An Introduction to the Study of South African Rainfall. 25 



tally fairly well. So do they also at Clanwilliam. At Aliwal North 

 only in the annual term. At Simonstown the annual term of rain- 

 fall is a month earlier, and the semi-annual term a month later than 

 the corresponding terms of cloud. Any correspondence at Port 

 Elizabeth and Mossel Bay is only in the semi-annual terms. At the 

 same time it seems not unlikely that the same annual cloud wave 

 whose crest passes over Durban about the middle of December 

 passes over Port Elizabeth and Mossel Bay a fortnight and five 

 weeks later respectively, and a fortnight later still over Kimberley 

 and Aliwal North. Nevertheless, it has lost its relative rank with 

 respect to the second term by the time it reaches the south coast. 

 It brings practically no rain, the annual wave of precipitation which 

 started from the east coast with it not arriving until three months 

 later. 



Where do our rains originate ? It has been stated pretty fre- 

 quently and positively that they come from the south-east in 

 summer and from the south-west in winter ; that enormous quantities 

 ■come from the South Indian Ocean in the summer, watering the 

 greater part of the land south of the Zambesi ; whereas the little 

 that comes from the South Atlantic during the winter is all deposited 

 within sight of Table Mountain." One view connects them with the 

 permanent anticyclones spreading across the ocean in these latitudes.! 

 It is not clear, however, that such explanations represent all, or even 

 more than a small portion, of the facts. 



If these rains originate entirely in the south, it is indeed strange 

 that practically all the cloud currents over Kimberley have a larger 

 or smaller northerly component ! Heavy thunder-clouds mostly 

 advance from the west or north-west. Other rain clouds, and 

 lighter thunderstorms, from somewhere between north-east and 

 east. Scarcely any clouds come from the south-east, and very 

 few from the south-west. The diurnal variation of the wind at 

 Kimberley may be arrested, and the vane point for two or three 

 days towards the south-east, and never a cloud obscure the sky. 

 The variation may be arrested, and the vane point anywhere 

 between east and north, and the skv be overcast and rain fall in 

 abundance. And in almost every case the direction of the cloud 

 movement is independent of the surface wind. One difficulty in 

 settling such a question is paucity of observations : saving for one 



* Maury claimed that the rain of the world came chiefly from the southern 

 hemisphere. The atmosphere " is an engine. The South Seas . . . are the boiler 

 for it, and the northern hemisphere is its condenser " {Physical Geography of the 

 Sea (1859), p. 52). 



f Buchan, "Rainfall of South Africa," p. 16. 



