Chapter III: Part 3

Owing to the distance from the chief source of supply of water vapour--the oceans--the air over the larger land areas is naturally drier and dustier than that over the oceans. Yet even in the arid continental interiors in summer the absolute vapour content is surprisingly large, and in the hottest months the percentages of relative humidity may reach 20% or 30%. At the low temperatures which prevail in the winter of the higher latitudes the absolute humidity is very low, but, owing to the cold, the air is often damp. Cloudiness, as a rule, decreases inland, and with this lower relative humidity, more abundant sunshine and higher temperature, the evaporating power of a continental climate is much greater than that of the more humid, cloudier and cooler marine climate. Both amount and frequency of rainfall, as a rule, decrease inland, but the conditions are very largely controlled by local topography and by the prevailing winds. Winds average somewhat lower in velocity, and calms are more frequent, over continents than over oceans. The seasonal changes of pressure over the former give rise to systems of inflowing and outflowing, so-called continental, winds, sometimes so well developed as to become true monsoons. The extreme termperature changes which occur over the continents are the more easily borne because of the dryness of the air; because the minimum temperatures of winter occur when there is little or no wind, and because during the warmer hours of the summer there is the most air-movement.

M, Madeira.
Bd, Bagdad.
V, Valentia.
N, Nerchinsk.]

_Desert Climate._--An extreme type of continental climate is found in deserts. Desert air is notably free from micro-organisms. The large diurnal temperature ranges of inland regions, which are most marked where there is little or no vegetation, give rise to active convectional currents during the warmer hours of the day. Hence high winds are common by day, while the nights are apt to be calm and relatively cool. Travelling by day is unpleasant under such conditions. Diurnal cumulus clouds, often absent because of the excessive dryness of the air, are replaced by clouds of blowing dust and sand. Many geological phenomena, and special physiographic types of varied kinds, are associated with the peculiar conditions of desert climate. The excessive diurnal ranges of temperature cause rocks to split and break up. Wind-driven sand erodes and polishes the rocks. When the separate fragments become small enough they, in their turn, are transported by the winds and further eroded by friction during their journey. Curious conditions of drainage result from the deficiency in rainfall. Rivers "wither" away, or end in sinks or brackish lakes.

Desert plants protect themselves against the attacks of animals by means of thorns, and against evaporation by means of hard surfaces and by a diminished leaf surface. The life of man in the desert is likewise strikingly controlled by the climatic peculiarities of strong sunshine, of heat, and of dust.

TEMPERATURE JANUARY
TEMPERATURE JULY
PRESSURE & WINDS JANUARY
PRESSURE & WINDS JULY]

_Coast or Littoral Climate._--Between the pure marine and the pure continental types the coasts furnish almost every grade of transition. Prevailing winds are here important controls. When these blow from the ocean, the climates are marine in character, but when they are off-shore, a somewhat modified type of continental climate prevails, even up to the immediate sea-coast. Hence the former have a smaller range of temperature; their summers are more moderate and their winters milder; extreme temperatures are rare; the air is damp, and there is much cloud. All these marine features diminish with increasing distance from the ocean, especially when there are mountain ranges near the coast. In the tropics, windward coasts are usually well supplied with rainfall, and the temperatures are modified by sea breezes. Leeward coasts in the trade-wind belts offer special conditions. Here the deserts often reach the sea, as on the western coasts of South America, Africa and Australia. Cold ocean currents, with prevailing winds along-shore rather than on-shore, are here hostile to rainfall, although the lower air is often damp, and fog and cloud are not uncommon.

_Monsoon Climate._--Exceptions to the general rule of rainier eastern coasts in trade-wind latitudes are found in the monsoon regions, as in India, for example, where the western coast of the peninsula is abundantly watered by the wet south-west monsoon. As monsoons often sweep over large districts, not only coast but interior, a separate group of monsoon climates is desirable. In India there are really three seasons--one cold, during the winter monsoon; one hot, in the transition season; and one wet, during the summer monsoon. Little precipitation occurs in winter, and that chiefly in the northern provinces. In low latitudes, monsoon and non-monsoon climates differ but little, for summer monsoons and regular trade-winds may both give rains, and wind direction has slight effect upon temperature.

