South America, and on the central plateau of
tropical Africa, the heat is tempered hy the altitude,
while the lowlands and coasts are very hot. The
rainfall on tropical mountains and highlands often
differs considerably in amount from that on the
lowlands, and other features common to mountain
climates the world over are also noted. But the
main emphasis is rightly laid upon the temperature.
Prometheus:
CHAPTER V
THE CHARACTERISTICS OF THE ZONES: II.—THE
TEMPERATE ZONES
General: “ Temperate ” Zones—Temperature—Pressure and Winds
—Rainfall—Humidity and Cloudiness—Seasons: Their Effects
on Man—Weather—Climatic Subdivisions—South Temperate
Zone—Sub-tropical Beits: Mediterranean Climates—North
Temperate Zone: Western Coasts—Interiors—Eastern Coasts
—Mountain Climates.
General: “Temperate" Zone». The so-called
“ temperate ” zones occupy about one-half of the
earth’s surface. As a whole, they are temperate only
in that their mean temperatures and their physiolo-
gical effects are intermediate between those of the
tropics and those of the polar zones. The modifica-
tions of solar climate which result from the distribu-
tion and influence of land and water are greatest in
the temperate zones. The north temperate zone in-
cludes the greatest known extremes of temperature.
If the use of the word “ temperate ” were not so
firmly established it would be well to change the name
to intermediate, or to middle.1
1 North-middle and south-middle would then distinguish the
zones in the two hemispheres. (See W. M. Davis: The Temperate
Zones, Joum. Geogr., vol. i, 1897, pp. 139-143.) “ Temperate ” does,
however, apply fairly well to the south temperate zone.
108
CHARACTERISTICS OF ZONES—TEMPERATE 109
A marked changeableness of the weather is a strik-
ing characteristic of these zones. Apparently irreg-
ular and haphazard, these continual weather changes
nevertheless run through a fairly systematic series,
although they are essentially non-periodic. Climate
and weather are by no means synonymous over most
of the extra-tropical latitudes.
Temperature. The mean annual temperatures at
the margins of the north temperate zone differ by
more than 70°. The ranges between the mean tem-
peratures of hottest and coldest months reach 120° at
their maximum in north-eastem Siberia, and 80° in
North America. A January mean of —60° and a
July mean of 95°, and maxima of over 120° and
minima of —90°, occur in the same zone. In the dis-
tricts of lowest winter minima, the mean summer tem-
peratures exceed 85°, and in portions of the districts
of highest mean summer maxima, the mean winter
minima fall below 82°. Such great ranges character-
ise the extreme land climates. Under the mild in-
fluence of the oceans, the windward west coasts have
much smaller ranges than the interiors; the seasonal
differences increase inland. The annual ranges in
the middle and higher latitudes exceed the diurnal,
the conditions in much of the torrid zone being ex-
actly reversed. Over much of the oceans of the tem-
perate zones the annual range is less than 10°. In
the south temperate zone there are no extreme ranges,
the maxima, slightly over 80°, being near the margin
of the zone in the interior of South America, south
110
CLIMATE
Africa, and Australia. In these same localities, the
diurnal ranges, however, rival those of the north
temperate zone.
The north-eastern Atlantic ocean and north-west-
em Europe are about 85° too warm for their latitude
in January, while north-eastem Siberia is 80° too cold.
The lands north of Hudson’s Bay are 25° too cold,
and the waters of the Alaskan Bay 20° too warm.
In July, and in the southem hemisphere, the anoma-
lies are small. The lands which are the centre of
civilisation in Europe average too warm for their lati-
tudes. These lands are the most truly “ temperate ”
portion of the north temperate zone. The north-
west coast of North America is much the same. The
diurnal variability of temperature is greater in the
north temperate zone than elsewhere in the world,
and the same month may differ greatly in its charac-
ter in different years. One winter in higher latitudes
may have much snow, and temperatures below
normal; the next may give much rain instead of snow,
and the ground remain unfrozen. One summer may
be very favourable for crops; the next may give a
poor harvest.
From the point of view of temperature, these zones
may be considered in three divisions: (1) the sub-
tropical, (2) the “temperate” latitudes, and (8) the
sub-polar. The annual temperature curve has one
maximum and one minimum. In the Continental
type, the times of maximum and minimum are about
one month behind the maximum and minimum in-
CHABACTERISTICS OF ZONES—TEMPERATE 111
gnlaftnn dates. In the marine type, the retardation
may amount to nearly two. months. Coasts and
islands have a tendency to a cool spring and warm
autumn; continents, to similar temperatures in both
spring and fall.
