An Introduction to the Study of South African Rainfall. 27 



gradients.' 1 ' Now Prof. Morrison has pointed out that barometric 

 depressions in most cases start from the south coast and travel in 

 some northerly direction. 



We may then summarise all that is known of the predominant 

 conditions determining South African rainfall, outside the Cape 

 Peninsula and West Coast, in the following paragraphs : — 



The rain decreases on the whole with distance from the coast. 



It comes with a high pressure at Durban, and a low pressure at 

 Kimberley. 



It comes chiefly with south-westerly winds at Durban, and with 

 north-easterly winds at Kimberley. 



The principal barometric disturbances come from the south. 



The clouds over the tableland come from some northerly point. 



We may be certain that the winds blowing on-shore along the west 

 coast carry comparatively little moisture, not so much because of the 

 short superoceanic path they are said to traverse (owing to the influ- 

 ence of the permanent anticyclone near by), as because of the coldness 

 of the water.f An interesting parallel is found on the coast of 

 California. It is referred to here because of the very apropos 

 explanation it has elicited : " On the coast of Southern California 

 the sea-breeze blows throughout the greater part of the year. It is 

 weak in winter and strong in summer. During the latter season 

 it is a dry wind even on the coast. . . . The cause is undoubtedly 

 to be found in the fact that the ocean near the coast is very cool, 

 while, on the other hand, the land is very much warmed. The air 

 that comes off the ocean must therefore seem relatively dry over the 

 land."]; The same argument holds with us, mutatis mutandis. 

 Port Nolloth, on the west coast, with only 2*5 inches of rain per 

 annum, is a far more humid place, because of its lower temperature, 

 than Port St. John's, on the east coast, with a rainfall ten or twelve 

 times greater. But Umtata, thirty-five miles inland from the east 



* It is interesting to compare these conditions with corresponding phenomena on 

 the coast of Syria : " The barometer is usually on the rise during a rain. A south- 

 west wind concurs with a rise from a low barometer." Moreover, " as long as the 

 wind blows steadily from the west or south-west there is usually no rain. But 

 when it blows for a day or two from the east, south, or south-south-west, and then 

 veers suddenly to the west or south-west, rain is very apt to fall. (G. E. Post, 

 "Notes on the Meteorology of Syria and Palestine," 'Trans. Vic. Inst., vol. xx., 

 pp. 284, 280.) At Manila the rain accompanies the depression, tending to its 

 greatest rate of fall after the passage of the minimum pressure. (See Loomis,. 

 " Contributions to Meteorology." Memoirs N.A.S., vol. iii., 1886. 



f " The temperature of the sea near Cape Town is sometimes 20° lower than in 

 the corresponding latitude on the eastern side of the continent " (Scott, Elementary 

 Meteorology , p. 299, 6th Edition). 



\ Hann's "Handbook of Climatology " (Ward's Edition), p. 156. 



