explanatory hypotheses 481 



Hypotheses in Explanation op Present climatic Changes 

 the meteorological hypothesis 



ISTo one can doubt the importance of purely meteorological causes in 

 producing the climatic variations which we observe from year to year. 

 The annual change in the sun^s altitude from season to season sets in 

 motion a train of consequences which can scarcely act in precisely the 

 same way each year. Accidental circumstances, such as the veiling of 

 certain areas by clouds, slight alterations in the strength or course of 

 oceanic currents, and many other circumstances, all may, and indeed must, 

 occur. If several accidents happen to lead in the same direction, we may 

 have a winter of unusual warmth or a summer with more than the usual 

 rainfall. Therefore students of meteorology and climatology almost uni- 

 versally believe that purely meteorological causes are adequate to produce 

 a large part of the variations by which t^*-^ weather of one year differs 

 from that of another. A few hold that such fortuitous causes may pro- 

 duce marked variations lasting scores of years. None, however, maintain 

 that they are the cause of great climatic changes lasting through long 

 periods. Hence this hypothesis would have little importance from the 

 geological point of view were it not that it has led geologists to search 

 for the cause of glaciation in conditions whose operation can not now be 

 observed. Because of this effect it seems necessary to consider the matter 

 somewhat further. 



Two important lines of reasoning seem to indicate that although the 

 hypothesis of purely accidental meteorological variations may explain 

 many minor phenomena, it does not fully explain the larger variations 

 from year to year. In the first place, the vast majority of climatic vicis- 

 situdes in temperate regions are due to variations in the number and 

 location of cyclonic storms. These in turn, as the Indian Meteorological 

 Service has shown,^ are associated with distinct and systematic changes 

 in the winds, which cause the equatorial rainfall of Abyssinia and the 

 Nile floods on the one hand, and the monsoon rains of India on the other 

 hand. Cyclonic storms, as will soon be shown in detail, are probably the 

 climatic phenomenon which varies most markedly in harmony with sun- 

 spots. Hence there may be ground for the opinion that although these 

 occurrences are apparently accidental, they are probably not really so. 



The second reason for not accepting the purely meteorological hy- 

 pothesis, except in respect to minor phenomena, lies in the fact that the 



"Memoirs Indian Meteorological Department, vol. xxi, part vii. 1018. The cold 

 weather storms of northern India, by G. T. Walker, 

 See also part ii, p. 34, in the same volume. 



