1902 Encyclopedia > Africa > African Climate. Climate - Africa. African Winds, Rainfall.

Africa
(Part 8)



(C) AFRICA - GEOGRAPHY (cont'd)

(g) African Climate. Africa - Climate.

Africa lies almost entirely in the torrid zone, and is the hottest continent of all. the greatest heat, however, is not found under the equator, since the whole of the central belt of the continent is protected by a dense covering of forest vegetation, supported by the heavy rainfall, and has in consequence a more equable climate, but in the dry, bare, exposed desert belts, which lie on the margins of the tropics, the Sahara in the north and the Kalahari in the south, where the climate is extreme. The highest temperature is found throughout the Sahara, particularly in its eastern portions towards the Red Sea. In Upper Egypt and Nubia eggs may be baked in the hit sands; and the saying of the Arabs is, "in Nubia the soil is like fire and the wind like a flame." The regions along the Mediterranean and Atlantic coasts are rendered more temperate by the influence of the sea. To the south of the Great Desert the temperature decreases, chiefly on account of the increasing moisture and protection of the land surface from extreme heating by its tree growth, but also because of the greater elevation of the land as the great southern plateau is approached. Both on account of its elevation and its narrower form, which gives greater access to the equalizing influence of the surrounding ocean, the southern half of the African continent has a less high temperature than the northern, though the same gradations of climate outward from the centre belt are clearly marked in each division. Regular snowfall does not occur even in the most southern or northern regions; and this phenomenon is only known in the most elevated points of the continent, as in the atlas Mountains in the north, the summits of which retain patches of snow even in summer, in the Abyssinian peaks, in the highest points of the mountains of the Cape Colony, and most remarkably in the lofty summits of Mounts Kenia and Kilima-njaro, which rise on the plateau beneath the equator. the intensity of radiation and its influence upon the temperature are very great in Northern Africa; while in the day time the soil of the Sahara rapidly absorbs the solar rays, during the night it cools so rapidly that the formation of ice has often been known to occur.

The observed average temperatures of the extreme months of the year at various points of Africa, from N. to S., are given in the following table:-

African Winds

Africa is not much under the influence of the regular winds, except the monsoons of the Indian Ocean, the great movement of the atmosphere depending chiefly on the oscillation of the continent beneath the sun during the sea sons, as will be afterwards explained. The wind currents over the whole continent have a prevailing direction from the east. There are the trade winds, modified by interruptions of changing heat and elevation of the land surface. In the northern part of the Indian Ocean the year is divided between the south-west monsoon, blowing from March till September, away from Africa, towards the then heated continent of Asia; and the north-east monsoon, or rather the normal trade wind, blowing towards and African coasts, from October till February. It will be seen in the next paragraph, that the monsoons, although they extend only to about a third portion of the East African shores, have an extremely important bearing upon the physical economy of the whole African continent. From hurricanes Africa is nearly exempt, except in its south-eastern extremity, to which at times the Mauritius hurricanes extend. At rare intervals these have visited the east coast as far as Zanzibar. Northern Africa is much exposed to the hot winds and storms from the Sahara, which are called in Egypt Khamsin, in the Mediterranean Scirocco, Shume or Asshume in Marocco, and Harmattan on the west coasts of the Sahara and in the countries bordering on the Gulf of Guinea. These always blow directly across the coast from the interior, and seem to move round the compass during the year, beginning in Egypt in April, in Algeria in July, in Marocco in August, in Senegambia in November. Similar dry electrical winds are experienced in the Kalahari desert in the south. Whirlwinds, frequently carrying sand up into the atmosphere, are of frequent occurrence in these deserts, and are also known in the dry region of Unyamuezi, between Zanzibar and the Tanganyika, and in the Limpopo basin farther south. Extreme heat and dryness are the characteristics of these winds, which, raising the sand, filling the air with dust, and prodigiously favouring the powers of evaporation, are often fatal to the vegetable and animal creation in the regions visited by them.

Africa - Rain. Africa - Rainfall. Rains Within the Tropics. Rains on the Coasts. Rains in the Interior.

In Africa the dependence of the winds and rains upon the movement of the land beneath the sun is more clearly marked than in any other intertropical region of the globe. The high temperature caused by the vertical heat of the sun over a particular area induces an indraught of air to that place, an ascending current is produced which carries up with it the warm and moist air; condensed in the higher regions of the atmosphere, the moisture falls as rain, and the condensation makes way for a further indraught. It is thus that in Africa the winds and rains follow as a rule the pendulating movement of the continent beneath the sun, and the rainy season of any space begins almost immediately after the sun has reached its zenith. Between the tropics and the equator the sun comes twice to the zenith of each belt during the year, at the tropical lines the sun is only once in the zenith; thus it follows that a double rainy season is observed in all places lying in the central belt of the tropics, and a single rainy season in those which are nearer the skirts of the zone. These wet and dry seasons correspond to the cooler and hotter periods of the year, and take the place of the summer and winter of the temperature regions. Various circumstances tend to interfere with and modify the working of this general rule of the rotation of seasons. In Southern Africa that rainy season which follows the apparent movement of the sun northward, is greater than that which ensues after his passage south, since in the former case the winds are drawn inwards from the ocean and carry greater quantities of moisture, whereas in the latter the winds are drawn from the land north of the equator, and their moisture is already in great part spent. In the northern and eastern regions of Africa the winds and rains are governed as much by the heating and cooling of the Asiatic continent as by that of Africa itself, but in the central and western portions of the continent the rule is well exemplified. Thus in Damara Land, bordering on the southern tropic, there is one short rainy season from February till April, beginning only with the northing sun; at Loanda in Angola the greater rains last from February till May, the lesser rainy season, when the sun has passed this place going south, occurs in November only. At Annobon island, surrounded by wide sea, April and May are the rainy months of the northing sun. October and November of the southing. The Guinea coast, facing the sea to southward, has its greater rainy season from March to June, when the northing sun draws the ocean winds on to the coast; and its lesser rains occur in October and November; when the sun has passed southward from the land. Nearing the northern tropical line, the coast-land from Sierra Leone to the Senegal river has a simple wet and dry season during the year.