The winter monsoon is off-shore and the summer monsoon on-shore under typical conditions, as in India. But exceptional cases are found where the opposite is true. In higher latitudes the seasonal changes of the winds, although not truly monsoonal, involve differences in temperature and in other climatic elements. The only well-developed monsoons on the coast of the continents of higher latitudes are those of eastern Asia. These are off-shore during the winter, giving dry, clear and cold weather; while the on-shore movement in summer gives cool, damp and cloudy weather.

_Mountain and Plateau Climate._--Both by reason of their actual height and because of their obstructive effects, mountains influence climate similarly in all the zones. Mountains as contrasted with lowlands are characterized by a decrease in pressure, temperature and absolute humidity; an increased intensity of insolation and radiation; usually a greater frequency of, and up to a certain altitude more, precipitation. At an altitude of 16,000 ft., more or less, pressure is reduced to about one-half of its sea-level value. The highest human habitations are found under these conditions. On high mountains and plateaus the pressure is lower in winter than in summer, owing to the fact that the atmosphere is compressed to lower levels in the winter and is expanded upwards in summer.

The intensity of insolation and of radiation both increase aloft in the cleaner, purer, drier and thinner air of mountain climates. The great intensity of the sun's rays attracts the attention of mountain-climbers at great altitudes. The vertical decrease of temperature, which is also much affected by local conditions, is especially rapid during the warmer months and hours; mountains are then cooler than lowlands. The inversions of temperature characteristic of the colder months, and of the night, give mountains the advantage of a higher temperature then--a fact of importance in connexion with the use of mountains as winter resorts. At such times the cold air flows down the mountain sides and collects in the valleys below, being replaced by warmer air aloft. Hence diurnal and annual ranges of temperature on the mountain tops of middle and higher latitudes are lessened, and the climate in this respect resembles a marine condition. The times of occurrence of the maximum and minimum temperature are also much influenced by local conditions. Elevated enclosed valleys, with strong sunshine, often resemble continental conditions of large temperature range, and plateaus, as compared with mountains at the same altitude, have relatively higher temperatures and larger temperature ranges. Altitude tempers the heat of the low latitudes. High mountain peaks, even on the equator, can remain snow-covered all the year round.

No general law governs the variations of relative humidity with altitude, but on the mountains of Europe the winter is the driest season, and the summer the dampest. At well-exposed stations there is a rapid increase in the vapour content soon after noon, especially in summer. The same is true of cloudiness, which is often greater on mountains than at lower levels, and is usually at a maximum in summer, while the opposite is true of the lowlands in the temperate latitudes. One of the great advantages of the higher Alpine valleys in winter is their small amount of cloud. This, combined with their low wind velocity and strong insolation, makes them desirable winter health resorts. Latitude, altitude, topography and winds are the determining factors in controlling the cloudiness on mountains. In the rare, often dry, air of mountains and plateaus evaporation is rapid, the skin dries and cracks, and thirst is increased.

Rainfall usually increases with increasing altitude up to a certain point, beyond which, owing to the loss of water vapour, this increase stops. The zone of maximum rainfall averages about 6000 to 7000 ft. in altitude, more or less, in intermediate latitudes, being lower in winter and higher in summer. Mountains usually have a rainy and a drier side; the contrast between the two is greatest when a prevailing damp wind crosses the mountain, or when one slope faces seaward and the other landward. Mountains often provoke rainfall, and local "islands," or better, "lakes," of heavier precipitation result.

Mountains resemble marine climates in having higher wind velocities than continental lowlands. Mountain summits have a nocturnal maximum of wind velocity, while plateaus usually have a diurnal maximum. Mountains both modify the general, and give rise to local winds. Among the latter the well-known mountain and valley winds are often of considerable hygienic importance in their control of the diurnal period of humidity, cloudiness and rainfall, the ascending wind of daytime tending to give clouds and rain aloft, while the opposite conditions prevail at night.