Presmre and Winds. The prevailing winds are
the “ westerlies,” which occupy about as much of the
earth’s surface as do the easterly trades. The wester-
lies are, however, much less regular than the trades.
They vary greatly in velocity in different regions and
in different seasons, from a light wind to a gale of
fifty or more miles an hour. They are stronger in
winter than in summer. They are much interfered
with, especially in the higher northem latitudes, by
seasonal changes of temperature and pressure over the
continents, whereby the latter establish, more or less
successfully, a system of obliquely outflowing winds
in winter and of obliquely inflowing winds in sum-
mer. On the eastern coast of Asia there is a com-
plete reversal in wind direction at the opposite
seasons, but usually the seasonal shift is much less
than 180°. In summer, when the lands have low
pressure, the northem oceans are dominated by great
oval areas of high pressure, with outflowing spiral
eddies, while in winter, when the northem lands have
high pressure, the northem portions of the oceans
develop cyclonic systems of inflowing winds over
their warm waters. All these great Continental and
oceanic systems of spiraling winds are important
climatic Controls.
CLIMATE
112
The westerlies are also much confused and inter-
rupted by storms. Hence their designation of stormy
•westerlies. A constant succession of cyclones, and
the accompanying anticyclones, travelling along with
the prevailing westerlies, causes the latter very fre-
quently to change direction in order to become part
of a cyclonic or an anticyclonic whirl. In these
storms, velocities of eighty or more miles an hour
may be reached at sea. So common are such in-
terruptions that the prevailing westerly wind direc-
tion is often difficult to discem without careful
observation. Cyclonic storms are most numerous
and best developed in winter. The irregular press-
ure changes usually wholly mask the faint diurnal
variation of the barometer which is so characteristic
of the tropics, and which becomes less and less marked
with increasing latitude. Although greatly inter-
fered with near sea-level by Continental changes of
pressure, by cyclonic and anticyclonic whirls, and by
local inequalities of the surface, the eastward move-
ment of the atmosphere remains very constant aloft.
The drift of the higher clouds, and wind observations
on mountains, show clearly that the upper currents
blow with great steadiness from westerly points, the
departures being temporary, and under the control
of passing cyclones or anticyclones. The south tem-
perate zone is chiefly water. Hence the westerlies
are but little distorted by Continental effects. They
are strong and steady, and almost as regular as the
trades. “ Roaring forties ” is a well-known designa-
CHARACTERISTICS OF ZONES—TEMPERATE 113
tion for the Southern middle latitudes, and between
latitudes 40° and 60° S. the “ brave west winds ” blow
with a constancy and a velocity found in the northern
hemisphere only on the oceans, and then in a modi-
fied form. Storms, frequent and severe, cfyaracter-
ise these southem hemisphere westerlies, and easterly
wind directions are temporarily noted during their
passage. Voyages to the west around Cape Horn
against head gales, and in cold, wet weather, are
much dreaded. South of Africa and Australia, also,
the westerlies are remarkably steady and strong.
The winter in these latitudes is stormier than the
summer, but the seasonal difference is less than that
north of the equator.
Between trades and westerlies lies a debatable belt
of high pressure, shifting seasonally. Within it,
stormy westerlies and drying trades alternately hold
sway. It is the sub-tropical belt, a favoured climatic
region, where invalids seek health, and an escape from
the rigors of a cold winter is found by many who
have time and means to leave their northem homes.
Rainfall. Rainfall is fairly abundant over the
oceans, where evaporation is large, and also over a
considerable part of the lands (30-80 inches, and
more). It comes chiefly in connection with the usual
cyclonic storms, or in thunderstorms, but altitude
often serves locally to increase this precipitation. So
great are the differences, geographic and periodic, in
rainfall, produced by differences in temperature,
topography, cyclonic conditions, etc., that none but
8
114
CLIMATE
the most general rules can be laid down. The
equatorward margin of the temperate zone rains is
clearly defined on the west coasts, at the points where
the coast deserts are replaced by beits of light or
moderate rainfall. Bold west coasts, on the polar
side of lat. 40°, are very rainy, having 100 inches and
more a year in the most favourable situations. The
hearts of the continents, far from the sea, and especi-
ally when well enclosed by mountains, or when blown
over by cool ocean winds which warm in Crossing the
land, have light rainfall (less than 10-20 inches).
