Africa Eastern Cost Lands
rainy rains south rain north west
AFRICA EASTERN COST LANDS On the eastern coast-land the rains are more dependant 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-cast 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. \V]iils the north-east m9n- soon 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 reason 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 5° 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 north 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 19° N., on the 30th Sept. 1850, there was a cry in the encampment, ' The 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 African. 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."
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 cooler 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 the 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 rain- less, 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.
The portions of the continent which lie beyond the tropics north and south, the outer slopes of the plateau of Parbary 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 neighbourhood, 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 as yet been measured at so few points, that no definite conclusions can be arrived at respecting it.