_Supan's Climatic Provinces._--The broad classification of climates into the three general groups of marine, continental and mountain, with the subordinate divisions of desert, littoral and monsoon, is convenient for purposes of summarizing the interaction 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 similar climates in different parts of the world are grouped together, and in which their geographic distribution receives particular consideration. An almost infinite number of classifications might be proposed; or we may take as the basis of subdivision either the special conditions of one climatic element, or similar conditions of a combination of two or more elements. Or we may take a botanical or a zoological basis. Of the various classifications which have been suggested, that of Supan gives a very rational, simple and satisfactory scheme of grouping. In this scheme there are thirty-five so-called climatic provinces.[3] It emphasizes the essentials of each climate, and serves to impress these essentials upon the mind by means of a compact, well-considered verbal summary in the case of each province described. Obviously, no classification of climates which is at all complete can approach the simplicity of the ordinary classification of the zones.

_The Characteristics of the Torrid Zone._

_General: Climate and Weather._--The dominant characteristic of the torrid zone is the simplicity and uniformity of its climatic features. The tropics lack the proverbial uncertainty and changeableness of the weather of higher latitudes. 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. 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. 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 mean temperature is high, and very uniform over the whole zone. There is little variation during the year. The mean annual isotherm of 68 deg. is a rational limit at the polar margins of the zone, and the mean annual isotherm of 80 deg. encloses the greater portion of the land areas, as well as much of the tropical oceans. The warmest latitude circle for the year is not the equator, but latitude 10 deg. N. The highest mean annual temperatures, shown by the isotherm of 85 deg., are in Central Africa, in India, the north of Australia and Central America, but, with the exception of the first, these areas are small. The temperatures average highest where there is little rain. In June, July and August there are large districts in the south of Asia and north of Africa with temperatures over 90 deg.

Over nearly all of the zone the mean annual range of temperature is less than 10 deg., and over much of it, especially on the oceans, it is less than 5 deg. Even near the margins of the zone the ranges are less than 25 deg., as at Calcutta, Hong-Kong, Rio de Janeiro and Khartum. The mean daily range is usually larger than the mean annual. It has been well said that "night is the winter of the tropics." 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 deg. Towards the margins of the zone, however, the minima on the continents fall to or even below 32 deg. Maxima of 115 deg. and even over 120 deg. occur over the deserts of northern Africa. A district where the mean maxima exceed 113 deg. 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 tropical oceans show remarkably small variations in temperature. The "Challenger" results on the equator showed a daily range of hardly 0.7 deg. in the surface water temperature, and P. G. Schott determined the annual range as 4.1 deg. on the equator, 4.3 deg. at latitude 10 deg., and 6.5 deg. at latitude 20 deg.

_The Seasons._--In a true tropical climate 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. Although the tropical rainy season is characteristically associated with a vertical sun, that season is not necessarily the hottest time of the year. It often goes by the name of winter for this reason. 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 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 from higher latitudes; the driest are the healthiest. The most energetic natives are the desert-dwellers. The monotonously enervating heat of the humid tropics makes man sensitive to slight temperature changes. The intensity of direct insolation, as well as of radiation from the earth's surface, may produce heat prostration and sunstroke. "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 twenty minutes if the reading of the barometer be known.

_Winds and Rainfall._--Along the barometric equator, where the pressure gradients are weakest, is the equatorial belt of calms, variable winds and rains--the doldrums. This belt offers exceptionally favourable conditions for abundant rainfall, and is one of the rainiest regions of the world, averaging probably about 100 in. Here the sky is prevailingly cloudy; the air is hot and oppressive; heavy showers and thunderstorms are frequent, chiefly in the afternoon and evening. Here are the dense tropical forests of the Amazon and of equatorial Africa. This belt of calms and rains shifts north and south of the equator after the sun. In striking contrast are the easterly trade winds, blowing between the tropical high pressure belts and the equatorial belt of low pressure. Of great regularity, and contributing largely to the uniformity of tropical climates, the trades have long been favourite sailing routes because of the steadiness of the wind, the infrequency of storms, the brightness of the skies and the freshness of the air. 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 on to windward coasts or over mountains, are drying winds. They cause the deserts of northern Africa and of the adjacent portions of Asia; of Australia, South Africa and southern South America. 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 deg. N. to and beyond the northern tropic, between Africa and the Philippines, giving rains over India, the East Indian archipelago and the eastern coasts of China. In winter, the north-east monsoon, the normal cold-season outflow from Asia combined with the north-east trade, and generally cool and dry, covers the same district, extending as far north as latitude 30 deg. 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.