East coasts, receiving rain from moist winds blowing
in from the adjacent oceans as monsoons, or in front
of cyclonic stormt, are wetter than interiors, but drier
than west coasts. Winter is the season of maximum
rainfall over oceans, islands, and west coasts, for the
westerlies are then most active, cyclonic storms are
then most numerous and best developed, and the cold
lands chili the inflowing damp air. At this season,
however, the low temperatures, high pressures, and
tendency to outflowing winds over the continents are
unfavourable to rainfall, and the interior land areas,
as a rule, then have their minimum. The warmer
months bring the maximum rainfall over the conti-
nents. Then conditions are favourable for inflowing
damp winds from the adjacent oceans; there is the
best opportunity for convection; thunder-showers
readily develop on the hot aftemoons; the capacity of
the air for water vapour is greatest. Continents,
from equator to higher latitudes, thus have a tend-
CHARACTERISTICS OF ZONES—TEMPERATE 115
ency to maximum rainfall in the warm season; sum-
mer rains, as a whole, predominate over the lands.
The marine type of rainfall, with a winter maximum,
extends in over the western borders of the continents,
and is also found in the winter rainfall of the sub-
tropical beits. These winter rains are in some respects
like the winter rains on windward coasts in the trades.
Coastal lands reached by them are well watered, and
droughts need not be feared. Rainfalls are heaviest
along the tracks of most frequent cyclonic storms.
For Continental stations, the typical daily march of
rainfall is shown in the accompanying data for Berlin
and New York.
DAILY MARCH OF RAINFALL (THOIJSANDTHS OF THB DAILY
MEAN).
I. Continental Type.
Hours. Berlin. New York.
12 p.m.—2 a.m...........................76
2-4 83
4-6 74
6-8 69
8-10 ..........................62
10-Noon ........................68
Noon—2 p.m.............................85
2-4 105
4-6 104
6-8 113
8-10 83
10-12 p.m.......................78
79
85
79
80
74
81
83
95
91
90
85
78
The chief maximum is in the afternoon, and the
secondary maximum comes in the night or early
moming. The chief minimum comes between 10 a.m.
and 2 p.m. Coast stations generally have a night
116
CLIMATE
maximum, and a minimum between 10 a.m. and 4
p.m., as illustrated in the following data for Valentia.
DAILY MARCH OF RAINFALL AT YALENTIA (THOUSANDTHS OF
DAILY MEAN)
II. Marine Type.
12 p.m—2 a.m..............................88
2-4 93
4-6 93
6-8 90
8-10 84
10-Noon ............................76
Noon—2 p.m................................74
2-4 75
4-6 80
6-8 ...............................82
8-10 ..............................82
10-12 p.m...........................83
Humidity and Cloudiness. Arrhenius gives the
mean cloudiness for different latitudes as follows:
70® N. 60° 50° 40° 30° 20° 10° Eq. 10° 20° 30° 40° 50° 60° S.
59 61 48 49 42 40 50 58 57 48 46 50 66 75
The higher latitudes of the temperate zones thus
have a mean cloudiness which equals and even exceects
that of the equatorial belt. The amounts over the
oceans and coasts are greater than inland. The beits
of minimum cloudiness are at about lat. 30° N. and
S. Over the Continental interiors, the cloudiest sea-
son is summer, but the amount is never very large.
Otherwise, winter is generally the cloudiest season,
with a fairly high mean annual amount.
The absolute humidity, as a whole, decreases as the
CHARACTERISTICS OF ZONES—TEMPERATE 117
temperature falls. The relative humidity averages
ninety per cent., more or less, over the oceans, and is
high under the clouds and rain of cyclonic storms, but
depends, on land, upon the wind direction; winds
from an ocean or from a lower latitude being damper,
and those from a continent or from a colder latitude
being drier.
Seaaons: Their Effects on Man. Seasons in the
temperate zones are classified according to tempera-
ture—not, as in the tropics, by rainfall. The four
seasons are important characteristics of these zones,
especially of the middle latitudes of the north tem-
perate zone. Here spring and autumn intervene as
transition seasons between the colder winter with
snow, and warmer summer with more or less rain.
