CLIMATE. Of all the great land divisions of the globe, Africa is characterized by the greatest uniformity of climate. It stretches into both the north temperate and south temperate zones, but the greater part of its area is included within the tropics; there is consequently a suc cessive decrease of average annual heat north ward and southward of the equatorial belt. hut the Hgularity of the decrease is modified by cer tain other factors, so that the region of greatest average heat for the year is located not at the equator hut considerably north of it. between the parallels of 10° and These modifying factors are mainly the direction of the winds and the distribution of the mountains. It is, of course, cooler here in certain seasons than in others; but theaverage• temperature of any given season shows little fluctuation. In summer the iso therm of SO° F. ineloses the whole of the Sahara Desert, and over a considerable portion of this area the average summer temperature is 9i° or more. This region of extreme heat, which is the largest in the world, may be delimited by a line drawn from Khartum west to Timbuktu, thence north to El-Golea in the Algerian Sahara, thence southeast to Mnrzuk and thence to Berber on the Nile. The mountain regions of Algeria and Morocco, and parts of British South Africa and of German South-West Africa have a sub tropical or temperate climate. Throughout a large portion of Africa, especially in the moun tains of the east, and in the Sahara and Kalahari deserts, the temperature varies widely between summer and winter and between day and night, as is characteristic of all desert regions. (See DESERT.) In the Kalahari Desert the extreme seasonal fluctuation reaches 113°, and in the Sahara Desert the temperature during the night often approaches the freezing point. In gen eral, the western coast of Africa is cooler than the eastern coast, owing to the conditions here tofore stated. and to the influence of the drift northward along that coast (south of the equator) of the cool water from the Ant arctic Ocean. (See article on CLIMATE.) WiM18.—Trade winds are characteristic of near ly the whole continent. The Sahara Desert is a region of high barometric pressure during the winter months, thus causing outward blow ing winds, while in the summer season the pressure is lowered, and there is an ind•aught from the surrounding territory. In the western part of the Sahara Desert and Sudan, north and northeast winds prevail during the greater part of the year, alternal ing with northwest and west winds for a few months in winter. The eastern Sahara region and Egypt have prevailing north and northeast winds. A devastating wind called the "khanisin" blows from the southeast across this region at times, carrying dust and sand and causing sudden rises of temperature. A
similar dust wind, but usually cooler. blows from the interior of the Sahara over Senegambia and Upper Guinea, and is called the "harmattan." During the summer, in the lower limb of Africa, an area of low pressure occurs in the interior, and the prevailing winds are from the east and southeast, on the eastern border. and south and soathwest on the Western. In winter there is a shorter period in which the winds blow out wardly. (See article on WIxo.) The principal factors governing rainfall are evaporation. of winds. and distribution of mountains. A combination of these factors most favorable to a large rainfall is found on the west coast of Africa near the equator. Ilere the humid atmosphere from the Atlantic is car ried landward by the winds and, becoming cooler, deposits the greater part of its moisture before passing the highland region. The maximum limit of precipitation is probably attained in where the total rainfall in the year may exceed 350 inches. while the Niger Delta and the coasts of Sierra Leone and Liberia also arc excessively humid. On the east equatorial coast the winds from the Indian Ocean deliver considerable moisture, hut not in such abun dance as on the west coast. As they pass into the interior, the winds from both the Atlantic and Indian oceans are deprived of their humid ity, especially in the mountains. which act as precipitating agents. Equatorial Africa. is a whole, is thus characterized by a heavy rainfall. North and south of this region, however, the conditions exhibit a striking contrast. In the north is the Sahara Desert, the largest arid region in the world, where the prevailing winds are from the northeast and are hot and dry, while the humidity of the southerly winds that may penetrate into the interior is diminished by the heat, and seldom falls as rain. A second arid region, the Kalahari Desert, is found in the southern limb of the continent, between the Zam bezi and Orange rivers and the eastern and western coastal highlands. It has a small spas modic rainfall, which is usually insufficient to support a constant growth of vegetation. The Alcditerranean coast. region and the extreme southern extension have a dry climate that is tempered by rains during certain seasons. Be sides the continental of rainfall, there is a seasonal variation in the amount re ceived in different latitudes. In the regions near the equator rain may fall during every month of the year, but the periods of greatest precipitation occur when the sun is nearly verti cal, in spring and fall. Away from the equator there is generally but one wet season. See arti cles on countries of Africa.