Tropical rains are in the main summer rains, 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, expecially 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. Local influences are, however, very important, and in many places night rainfall maxima are found.

_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.

_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.

_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. 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. 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 on the coast of Peru and on parts of the west coast of Africa.

_Intensity of Sky-Light and Twilight._--The light from tropical skies by day is trying, and the intense insolation, together with the reflection from the ground, increases the general dazzling glare under a tropical sun. 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 of a deeper blue. Twilight within the tropics is shorter than in higher latitudes, but the coming on of night is less sudden than is generally assumed.

_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: (1) the equatorial belt; (2) the trade-wind belts; (3) the monsoon belts. In each of these subdivisions there are modifications due to marine and continental influences. In general, both seasonal and diurnal phenomena are more marked in continental interiors than on the oceans, islands and windward coasts. Further, the effect of altitude is so important that another group should be added to include (4) mountain climates.

1. _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 _equatorial_ type of annual march of temperature is illustrated in the three curves for the interior of Africa, Batavia and Jaluit (fig. 4). 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. This double maximum is not a universal phenomenon, there being many cases where but a single maximum occurs.

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

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 deg.-12 deg.) 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. This rainfall type with double maxima and minima has been called the _equatorial_ type, and is illustrated in the following curves for South Africa and Quito (fig. 5). The monthly rainfalls are given in thousandths of the annual mean. The mean annual rainfall at Quito is 42.12 in. These double rainy and dry seasons are easily modified by other conditions, as by the monsoons of the Indo-Australian area, so that 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 curve following, E (fig. 6), is fairly typical, but the annual period varies greatly under local controls.

At greater distances from the equator than about 10 deg. or 12 deg. 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, 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 curves for Sao Paulo, Brazil, and for the city of Mexico (fig. 5). The mean annual rainfall at Sao Paulo is 54.13 in. and at Mexico 22.99 in. 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.

The so-called _tropical_ type of temperature variation, with one maximum and one minimum, is illustrated in the accompanying curves for Wadi Halfa, in upper Egypt; Alice Springs, Australia; Nagpur, India; Honolulu, Hawaii; and Jamestown, St Helena (fig. 7). 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.

2. _Trade-Wind Belts._--The trade belts near sea-level are characterized 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. The climate of the ocean areas in the trade-wind belts is indeed the simplest and most equable in the world, the greatest extremes over these oceans being found to leeward of the larger lands. On the lowlands swept over by the trades, beyond the polar limits of the equatorial rain belt (roughly between lats. 20 deg. and 30 deg.), 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.

The ranges and extremes of temperature are much greater over the continental interiors than over the oceans of the trade-wind belts. Minima of 32 deg. or less occur during clear, quiet nights, and daily ranges of over 50 deg. are common. The midsummer mean temperature rises above 90 deg., with noon maxima of 110 deg. 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, 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.

While the trades are drying winds as long as they blow strongly over the oceans, or over lowlands, they readily become rainy if they are cooled by 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, &c., show marked differences 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 dry. 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 close to 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 near the equator to about lat. 30 deg. S. The cold ocean waters, with prevailing southerly (drying) winds alongshore, are additional factors causing this aridity. Highlands in the trade belts are therefore moist on their windward slopes, and become 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 side of Central America was from the earliest days of European occupation left to the natives, while the centre of civilization 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. 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 real dry season. The curve for Hilo (mean annual rainfall 145.24 in.) on the windward side of the Hawaiian Islands, shows typical conditions (see fig. 5). The trade type of rainfall is often much complicated by the combination with it of the _tropical_ type and of the _monsoon_ type. In the Malay archipelago there are also complications of equatorial and trade rains; likewise in the West Indies.