Towards the equatorial margins of the zones, the dif-
ference in temperature between summer and winter
becomes smaller, and the transition seasons weaken
and even disappear. At the polar margins, the
change from winter to summer, and vice versa, is so
sudden that there also the transition seasons dis-
appear. These seasonal changes are of the greatest
importance in the life of man.
Weather. An extreme changeableness of the
weather, depending on the succession of cyclones and
anticyclones, is another characteristic. For most of
the year and most of the zone, settled weather is un-
known. The changes are most rapid in the northern
portion of the north temperate zone, especially on
the continents, where the cyclones travel fastest. The
118
CLIMATE
nature of these changes depends on the degree of de-
velopment, the velocity of progression, the track, and
other conditions of the disturbance which produces
them. The changes may be sudden and marked, or
faint and slow; the wind may back or veer; the pre-
cipitation may be heavy or light; the wind velocity
may be light, or of hurricane force; anticyclones may
be clear, or may have clouds, and not infrequently,
precipitation. There is an almost endless variety of
such examples. The detailed study of these varying
phases of cyclonic and anticyclonic weather Controls
belongs to meteorology. It suffices here to say that
the particular weather types resulting from this con-
trol give the climates their distinctive character, and
that the study of climate through these types is the
only method of appreciating the actual conditions.
Annual and monthly averages of the different cli-
matic elements alone are misleading, and give but a
lifeless picture. The cyclonic unit, although its period
is irregular and of varying length, is an essential
basis of computation and comparison.
The weather types vary with the season and with
the geographical position. They result from a com-
bination, more or less irregular, of periodic, diurnal
elements, under the regular control of the sun, and
of non-periodic cyclonic and anticyclonic élements.
In summer, on land, when the cyclonic element is
weakest and the solar control is the strongest, the
dominant types are associated with the regular
changes from day to night. Daytime cumulus
CHARACTERISTICS OF ZONES—TEMPERATE 119
clouds; diurnal variation in wind velocity; afternoon
thunderstorms, with considerable regularity, char-
acterise the warmest months over the continents and
present an analogy with tropical conditions. Cy-
clonic and anticyclonic spells of hotter or cooler,
rainy or dry, weather, with varying winds differing
in the temperatures and the moisture which they
bring, serve to break the regularity of the diurnal
types. On the oceans, the diurnal characteristics are
much less marked.
In winter, the non-periodic, cyclonic control is
strongest. Local conditions of heat and cold become
subordinate to the general control by the cydone,
which imports weather from a distance. The irregu-
lar changes from clear to cloudy, from warmer to
colder, from dry air to snow or rain, extend over
large areas and show little diurnal control. Spring
and autumn are transition seasons and have transition
weather types. In spring, the growing diurnal
quality is marked by the increasing importance of
local Controls; the appearance of convectional pheno-
mena such as spring rains; the struggle between the
cyclonic and the solar Controls of temperature, now
one and now the other being paramount, but the lat-
ter gaining and the former losing. Cold spells, with
cyclonic winds and clouds, recall winter. Warm
spells, with marked diurnal temperature range, pre-
sage summer. In autumn, the decreasing frequency
and importance of diurnal phenomena, such as thun-
der-showers, high afternoon temperatures, and the
120
Prometheus:
CLIMATE
like; the active radiation and cooling during the
longer nights, with resulting fogs; and the increas-
ing control by the cyclone, point to winter’s coming.
Weather types thus differ with the seasons. They
differ also in Continental and marine climates. They
differ according to topography and cyclonic and anti-
cyclonic tracks. The oceans in the south temperate
zone have a constancy of non-periodic cyclonic
weather changes through the year which resembles
only faintly that over the oceans of the northem hemi-
sphere. Winter types differ little from summer
types. The diurnal control is never very strong.
Stormy weather prevails throughout the year, al-
though the weather changes are more frequent and
stronger in the colder months.
Climatic Subdivisions. From whatever point of
view the temperate zones be considered, it is clear that
there are fundamental differences between the north
and south temperate. The latter is sufficiently in-
dividual to be given a place by itself. The marginal
sub-tropical beits must also be considered as a separ-
ate group by themselves. The north temperate zone
as a whole includes large areas of land, stretching over
many degrees of latitude, as well as of water. Hence
it embraces so remarkable a diversity of climates that
no single district can be taken as typical of the whole.
lts climate has been called “ a crazy quilt of patches.”
