THE CHARACTERISTICS 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—Cloudiness—Intensity of Skylight and Twilight—Climatic Subdivisions:—I. The Equatorial Belt.—II. Trade Wind Belts.—III. Monsoon Belts.—IV. Mountain Climate.

_General: Climate and Weather._ The so-called “torrid zone” has been variously bounded. Its limits have been set at the tropics (lat. 231⁄2°); 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 remarkable simplicity and uniformity 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 uncertainty 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 subordinate. The succession of daily weather changes is even more regular, and the distribution of the climatic elements 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 infrequent, are notable features of the climate of the areas in which they occur. Generally bringing very heavy rains, and thus locally increasing the total annual 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 economic 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 uniform 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 tropical 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 Australia and Central America, but, with the exception 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 temperature is less than 10°, and over much of it, especially 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. 3.7° S.), in Peru, it is 4.3°. Even near the margins of the zone, where the seasonal differences are greatest, 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 Zanzibar to New Guinea, as well as on the Guiana coast, the minimum temperatures do not normally fall below 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, 32°. Maxima of 115°, and even over 120° (122°), occur over the deserts of northern Africa. A district where the mean maxima exceed 113° extends from the western Sahara to north-western 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 _Challenger_ results showed a daily range of hardly 0.7° in the surface water temperature on the equator, and Schott determined 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 distribution 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 latitudes; (3) the oblique course of the trades, which are well warmed on their indirect road towards the equator; (4) the slight nocturnal cooling, where the air 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 variations in temperature throughout the year are so slight that the seasons are not classified according to temperature, 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 prospers, or fails, according to the sufficiency and punctual appearance of the rains. After a long dry season, when the rains come, there is an extraordinarily sudden awakening of the parched and dusty vegetation. 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 somewhat 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 maximum temperature is also controlled by the rainy season. Towards the margins of the zone, with increasing annual ranges of temperature, seasons in the extra-tropical sense gradually appear.

_Physiological Effects of Heat and Humidity._ Tropical monotony of heat is associated with high relative humidity, except over deserts and in dry seasons. 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 difficult, 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 temperature 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 borne. 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 variation of the barometer is so regular and so marked that, as von Humboldt said, the time of day can be told within about fifteen minutes if the reading of the barometer be known. Even severe thunderstorms do not overcome the regular diurnal march of the pressure, 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 deficient precipitation. Along the barometric equator, where the pressure gradients are weakest, is the equatorial belt of calms, variable winds and rains—the doldrums. 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 actually 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 unlike those which exist during certain spells of summer weather in the north temperate zone. There are the dense tropical forests of the Amazon and of equatorial 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 somewhat narrower than the belt of rains, the warm ascending air being carried north and south, and giving precipitation beyond the limits of the calm zone. In striking contrast are the easterly trade winds, blowing between the tropical high pressure belts 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 steadiness 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 desirable 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 southern margins shift north and south after the sun; at certain seasons they are interrupted, often over wide areas near their equatorward margins, by the migrating belt of equatorial 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 naturally 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 belts are in the form of passing showers.

The monsoons on the southern 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 northern 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 north-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, as a general rule, soon after the “vertical sun,”[14] 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 accompanying data, are fairly typical.[15]

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

[15] Seven year record; expressed in thousandths of the daily mean.

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 8-10 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 mornings 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 intensity of tropical rains averages higher than in middle latitudes, but the difference is not great; (2) the heaviest short downpours have, so far as observation now goes, occurred in middle latitudes; (3) 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 rainfalls have been noted outside the tropics, as at Cherrapunji, 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 important 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 desirable locations, where the sea breeze has a free entrance, 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 frequent in the humid portions of the tropics. They have a marked diurnal periodicity; find their best opportunity in the equatorial belt of weak pressure 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 their 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 discharges 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 decomposition. In higher latitudes, where the ground may be frozen part of the year, and where the decomposing action of rain water is less, there is less of this effect. In northern 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.[16] The equatorial cloud belt moves north and south after the sun. Wholly clear days are very rare in the tropics generally, especially near the equator, and during the rainy season heavy clouds usually cover the sky.

[16] Supan, _Grundzüge der Physischen Erdkunde_, 3d ed., 1903, Fig. 13, page 53, gives a diagram showing the distribution of rainfall and cloudiness (also of other elements) according to latitude.

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 thunderstorms—these usually characterise the rainy season. Skies clear, or flecked with scattered small cumuli, are typical of the dry season. Wholly overcast, 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 intensity 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, whitish appearance. In the heart of the trade wind belts 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 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 Subdivisions._ 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 belts; III. The monsoon belts. 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 subdivision should be added to include 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 interior of Africa; the curve for Batavia illustrates insular conditions with less range; and the oceanic type, 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 double maximum is not a universal phenomenon, there being many cases where but a single maximum occurs, as will be seen later.