3. _Monsoon Belts._--In a typical monsoon region the rains follow the vertical sun, and therefore have a simple annual period much like that of the tropical type above described. This monsoon type of rainfall is well illustrated in the curve for Port Darwin (mean annual rainfall 62.72 in.), in Australia (see fig. 5). This summer monsoon rainfall results from the inflow of a body of warm, moist air from the sea upon a land area; there is 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. In India, the precipitation is heaviest at the head of the Bay of Bengal (where Cherrapunji, at the height of 4455 ft. in the Khasi Hills, has a mean annual rainfall of between 400 and 500 in.), along the southern base of the Himalayas (60 to 160 in.), on the bold western coast of the peninsula (80 to 120 in. and over), and on the mountains of Burma, (up to 160 in.). 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. The prevailing direction of the rainy monsoon wind in India is south-west; on the Pacific coast of Asia, it is 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. A typical temperature curve for a monsoon district is that for Nagpur, in the Indian Deccan (fig. 7), and a typical monsoon cloudiness curve is given in fig. 6, the maximum coming near the time of the vertical sun, in the rainy season, and the minimum in the dry season.

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 deg.-10 deg. S., the monsoon rains come with north-west winds in the period between November and March or April.

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 monsoon 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 Mountains. Goree, lat. 15 deg. N., on the coast of Senegambia, gives a fine example of a rainy (summer) and a dry (winter) monsoon. Numerous combinations of equatorial, trade and monsoon rainfalls are found, often creating great complexity. The islands of the East Indian archipelago furnish many examples of such curious complications.

4. _Mountain Climate._--In the torrid zone 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 all the year round, 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 healthy conditions for white settlement. They are thus often sought 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. 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 plateaus 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.

_The Characteristics of the Temperate Zones._

_General._--As a whole, the "temperate zones" are _temperate_ only in that their mean temperatures and their physiological effects are intermediate between those of the tropics and those of the polar zones. A marked changeableness of the weather is a striking characteristic of these zones. Apparently irregular and haphazard, these continual weather changes, although they are essentially non-periodic, nevertheless run through a fairly systematic series. 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 deg.. The ranges between the mean temperatures of hottest and coldest months reach 120 deg. at their maximum in north-eastern Siberia, and 80 deg. in North America. A January mean of -60 deg. and a July mean of 95 deg., and maxima of over 120 deg. and minima of -90 deg., occur in the same zone. Such great ranges characterize the extreme land climates. Under the influence of the oceans, the windward coasts have much smaller ranges. The annual ranges in middle and higher latitudes exceed the diurnal, the conditions of much of the torrid zone thus being exactly reversed. Over much of the oceans of the temperate zones the annual range is less than 10 deg.. In the south temperate zone there are no extreme ranges, the maxima, slightly over 30 deg., being near the margin of the zone in the interior of South America, South Africa and Australia. In these same localities the diurnal ranges rival those of the north temperate zone.

The north-eastern Atlantic and north-western Europe are about 35 deg. too warm for their latitude in January, while north-eastern Siberia is 30 deg. too cold. The lands north of Hudson Bay are 25 deg. too cold, and the waters of the Alaskan Bay 20 deg. too warm. In July, and in the southern hemisphere, the anomalies are small. The lands which are the centre of civilization in Europe average too warm for their latitudes. 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 character in different years. 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 dates of maximum and minimum insolation. 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.

_Pressure and Winds._--The prevailing winds are the "westerlies," which are much less regular than the trades. They vary greatly in velocity in different regions and in different seasons, and are stronger in winter than in summer. They are much interfered with, especially in the higher northern 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 summer. In summer, when the lands have low pressure, the northern oceans are dominated by great oval areas of high pressure, with outflowing spiral eddies, while in winter, when the northern lands have high pressure, the northern portions of the oceans develop cyclonic systems of inflowing winds over their warm waters. All these great continental and oceanic systems of spiralling winds are important climatic controls.

The westerlies are also much confused and interrupted by storms, whence their designation of _stormy westerlies_. So common are such interruptions that the prevailing westerly wind direction is often difficult to discern without careful observation. Cyclonic storms are most numerous and best developed in winter. Although greatly interfered with near sea-level by continental changes of pressure, by cyclonic and anticyclonic whirls, and by local inequalities of the surface, the eastward movement of the atmosphere remains very constant aloft. The south temperate zone being chiefly water, the westerlies are but little disturbed there by continental effects. Between latitudes 40 deg. and 60 deg. S. the "brave west winds" blow with a constancy and velocity found in the northern hemisphere only on the oceans, and then in a modified form. Storms, frequent and severe, characterize these southern 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 north of the equator.