On the eastern coast-land the rains are more dependent on the direction of the monsoon winds; about the mouths of the Zambeze and on the Mozambique coast the rains begin in November, after the north-east monsoon wind has set in over the northern part of the Indian Ocean, bringing with it the vapours drawn from the sea to condense on the coast slopes. The rains continue here till March, when the south-west monsoon begins to blow away from the land towards the then heated surface of Asia. At Zanzibar there is a double rainy season, a stronger in the months of March, April, and May with the northing sun, beginning immediately after the south-west monsoon has set in, and a weaker in September and October with the southing sun. under the equator on the east coast the rains begin in April with the south-west monsoon, continuing till June, and during this period the sky is obscured by heavy clouds. The second rainy season here is only marked by a few showers in September and October. While the north-east monsoon is blowing the sky remains of a cloudless blue. In the interior of the continent, between these tropical coasts, the rainy seasons appear rather to precede than follow the advancing sun. in the region of the central Zambeze the greater rains last through February, March, and April, the lesser occurring in October and November. The worst droughts are experienced in December and January. Nearer the centre of the continent the two rainy seasons become so lengthened as almost to merge into one period of rains, extending over about eight months of the year. In the newly-explored country south-west of the Tanganyika, Dr Livingstone found that the rains began in October, and that the last showers fell in May; but there is probably a drier period between these limits. At the Tanganyika Lake the rainy season begins in September, lasting till May, and the same rainy season has been observed in the interior country of the west coast immediately north of the equator. between these points, in Manuyema country, Dr Livingstone found that the rains continued till July, or almost through the year. Northward in the interior the rainy seasons are again clearly divided into a greater and lesser, and in the regions west of the Upper Nile between 5o and 10
° N. lat., the stronger rains occur from August till October, the weaker come with the northing sun in April and May. The plateau of Abyssinia, rising high above the general level of the north of Africa, and intercepting and condensing the moist winds, has also a double rainy season, -- a greater from June to September, when the sun is passing southward; a lesser in February and April, with the northing sun. The rainy seasons in Central Africa are ushered in and accompanied by violent thunderstorms and by occasional falls of hail. The quantity of the rainfall, which is excessive in the regions near the equator, diminishes rapidly to nortrh, and south of this belt as the dry regions on the borders of the tropics are approached.

The Sahara, and also the Kalahari of Southern Africa, are almost rainless regions, but wherever a sufficient elevation occurs to intercept a cooler stratum of the atmosphere, rain is not wanting, even in the midst of the Great Desert. A striking instance of this is related by Mr Richardson. That traveller relates that when on the borders of the mountain knot of "Air, in about latitude 19o N., on the 30th Sept. 1850, there was a cry in the encampment. ÎThe wady is coming.' Going out to look, I saw a broad white sheet of foam advancing from the south, between the trees of the valley. In ten minutes after a river of water came pouring along, and spread all around us, converting the place of our encampment into an isle of the valley. The current in its deepest part was very powerful, capable of carrying away sheep and cattle, and of uprooting trees. This is one of the most interesting phenomena I have witnessed during my present tour in Africa. The scene, indeed, was perfectly Africa. Rain had been observed falling in the south; black clouds and darkness covered that zone of the heavens, and an hour afterwards came pouring down this river of water into the dry parched-up valley."

Rainfall in the Dry Regions of Africa

The causes of want of rainfall in the vast region of the Sahara appear to be mainly these-that the winds advancing towards it come from a cooled and moister to a warmer and drier region, indeed to the hottest and driest of all, and so are constantly losing in moisture and gaining in temperature as they approach; the high plateau of Abyssinia forms an effective screen from the winds of t he Indian Ocean, wringing out their moisture before the Sahara is reached, and on the Atlantic side the north-east trade wind constantly blows away from the land; a barrier of mountains also deprives the Sahara of rain from the south-west. Another cause of dryness is the low level of great areas of the Sahara. We have seen that wherever there is a considerable elevation, even in its midst there is a periodical rainfall. The Kalahari region is almost rainless, on account of the great heat to which it is subjected; but specially because the winds coming towards it from the eastward, the prevailing winds, expend their moisture on the high slopes of the plateau which face the Indian Ocean. Heavy dews, consequent on the rapid changes of day and night temperature in these bare regions, partly compensate the deficiency of rain.

Extratropical Rains

The portions of the continent which lie beyond the tropics north and south, the outer slopes of the plateau of Barbary and of the Cape Colony, have no marked rainy season, and the times of the occurrence of rain are altered, the summers of both being drier, the showers more frequent in winter. In Natal, and on the slopes of the plateau in its neighborhood, rain may be expected in any month; but the greatest falls occur from October to March. The absolute quantity of rain which falls in Africa has a yet been measured at so few points, that no definite conclusions can be arrived at respecting it.





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