TABLE OF MEAN MONTHLY TEMPERATURES FOR SELECTED TROPICAL STATIONS[17] ---------++-----------------------------------+ || I. Equatorial Type | ---------++-----------+-----------+-----------+ ||Continental| Insular | Marine | ---------++-----------+-----------+-----------+ || Africa | Batavia | Jaluit, | || interior | | Marshall | || | | Islands | ---------++-----------+-----------+-----------+ Lat. || 8.1° N. | 6° 11′ S.| 5° 55′ N.| Long. || 23.6° E. |106° 50′ E.|169° 40′ E.| Altitude || 1837 ft. | 23 ft. | 10 ft. | Jan. || 73.4° | 77.5° | 80.8° | Feb. || 77.2° | 77.7° | 81.0° | Mar. || 83.8° | 78.4° | 80.6° | April || 85.3° | 79.3° | 80.4° | May || 83.7° | 79.5° | 80.4° | June || 81.5° | 78.8° | 80.2° | July || 78.4° | 78.3° | 80.2° | Aug. || 75.7° | 78.6° | 80.4° | Sept. || 77.7° | 79.3° | 80.4° | Oct. || 78.1° | 79.5° | 80.8° | Nov. || 75.7° | 79.0° | 80.8° | Dec. || 72.9° | 78.1° | 80.6° | Mean || 78.6° | 78.8° | 80.6° | Range || 12.4° | 2° | 0.8° | ---------++-----------+-----------+-----------+

---------++--------------------------------------------------------- || II. Tropical Type ---------++----------+-----------+-----------+-----------+---------- || Continental | Monsoon | Insular ---------++----------+-----------+-----------+-----------+---------- || Wadi | Alice | Nagpur | Honolulu |Jamestown || Halfa | Springs | | | || | | | | ---------++----------+-----------+-----------+-----------+---------- Lat. ||21° 53′ N.| 23° 38′ S.| 21° 9′ N. | 21° 18′ N.|15° 55′ S. Long. ||31° 20′ E.|133° 37′ E.| 79° 11′ E.|157° 50′ E.| 5° 43′ W. Altitude || 426 ft. | 1926 ft. | 1093 ft. | 49 ft. | 39 ft. Jan. || 61.3° | 85.6° | 68.2° | 70.0° | 74.7° Feb. || 66.6° | 83.3° | 73.8° | 70.3° | 75.9° Mar. || 73.0° | 77.9° | 83.7° | 70.9° | 73.6° April || 81.0° | 68.5° | 90.3° | 72.9° | 75.0° May || 87.1° | 60.6° | 94.3° | 74.3° | 68.9° June || 91.4° | 54.0° | 85.6° | 76.1° | 70.5° July || 93.4° | 51.8° | 80.1° | 77.2° | 71.8° Aug. || 91.6° | 59.4° | 80.2° | 77.5° | 69.4° Sept. || 87.1° | 66.6° | 80.4° | 77.2° | 67.6° Oct. || 83.1° | 73.4° | 78.6° | 76.5° | 65.7° Nov. || 71.4° | 79.7° | 72.3° | 73.8° | 67.8° Dec. || 64.8° | 82.8° | 66.7° | 71.4° | 71.8° Mean || 79.3° | 70.3° | 79.5° | 73.9° | 71.1° Range || 32.1° | 33.8° | 27.6° | 7.5° | 10.2° ---------++----------+-----------+-----------+-----------+----------

[17] Given to nearest tenth of a degree Fahr.

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

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 the trades blow. These conditions prevail on the equator, and as far north and south of the equator (about 10°-12°) as sufficient time elapses between the two zenithal positions of the sun for the two rainy seasons to be distinguished from one another. In this belt, under normal conditions, there is, therefore, no dry season of any considerable duration. The double rainy season is clearly seen in equatorial Africa and in parts of equatorial South America. The maxima lag somewhat behind the vertical sun, coming in April and November, and are unsymmetrically developed, the first maximum being the principal one. The minima are also unsymmetrically developed, 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 western portion of the Malay Archipelago 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 régime of rainfall, there being precipitation at all seasons.

S. A: South Africa. Q: Quito. S. P: Sâo Paulo. M: Mexico. H: Hilo. P. D: Port Darwin.]

This rainfall type with double 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 double rainy and dry seasons are easily modified by other conditions, as by the monsoons of the Indo-Australian 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 cloudiness is high throughout the year, averaging .7 to .8, with a relatively small annual period. The data and curve following are fairly typical, but the annual period varies greatly under local controls. (Fig. 21).