_Rainfall._--Rainfall is fairly abundant over the oceans and also over a considerable part of the lands (30-80 in. and more). It comes chiefly in connexion with the usual cyclonic storms, or in thunderstorms. So great are the differences, geographic and periodic, in rainfall produced by differences in temperature, topography, cyclonic conditions, &c., that only 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 belts of light or moderate rainfall. Bold west coasts, on the polar side of lat. 40 deg., are very rainy (100 in. and more a year in the most favourable situations). The hearts of the continents, far from the sea, and especially when well enclosed by mountains, or when blown over by cool ocean winds which warm while crossing the land, have light rainfall (less than 10-20 in.). East coasts 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 most numerous and best developed, and the cold lands chill 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 continents. Conditions are then favourable for inflowing damp winds from the adjacent oceans; there is the best opportunity for convection; thunder-showers readily develop on the hot afternoons; the capacity of the air for water vapour is greatest. 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 belts. Rainfalls are heaviest along the tracks of most frequent cyclonic storms.

For continental stations the typical daily march of rainfall shows a chief maximum in the afternoon, and a secondary maximum in the night or early morning. The chief minimum comes between 10 A.M. and 2 P.M. Coast stations generally have a night maximum and a minimum between 10 A.M. and 4 P.M.

_Humidity and Cloudiness._--S.A. Arrhenius gives the mean cloudiness for different latitudes as follows:--

+-----------+----+
| 70 deg. N.| 59 |
| 60 deg. | 61 |
| 50 deg. | 48 |
| 40 deg. | 49 |
| 30 deg. | 42 |
| 20 deg. | 40 |
| 10 deg. | 50 |
| Eq. | 58 |
| 10 deg. | 57 |
| 20 deg. | 48 |
| 30 deg. | 46 |
| 40 deg. | 56 |
| 50 deg. | 66 |
| 60 deg. S.| 75 |
+-----------+----+

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

The absolute humidity as a whole decreases as the temperature falls. The relative humidity averages 90%, 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.

_Seasons._--Seasons in the temperate zones are classified according to temperature; not, as in the tropics, by rainfall. The four seasons are important characteristics, especially of the middle latitudes of the north temperate zone. Towards the equatorial margins of the zones the difference 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 disappear.

These seasonal changes are of the greatest importance in the life of man. The monotonous heat of the tropics and the continued cold of the polar zones are both depressing. Their tendency is to operate against man's highest development. The seasonal changes of the temperate zones stimulate man to activity. They develop him, physically and mentally. They encourage higher civilization. A cold, stormy winter necessitates forethought in the preparation during the summer of clothing, food and shelter. Development must result from such conditions. In the warm, moist tropics life is too easy; in the cold polar zones it is too hard. Near the poles, the growing season is too short; in the moist tropics it is so long that there is little inducement to labour at any special time. The regularity, and the need, of outdoor work during a part of the year are important factors in the development of man in the temperate zones.

_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 unknown. The changes are most rapid in the northern portion of the north temperate zone, especially on the continents, where the cyclones travel fastest. The nature of these changes depends on the degree of development, the velocity of progression, the track, and other conditions of the disturbance which produces them. The particular weather types resulting from this control give the climates their distinctive character.

The types vary with the season and with the geographical position. They result from a combination, more or less irregular, of periodic diurnal elements, under the regular control of the sun; and of non-periodic cyclonic and anticyclonic elements. 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 clouds; diurnal variation in wind velocity; afternoon thunderstorms, with considerable regularity, characterize the warmest months over the continents and present an analogy with tropical conditions. Cyclonic 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. In winter the non-periodic, cyclonic control is strongest. The irregular 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 fall are transition seasons, and have transition weather types. The south temperate zone oceans have a constancy of non-periodic cyclonic weather changes through the year which is only faintly imitated over the oceans of the northern hemisphere. Winter types differ little from summer. The diurnal control is never very strong. Stormy weather prevails throughout the year although the weather changes are more frequent and stronger in the colder months.