It is a zone of marked seasonal variations and of
great extremes, annual, diurnal, cyclonic. The
simplest and most rational scheme for a classification
CHARACTERISTICS OF ZONES—TEMPERATE 121
of these climates is based on the fundamental differ-
ences which depend upon land and water, upon the
prevailing winds, and upon altitude. Thus there are
the ocean areas and the land areas. The latter are
then subdivided into western (windward) and eastern
(leeward) coasts, and interiors. Mountain climates
remain as a separate group.
South Temperate Zone. If the climate of the
north temperate zone is “ a crazy quilt of patches,”
that of the south temperate is a piece of fairly uni-
form texture and appearance throughout. This is
the effect of the large ocean surface. The whole
meteorological régime is more uniform than in the
northem zone. Although the solar climate of the
southem hemisphere is more severe than that of the
northem, the physical climate is very much less ex-
treme. It has been pointed out that this zone may
properly be called “temperate”; that its tempera-
ture changes are small; its prevailing winds are
stronger and steadier than in the northern hemisphere;
its seasons more uniform; its weather prevailingly
stormier, more changeable, and more under cyclonic
control. The uniformity of the climatic conditions
over the far southem oceans is monotonously unat-
tractive. The Continental areas are small, and de-
velop to a limited degree only the more marked
seasonal and diurnal changes which are characteristic
of lands in general. The summers are less stormy
than the winters, but even the summer temperatures
are not high. Such an area as that of New Zealand,
122
CLIMATE
with its mild climate and fairly regular rains, is really
at the margins of the zone, and has, much more
favourable conditions than do the islands farther
south. These islands, in the heart of this zone, have
dull, cheerless, and inhospitable climates, with snow
sometimes in midsummer. The zone enjoys a good
reputation for healthfulness, which fact has been
ascribed chiefly to the strong and active air move-
ment, the relatively drier air than in corresponding
northern latitudes, and the cool summers. It must
be remembered, also, that the lands are mostly in the
sub-tropical belt, which possesses peculiar climate ad-
vantages, as will be seen. The northem oceans
repeat, in a much modified fora», many of the charac-
teristics of the south temperate oceans. Except to
leeward of the broad lands, the northern oceans have
the conservative features typical of ^ marine climates
the world over.
Sub-tropical Belts: Mediterranean Climates. At
the tropical margins of the temperate zones, in the
latitudes of the tropical high pressure areas, are
the so-called sub-tropical beits. Far enough from the
equator to be free from continued high temperatures,
and near enough to it to be spared the extreme cold
of higher latitudes, these transition beits are among
the most favoured of the world. Their rainfall ré-
gime is altemately that of the westerlies and of the
trades. They are thus associated, now with the tem-
perate, and now with the tropical zones. In winter,
the equatorward migration of the great pressure and
CHABACTERISTICS OF ZONES—TEMPERATE 123
wind systems brings these latitudes under the control
of the westerlies, whose frequent irregular storms
give a moderate winter precipitation. These winter
rains recall the winter trade rains of the tropics,
although their origin is different. They are not
steady and continuous, but are separated by spells of
fine, sunny weather. The amounts vary greatly.1
In summer, when the trades are extended pole-
wards by the outflowing equatorward winds on the
eastern side of the ocean highs, mild, dry, and nearly
continuous fair weather prevails, with general north-
erly winds.
The sub-tropical beits of winter rains and dry
summers are not very clearly defined. They do not
extend continuously around the world. They are
mainly limited to the western coasts of the continents,
and to the islands off these coasts, in latitudes between
about 28° and 40°. Their degree of development
and their importance vary in different longitudes.
The sub-tropical belt is exceptionally wide in the east-
em hemisphere, and reaches far inland there, em-
bracing the countries bordering on the Mediterranean
in Southern Europe and northern Africa, including
the Azores and the famous Riviera, and then extend-
ing eastward across the Dalmatian coast and the
southem part of the Balkan peninsula into Syria,
Mesopotamia, Arabia north of the tropic. Persia, and
the adjacent lands. In the great eastward extension
1 In round numbers, Lisbon has 28.60 inches; Madrid, 16.60;
Algiers, 28.15; Nice, 33.00; Rome, 29.90; Ragusa, 63.90.
