SEASONAL WEATHER AND ITS PREDICTION^ 



By SiE GiLBEXT T. Walker, C. S. I., F. R. S. 



I have chosen the subject of seasonal weather for my address, 

 because its economic importance is obvious to most men who have 

 lived in the Tropics, and its scientific problems are full of interest. 

 Unfortunately there is an additional motive, the need of warning 

 against dangers ahead. For the difficulties of long-range forecast- 

 ing are not in general adequately recognized, so that some of the 

 most progressive countries in the world are inclined to make pre- 

 dictions on an insecure basis; their technical stafF does not realize 

 that though the prestige of meteorology may be raised for a few 

 years by the issue of seasonal forecasts, the harm done to the science 

 will inevitably outweigh the good if the prophecies are found un- 

 reliable. We only learn from experience that while the forecasting 

 efforts of a charlatan are judged by their occasional successes, it is 

 the occasional failures of a government department which are re- 

 membered against it. 



In a country where conditions are as changeable from day to day 

 as they are here, it is natural that we should think in terms of wet 

 or fine days rather than of wet or dry periods; but in the greater 

 part of our empire the different seasons are much more sharply 

 defined, and so their dominant features stand out more clearly. Also 

 the variability of their seasons is in general materially greater than 

 here. Thus, in the annual rainfall measurements of the last half 

 century the smallest rainfall of Great Britain has been 23 percent 

 below normal; but that of large areas in South Africa has been in 

 defect by 40 percent, in northeast Australia by 50 percent, and in 

 the Punjab by as much as 58 percent, or two and a half times that 

 of this country. 



Now, a season that is unusual seems to have some abnormal factor 

 permanently at work diverting the weather from its ordinary course ; 

 in India I found, when issuing the daily forecast in a dry winter. 



^ Presidential address before tlie section of mathematical and physical sciences, British 

 Association for the Advancement of Science. Reprinted by permission from the Report 

 of the Association for 1933. 



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