AFRICA - CLIMATE Extending from latitude 37° N. to 35° S., the greater part of Africa lies within the tropics, and except on mountains and plateaux the average annual temperature is high, ranging from 62° at Cape Town to 86° at Massaua on the Red Sea. On the other hand there are great extremes of rainfall; in the Egyptian desert appreciable falls occur only once in a number of years, while the western slopes of the Cameroon mountains are among the wettest parts of the world, Debundscha at the foot of Camer oon Peak having an average of 369in. a year. Africa may be con veniently divided into five climatic divisions, of which four run roughly parallel with the lines of latitude, while the fifth occupies a large part of the eastern coastal districts. On either side of the equator is a belt of heavy rainfall which is distributed fairly uni formly throughout the year, but with a tendency for April and October to be the wettest months, those following the annual passage of the sun across the equator. This belt roughly coincides with the Congo valley and Gabon. Northwards the first rainy season is retarded into May and June, while the second rainy sea son is advanced into September and August, giving a long dry season, a principal wet season, a short dry season and a second ary wet season which is not much inferior in intensity to the first. In latitude 8° or 9° N. these two rainy seasons coalesce into one wet season lasting from May or June until September, while the rest of the year is dry; at the same time the total rainfall de creases. North of 15° N. the rainfall is small and irregular and we pass into the belt of desert which includes the Sahara and Sudan. This extends to the Mediterranean in the north-east, but in the north-west there is a coastal strip in Algeria and Morocco which has a long dry summer but an appreciable winter rainfall and forms part of the Mediterranean climatic province.
In South Africa there is a similar succession, the April rainfall maximum of the equatorial belt advancing into March and Feb ruary, while the October maxi mum is retarded into November and December, until the two co alesce into the summer rainfall region of the southern hemi sphere, which includes Angola and southern Rhodesia. South of this is the Kalahari desert, the coun terpart of the Sahara, while in the extreme south-west is a small area round Cape Town with a dry summer but appreciable rainfall in winter (April to September).
This zonal division does not apply to eastern Africa south of the equator, which has summer mon soon rains and a winter dry season over its whole extent.
Belt.—This region includes the greater part of the Congo basin above Leopoldville, Gabon, Cameroon and the coast of Nigeria as far as the mouth of the Niger. Its characteristics are uniformly high temperature and humidity, a large amount of cloud and a rainfall generally exceed ing 6oin. a year and rarely falling below 2in. in any month. The temperature is rarely extreme, and probably many parts of the region have never reached the Greenwich maximum of i oo°, but on the other hand the temperature rarely falls below 6o° on the low ground. The annual average may be taken as 80°, rising to 87° during the day and falling to 73° at night. The hottest months generally fall in the early part of the year, while July and August tend to be coolest, but the annual range is rarely as much as o°. The relative humidity averages 8o%, and this continual moist heat is very enervating to Europeans. The greater part of the region is densely forested.
North and south of the equatorial rain forest is a zone of moderate rainfall, the whole of which falls in a short summer rainy season, while the remainder of the year is dry. These belts may be regarded as extending as far as the isohyet of 20 inches. The northern belt includes Senegal, Bathurst, Sierra Leone, the Gold Coast, Dahomey, the greater part of Ni geria, the Sudan and Abyssinia. The rainfall varies greatly, rising to as much as 154in. a year at Sierra Leone, but even there the months of December to March are almost rainless. The drier parts are occupied by open grass-land but the wetter areas are forested. The rainy season from May or June to September in clusive, in spite of being summer in the astronomical sense, is the coolest part of the year, the temperature being kept down `by the great cloudiness and heavy rainfall. This season is unpleasant because of the high humidity, the small daily range of Tempera ture and the frequent thunderstorms and tornadoes, especially at the beginning and end of the season. The prevailing wind is south-westerly at the surface, but is very light; at a height of a few thousand feet the wind becomes north-easterly, and the thunderstorms advance from the north-east. From late October until the end of February is the most pleasant season, with hot days but cool nights, dry air and clear skies ; the prevailing wind is north-easterly except on the coast, where sea-breezes occur during the day. Over the interior the wind is often laden with penetrating fine dust (Harmattan wind). In March, April and the beginning of May, the temperature is very high, maxima some times exceeding Ioo° even on the coast, and reaching iio° in the interior. The humidity of the air is still low but is rising steadily, and the wind is light and variable. The rainfall of the Abyssinian plateau is of great importance as the source of the annual Nile floods, and so of the fertility of Egypt. The annual rainfall of Addis Ababa averages 5oin., but more than three-fifths of , this amount falls in July, August and September, while October to January inclusive are almost rainless. In spite of the position of Abyssinia near the eastern margin of Africa, it is believed that most of the moisture which falls as rain is derived from the Atlantic ocean, and is carried across the whole width of the con tinent by the south-west winds, and that only a comparatively small part of the rainfall is derived from the Indian Ocean. The southern savanna belt includes Angola and Rhodesia ; the cli mate is generally similar to that of the northern belt, but the summer rainy season occurs from December to February. The rainfall nowhere exceeds 6oin. and owing to the generally high elevation the climate is comparatively pleasant and healthy. The amount of cloud is comparatively small, and the lowest tempera tures occur in winter and not, as in the north, in the rainy season.