124
CLIMATE
of the winter rains in this area, the development of
secondary lows over the Mediterranean Sea is an im-
portant factor. The fact that the Mediterranean
countries are so generally included in this belt has led
to the use of the name “ Mediterranean climates.”
Owing to the great irregularity of topography and
outline, the Mediterranean province embraces many
varieties of climate, but the dominant characteristics
are the mild temperatures, except on the higher ele-
vations, and the sub-tropical rains.
On the west coasts of the two Americas, the sub- '
tropical belt of winter rains is clearly seen in Cali-
fomia and in northem Chile, on the west of the coast
mountain ranges. Between the region which has rain
throughout the year from the stormy westerlies, and
the districts which are permanently arid under the
trades, there is an indefinite belt over which rains fall
in winter. In Southern Africa, which is controlled
by the high pressure areas of the South Atlantic and
South Indian oceans, the south-westem Coastal belt
has winter rains, decreasing to the north, while the
east coast and adjoining interior have summer rains,
from the south-east trade. There is sub-tropical veg-
etation on the south-east coast, and a cool, dry climate
on the south-west coast. Southern Australia is cli-
matically similar to south Africa. In summer, the
trades give rainfall on the eastern coast, which de-
creases inland. In winter, the westerlies give mod-
erate rains, chiefly on the south-westem coast.
Northern Chile, California, south-westem Australia,
CHARACTERISTICS OF ZONES—TEMPERATE 125
and the Cape province of Africa are thus all in the
sub-tropical belt.
Fig. 23. Monthly Distribution of Rainfall: Sur-tropical
Winter Rains
M: Malta. W.A: Western Australia
The sub-tropical climates follow the tropical high
pressure beits across the oceans, but they do not re-
tain their distinctive character far inland from the
west coasts of the continents (except in the Mediter-
ranean case), nor on the east coasts. On the latter,
summer monsoons and the occurrence of general sum-
mer rains interfere, as in eastern Asia and in Florida,
and to some extent in South America east of the
Andes.
Strictly winter rains, with a maximum in Decem-
ber or in June, according to the hemisphere, are typi-
126
CLIMATE
cal of the coasts and islands of this belt. The more
Continental areas have a tendency to spring and
autumn rains. The rainy and dry seasons are most
marked at the equatorward margins of the belt, and
thus recall the tropical characteristic of dry and wet,
rather than cold and hot seasons. With increasing
latitude, the rain is more evenly distributed through
the year, the summer becoming more and more rainy
until, in the Continental interiors of the higher lati-
tudes, the summer becomes the season of maximum
rainfall. The monthly distribution of rainfall in two
sub-tropical regions is shown in the accompanying
data and curves (see Fig. 28).
ANNUAL MABCH OF RAINFALL! SUB-TROPICAL TYPE (in thoUS-
andths of the annual mean).
Southern Italy
Western Sicily
Australia Malta
Latitude............................. 32.3° S. About 38° N.
January............................... 14 130
February.............................. 18 93
March................................. 30 98
April................................. 64 75
May...................................150 35
June..................................183 23
July..................................168 8
August................................166 28
September............................. 93 73
October............................... 59 133
November.............................. 32 144
December.............................. 23 160
The following table (from Supan), giving the sea-
sonal distribution of rainfall in Southern Europe, in
CHARACTERISTICS OF ZONES—TEMPERATE 127
percentages of the annual mean, shows very clearly
the change in the rainfall season in going from north
to south. In the northem Tyrol, the normal type of
central Europe prevails. In Sicily, the summer is
almost rainless: the sub-tropical type is fully
developed.
SEASONAL DISTRIBUTION OF RAINFALL IN CENTRAL AND SOUTH-
ERN europe (in percentages of the annual mean).
Winter Spring Summer Autumn
Northern Tyrol 16 24 37 23
Southem Tyrol 14 26 28 32
Po Valley 20 26 24 30
Central Italy 25 24 17 34
Southem Italy 31 25 11 33
Sicily 39 22 3 36
Malta 48 14 2 36
In Alexandria the dry season lasts nearly eight
months; in Palestine, from six to seven months; in
Greece, about four months.