TABLE SHOWING MONTHLY DISTRIBUTION OF RAINFALL FOR SELECTED TROPICAL STATIONS[18]

--------+----------------------------------------------------- | Tropics +----------------+------------------------------------ | Double Rainy | Single Rainy Season | Season +-----------------+--------+--------- | Equatorial |Margin of Tropics| Trade | Monsoon | +--------+--------+ Rains | Rains | |Southern|Northern| | +--------+-------+--------+--------+--------+--------- |Southern| Quito | Sâo | Mexico | Hilo | Port | Africa | | Paulo | | | 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 | 195 | 165 | 58 | 26 | 94 | 61 May | 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 --------+--------+-------+--------+--------+--------+---------

[18] The figures in this table are thousandths of the mean annual rainfall. In the first column of the table, the average of a considerable number of stations is given.

TABLE SHOWING MONTHLY DISTRIBUTION OF CLOUDINESS IN AN EQUATORIAL 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 year within 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 winter. This is the so-called _tropical_ type of rainfall,[19] and is found where the trade belts are encroached upon by the equatorial rains during the migration of these rains into each hemisphere. It is illustrated in the data and curves for Sâo Paulo, Brazil, and for the city of Mexico (see rainfall table above and Fig. 20). The mean annual rainfall at Sâo Paulo is 54.13 inches, and at Mexico 22.99 inches.

[19] Supan calls it the _marginal_ type of the tropics.

E: Equatorial type. M: Monsoon type]

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 seasons is strongly contrasted. The single dry season lasts longer than either dry season in the equatorial 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 vegetation grows actively, when the doldrum belt is north of the equator (May-August). But when the trades blow (December-March), the ground is parched and dusty. The Venezuelan llanos have a dry season in the northern 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 mountains 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 infrequently complicated by association with trade or monsoon rains, as in the West Indies, Central America, 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.

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

The so-called tropical type of temperature variation, 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 Upper 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 Belts._ The trade belts near sea-level are characterised by fair weather, steady winds, infrequent light rains or even an almost complete 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 belts 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 30°), are most of the great deserts of the 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 latter regions, the desert conditions are further helped by the presence of cold ocean currents offshore. Because 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 interiors, than over the oceans of the trade wind belts. Minima of 32° or less occur during clear, quiet nights, and daily ranges of over 50° are common. The midsummer 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, accentuated 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 temperature ranges in tropical deserts than to the smaller ones in the equatorial belt, because of the lower relative humidity in the former case. In the trade wind deserts, as in other arid regions, man is nomadic.

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 become 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 belts 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, Borneo, Ceylon, Madagascar, Teneriffe, etc., show marked divergences of this sort. The eastern coasts of Guiana, Central America, south-eastern Brazil, south-eastern Africa, and eastern Australia are well watered, while the interiors are very dry in the two last-named countries. 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 arid portion 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 abnormally cool. Highlands in the trade belts are therefore 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 occupation, left to the natives, while the centre of civilisation was naturally established on the more open and sunny south-western side.

The rainfall associated with the conditions just described is known as the _trade_ type. These rains have a maximum in winter, when the trades are most active, 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 mountains or bold coasts, as on the Atlantic coast of Central 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.

Many combinations of different rainfall types are found in the trade wind latitudes. Zanzibar, for example (lat. 6° 12′ S., long. 39° 19′ E.), has two _equatorial_ rainfall maxima (April-May; November), which come soon after the vertical sun, but are associated with monsoon winds. 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 scattering showers. In the Malay archipelago, there are complications of equatorial and trade rains; likewise in the West Indies. Trade rains often have a tendency to give precipitation both day and night, while torrid zone rains generally prefer the day.

III. _Monsoon Belts._ In a typical monsoon region, such as that of India, eastern Asia, and the adjacent 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 monsoon 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 consequent 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 southern base of the Himalayas (60 to 160 inches); on the bold western coast of the peninsula (80 to 120 inches and 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 distributed, 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 district is very large, including the Indian Ocean, Arabian Sea, Bay of Bengal, and adjoining continental areas; the Pacific coast of China, the Yellow and Japan seas, and numerous islands from Borneo 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 northern provinces, where moderate cyclonic storms occasionally 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 thunderstorms. 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.

TABLE SHOWING MONTHLY DISTRIBUTION OF CLOUDINESS IN A MONSOON CLIMATE (BENGAL, LAT. 23.5° N.).[20]

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

[20] Five stations.

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 under the influence of this north-west monsoon. The essential features of the whole Indo-Australian monsoon 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 similar 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 double 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 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 seasons (_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 vertical 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 monsoon maximum in January, two months before 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 summer 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 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 maximum 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, altitude is chiefly important because of its effect in tempering 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 summits. 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 stations 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.” Thus, on the interior plateau of the tropical Cordilleras of South America, and on the central plateau of tropical Africa, the heat is tempered by 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.