The greater part of Africa north of 16° N. is occu pied by an arid climate which gives rise to the deserts of the Sahara and Egypt. The rainfall is less than loin. a year, and over a large part of the area is probably less than an inch, being con fined to a few drops once or twice a year and isolated heavy falls at intervals of many years ; under these conditions the term "average" has no meaning. The temperature is very high in sum mer, the average in July exceeding 9o° over a large part of the interior; it remains very hot from April to September inclusive, but the winter months are relatively cool. The daily range of temperature is very great, exceeding 3o° at many places; for ex ample at El Obeid in the Anglo-Egyptian Sudan in January the mean daily maximum is 86° and the mean daily minimum 52°, and frosts are occasionally experienced. Very high maxima are recorded in summer, exceeding 11o° on the coast and 12o° in the interior, while at Azizia, in Tripoli, a shade temperature of 136° has been recorded, the highest known on the earth's surface. The air over the interior is dry, less than 4o% average humidity for the year, and figures of 2 or 3% have been recorded. On the coast special conditions prevail, which will be referred to later. The amount of cloud is very small, generally less than one-fifth of the sky covered, while in parts of the Egyptian desert no cloud is seen for months together. The winds are generally light and irregular over the Sahara, but dust storms, sand pillars and other local violent winds occur. In the Nile valley the prevailing wind is southerly from November to February and northerly from April to September and reaches considerable strength in the midsummer months, greatly moderating the temperature ; occasional very hot, dry, dust-laden southerly winds are known as the Khamsin ; these are very unpleasant, especially when accompanied by sand storms. Similar winds from the desert occur also under local names in Tripoli, Tunis and Algeria ; they frequently cross the Medi terranean to southern Italy as the scirocco. A severe form is the dreaded simoom or samun, a hot blast of air (125° or more), often accompanied by heavy clouds of dust or sand. Land and sea breezes are well developed on the coast of Tripoli, Tunis and Algeria, but not of Egypt. The corresponding desert of southern Africa, the Kalahari, is of much less extent, being limited to the western half of the continent, and the heat and dryness are not so excessive as in the Sahara.
The climate of the coast of the Red Sea and Gulf of Aden is peculiar. The rainfall is exceedingly scanty, the sky almost cloud less, and the temperature very high, but this is associated with an abnormally high humidity, the moisture being derived from the warm waters of the Red Sea, which reach
The highest known temperature of the wet-bulb thermometer, oo°, has occurred at Kamaran island and Berbera ; this moist heat is very unpleasant. Where the Sahara and Kalahari meet the Atlantic, different conditions occur ; here the presence of cold currents makes the surface of the sea very cold, while the land a few miles inland is very hot; these regions have a very small rainfall, clear skies but frequent fogs over the sea, and a moderate temperature which changes very little throughout the year. The rainfall of the South African west coast from 17 to 28° S. is less than an inch a year.
Morocco, northern Algeria and northern Tunis have pleasantly mild, rather rainy winters and long, hot, dry, almost cloudless summers. The annual rainfall averages about 3oin., but is very variable. The mean temperature is between 50° and 6o° in January and about 8o° in July and August ; the daily range is about 15°. A similar climate is found over a small area near Cape Town, which has a temperature rang ing from 55° in July to 7o° in January and February, and a rain fall of 25in. falling mainly from May to September inclusive.
Kenya Colony, Tanganyika, Uganda, Nyasa land and Portuguese East Africa have an equable climate with a temperature between 75° and 80° near sea-level and a rainfall of 3o to 6o inches. There is generally a fairly well-marked rainy sea son; near the equator this occurs in April and May with a minor rainy season in November and December, but further south there is only one rainy season, falling in summer. Thus at Zanzibar the rainiest months are April, May and November, in that order, and at Nairobi, April, November and May, but at Lourenco Marques in 26° S., January and February. The dry season or seasons are not so extreme as in West Africa; months entirely without rain are rare and the relative humidity remains fairly high throughout the year. The prevailing winds are easterly. Thunderstorms are very frequent during the rainy season, re curring with great regularity almost every afternoon. The low lying coastal areas are enervating and in places unhealthy, but large parts of the interior, including the whole of Uganda, form plateaux with an elevation of several thousand feet, and as the average temperature decreases upwards at a rate of about 3° per I,000ft., these uplands are much cooler and more bracing than the coastal regions, while their distance from the sea makes them somewhat drier except on slopes directly exposed to the rain-bear ing easterly winds. These plateaux are rather more suitable for European habitation than the coastal areas. Natal and the eastern parts of the Cape Province have warm, rainy summers, with a tem perature of 7o° to 75°, and moderately cool winters, relatively dry but very seldom rainless. The annual rainfall is between 3o and 4oin. ; the average July temperature is 63° at Durban and 59° at East London, but over the high ground of the interior frosts are sometimes experienced in winter. This is one of the finest and healthiest climates in Africa. (C. E. P. B.)