The sub-tropical rains are peculiarly well developed
on the eastern coast of the Atlantic Ocean, and are
clearly illustrated in the accompanying diagram,
after Supan (see Fig. 24).
The different types of rainfall are as follows:
I. North of lat. 40° N. Rain throughout the year.
II. Lats. 40°_27° N. Dry in summer (sub-tropical
rains).
III. Lats. 27°-19° N. Always deficiënt in rainfall.
IV. Lats. 19°-7° N. Dry in winter (tropical
rains).
V. Lats. 7°-l° N. Always rainy (equatorial belt).
128
CLIMATE
VI. Lats. 1° N.-17° S. Dry in winter (tropical rains).
VII. Lats. 17°-30° 8. Always dry.
VIII. Beyond lat. 30° S. Dry in summer (sub-tropical
rains).
(IX. Always rainy on the oceans. The African west
coast does not extend into this zone.)
50W
40
30
ZO
10
0
m
20
30
Winter Summer
I
II
m
w
v
vi
m
| m
Summer Winter
Fig. 24. Rainy and Rainless Zones on
Eastern Atlantic Coast
The winter rains which migrate equatorward are
separated by the Sahara from the equatorial rains
which migrate poleward. An unusually extended
migration of either of these rain beits may bring them
CHARACTERISTICS OF ZONES—TEMPERATE 129
close together, leaving but a small part, if any, of the
intervening desert actually rainless. The Arabian
desert occupies a somewhat similar position. Large
variations in the annual rainfall, and droughts, may
be expected towards the equatorial margins of the
sub-tropical beits. Irrigation is practised in many
places.
TABLB OF MEAN MONTHLY TEMPERATURES FOR SELECTED SUB-
TROPICAL STATIONS.1
Continental Insular
Bagdad Cordoba Bermuda Auckland
Lat. 33°19' N. 31°25' S. 32°20' N. 36°50' N.
Long. 44°26' E. 64°12' W. 64°43' W. 174°51' E.
Altitude 39 ft. 1440 ft. 148 ft. 276 ft.
January 50.9° 73.4° 62.4° 67.1°
February 53.1 72.3 61.9 67.5
March 62.1 68.4 61.7 65.5
April 69.3 61.0 64.4 61.5
May 82.0 54.5 69.6 56.7
June 89.6 49.1 74.8 53.2
July 92.8 50.0 78.8 51.8
August 92.7 54.3 80.1 52.2
September 85.6 58.6 78.1 54.5
October 76.5 63.5 73.4 57.4
November 62.1 68.5 67.6 60.3
December 52.5 71.8 63.7 64.8
Mean 72.4 62.1 69.7 59.4
Range 41.9 24.3 18.4 15.7
The main features of the sub-tropical rains east of
the Atlantic are repeated on the Pacific coasts of the
1 Given to nearest tenth of a degree Fahr.
9
m
CLIMATE
two Americas. In North America, the rainfall de-
creases from Alaska, Washington, and northern Ore-
gon southwards to Lower California, and the length
of the summer dry season increases. The mean an-
nual rainfall (1871-1901) at Neah Bay, Wash., is
112.40 inches; at San Francisco, Cal., 22.88 inches,
and at San Diego, Cal., 9.40 inches. At San Diego,
six months (May-October) have each less than five
per cent. of the annual precipitation, and four of
these have one per cent. The southem extremity of
Chile, from about latitude 88° S. southward, has
heavy rainfall throughout the year from the wester-
lies, with a winter maximum. Northem Chile is per-
sistently dry. In the intermediate area there are
winter rains and dry summers. Neither Africa nor
Australia extends far enough south to show the dif-
ferent members of this system well. New Zealand
is almost wholly in the prevailing westerly belt.
Northem India is unique in having summer monsoon
rains, and also winter rains from weak cyclonic
storms, which correspond to the sub-tropical winter
rains.
From the position of the sub-tropical beits to lee-
ward of the oceans, and at the equatorial margins of
the temperate zones, it follows that their temperatures
are not extreme. Further, the protection afforded
by mountain ranges, as by the Alps in Europe and
the Siërra Nevada in the United States, is an im-
portant factor in keeping out extremes of winter cold.
The annual march, and ranges, of temperature de-
CHARACTERISTICS OF ZONES—TEMPERATE 131
pend upon position with reference to Continental or
marine influences. This is seen in the accompanying
data and curves for Bagdad, Cordoba, Bermuda, and
Auckland (see Fig. 25).
