no SCIENCE IN AFRICA 



ATMOSPHERIC CIRCULATION 



Special reference must be made to the circulation of the atmos- 

 phere in view of its importance for understanding local variation 

 in weather and for forecasting. Since Africa is bisected by the 

 equator it is in a syrtimetrical position in relation to the equatorial 

 rain-belt. This is flanked by sub-tropical belts of high pressure, 

 dominated by the trade winds. In the north the trade wind desert 

 belt stretches from the Atlantic to the Red Sea, but in the south 

 desert conditions are confined to the Atlantic Coast, because the 

 Drakensberg scarp obstructs the passage of the moisture-laden 

 trades from the warmer Indian Ocean, and induces abundant 

 rainfall. The extreme north and south of Africa are influenced by 

 the westerly cyclonic belt during the winter months, and enjoy 

 higher rainfall. 



Two factors, however, alter this symmetry of climate: the greater 

 width of North Africa, which causes the actual thermal equator 

 to deviate northward because in tropical latitudes the land is 

 warmer than the sea, and the greater height of South Africa 

 (average of 4,000 feet above sea-level, compared with 1,000 feet 

 in the north), which increases this asymmetrical effect. 



It is in analysing the influence of these factors on the climate 

 that the recording of atmospheric circulation has the greatest 

 interest. Since the war studies have been made by Lyons (191 7) 

 on North Africa, by Brooks and Mirrlees (1932) for Central Africa, 

 and by Cox (1935) for South Africa. The authors have considered 

 conditions in the upper air as well as at sea-level, making use of 

 modern methods of upper air research. Thus Sir Henry Lyons 

 shows that the south-west monsoon or surface air-current of the 

 Guinea Coast has a vertical thickness of not more than 5,000 feet, 

 being overlain by the north-east harmattan. Mr. Cox points out 

 that the South African plateaux interfere so much with the surface 

 circulation that at some places, such as Durban, the wind often blows 

 in complete opposition to the barometric gradient. In summer 

 apparently the south-east trade, which brings much rain to the 

 Drakensberg scarp, does not succeed to any great extent in passing 

 over the range into the Transvaal, but is deflected and reaches 

 that country from the north and passes out to sea in Natal from the 