_Climatic Subdivisions._--There are fundamental differences between the north and south temperate zones. The latter zone is sufficiently individual to be given a place by itself. The marginal sub-tropical belts must also be considered as a separate 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. The simplest and most rational scheme for a classification of these climates is based on the fundamental differences 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._--Because of the large ocean surface, the whole meteorological regime in the south temperate zone is more uniform than in the northern. The south temperate zone may properly be called "temperate." Its temperature changes are small; its prevailing winds are stronger and steadier than in the northern hemisphere; its seasons are more uniform; its weather is prevailingly stormier, more changeable, and more under cyclonic control. The uniformity of the climatic conditions over the far southern oceans is monotonously unattractive. The continental areas are small, and develop 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, with its mild climate and fairly regular rains, is really at the margins of the zone, and has much more favourable conditions than the islands farther south. These islands, in the heart of this zone, have dull, cheerless and inhospitable climates. The zone enjoys a good reputation for healthfulness, which fact has been ascribed chiefly to the strong and active air movement, 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 climatic advantages, as will be seen.

_Sub-tropical Belts: Mediterranean Climates._--At the tropical margins of the temperate zones are the so-called sub-tropical belts. Their rainfall regime is alternately that of the westerlies and of the trades. They are thus associated, now with the temperate and now with the torrid zones. In winter the equatorward migration of the great pressure and wind systems brings these latitudes under the control of the westerlies, whose frequent irregular storms give a moderate winter precipitation. These winter rains are not steady and continuous, but are separated by spells of fine sunny weather. The amounts vary greatly.[4] In summer, when the trades are extended polewards by the outflowing equatorward winds on the eastern side of the ocean anticyclones, mild, dry and nearly continuous fair weather prevails, with general northerly winds.

The sub-tropical belts of winter rains and dry summers are not very clearly defined. They are mainly limited to the western coasts of the continents, and to the islands off these coasts in latitudes between about 28 deg. and 40 deg.. The sub-tropical belt is exceptionally wide in the old world, and reaches far inland there, embracing the countries bordering on the Mediterranean in southern Europe and northern Africa, and then extending eastward across the Dalmatian coast and the southern part of the Balkan peninsula into Syria, Mesopotamia, Arabia north of the tropic, Persia and the adjacent lands. The fact that the Mediterranean countries are so generally included has led to the use of the name "Mediterranean climate." 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 elevations, and the sub-tropical rains.

On the west coasts of the two Americas the sub-tropical belt of winter rains is clearly seen in California and in northern 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-western coastal belt has winter rains, decreasing to the north, while the east coast and adjoining interior have summer rains, from the south-east trade. Southern Australia is climatically similar to South Africa. In summer the trades give rainfall on the eastern coast, decreasing inland. In winter the westerlies give moderate rains, chiefly on the south-western coast.

The sub-tropical climates follow the tropical high pressure belts across the oceans, but they do not retain their distinctive character far inland from the west coasts of the continents (except in the Mediterranean case), nor on the east coasts. On the latter, summer monsoons and the occurrence of general summer rains interfere, as in eastern Asia and in Florida.

Strictly winter rains are typical 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. 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 latitudes, the summer becomes the season of maximum rainfall. The monthly distribution of rainfall in two sub-tropical regions is shown in the accompanying curves for Malta and for Western Australia (fig. 8). 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.

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 belts may bring them 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 may be expected towards the equatorial margins of the sub-tropical belts.

The main features of the sub-tropical rains east of the Atlantic are repeated on the Pacific coasts of the two Americas. In North America the rainfall decreases from Alaska, Washington and northern Oregon southwards to lower California, and the length of the summer dry season increases. At San Diego, six months (May-October) have each less than 5% of the annual precipitation, and four of these have 1%. The southern extremity of Chile, from about latitude 38 deg. S. southward, has heavy rainfall throughout the year from the westerlies, with a winter maximum. Northern Chile is persistently dry. Between these two there are winter rains and dry summers. Neither Africa nor Australia extends far enough south to show the different members of this system well. New Zealand is almost wholly in the prevailing westerly belt. Northern India is unique in having summer monsoon rains and also winter rains, the latter from weak cyclonic storms which correspond with the sub-tropical winter rains.