Fig. 25. Annual March of Temperature for
Selected Sub-tropical Stations
Bd: Bagdad. Ba: Bermuda. A: Auckland.
C: Cordoba
Autumn is, as a rule, a good deal warmer than
spring, as in all the eastern Mediterranean basin, the
Canaries, and Madeira. This basin is particularly
132
CLIMATE
favoured in winter, not only in the protection against
cold afforded by the mountains, but also in the high
temperature of the sea itself. The Southern Alpine
valleys and the Riviera are well situated, having good
protection and a Southern exposure. The coldest
month usually has a mean temperature well above
82°. Mean minimum temperatures of about, and
somewhat below, freezing occur in the northem por-
tion of the district,1 and in the more Continental lo-
calities minima a good deal lower have been observed.
(At San Diego, Cal., the absolute minimum is 82°;
at San Francisco, 29°.) Mean maximum tempera-
tures of about 95° occur in northem Italy, and of
still higher degrees in the southem portions. Some-
what similar conditions exist in the sub-tropical
district of North America. Under the control of
passing cyclonic storm areas, hot or cold winds, which
often owe some of their special characteristics to the
topography, bring into the sub-tropical beits, from
higher or lower latitudes, unseasonably low or high
temperatures. These winds have been given special
names (mistral, sirocco, bora, chamsin, leste, leveche,
pampero, southerly burster, etc.)
These beits enjoy abundant sunshine, being among
the least cloudy districts in the world. The accom-
panying data and curve, giving an average for ten
stations, show the small annual amount of cloud, the
winter maximum and the marked summer minimum,
in a typical sub-tropical climate. (Fig. 26).
i Nice, 30.4°; Rome, 25.7°; Palermo, 32°; Athens, 28.8°.
CHARACTERISTICS OF ZONES—TEMPERATE 133
MONTHLY DISTRIBUTION OP CLOUDINESS IN A SUB-TROPICAL
CLIMATE (EASTERN MEDITERRANEAN, LAT. 33.8° N.)
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Year
4.9 4.6 3.8 3.7 2.8 1.3 1.1 1.3 1.8 2.5 4.0 4.7 3.0
The winter rains do not bring continuously over-
cast skies; and it has been well said that the problem
of securing a maximum rainfall with a maximum
number of clear days has been solved on the southem
Alpine slopes. A summer month with a mean
10
9
8
7
6
5
4
3
2
I
0
Fig. 26. Annual March of Cloudiness in a Sub-tropical Climate
(Eastern Mediterranean)
cloudiness of 0.1 is not exceptional in the drier parts
of the sub-tropics. The winter cloudiness in northem
Italy is 5.0 to 6.0; in summer, 8.0 to 4.0. Cairo
has an annual mean of 1.9, and in June it has
0.8. Biskra, on the northem margin of the Sahara,
has 264 clear days. In the central valley of Cali-
134
CLIMATE
fornia, the number of clear days is similarly very
large.
With prevailingly fair skies, even temperatures, and
moderate rainfall, the sub-tropical beits possess many
climatic advantages which fit them for health resorts.
The long list of well-known resorts on the Mediter-
ranean coast, and the shorter list for Califomia, bear
witness to this fact.
North Temperate Zone: Weet Coaete. Marine
climatic types are carried by the prevailing wester-
lies on to the western coasts of the continents, giving
them mild winters and cool summers, abundant rain-
fall and a high degree of cloudiness and relative
humidity. North-western Europe is particularly
favoured because of the remarkably high tempera-
tures of the North Atlantic Ocean, and because of the
influence of the winds controlled by the low pressure
area off Iceland. In January, north-western Europe
has temperatures from 20° to 40° in excess of the
normal for the latitude. The north-western coast of
North America has temperatures more than 10° too
warm for the latitude. January means of 40° to 50°
in the British Isles and on the northem French coast
occur in the same latitudes as those of 0° and 10° in
the far interior of Asia. In July, means of 60° to
70° in the former contrast with 70° and 80° in the lat-
ter districts. The conditions are somewhat similar
in North America. Along the western coasts of
North America and of Europe the mean annual
ranges are under 25°,—actually no greater than some
Fig. 27. annual March op
Temperature for Selected
Stations in the Temperate
Zones
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