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XXX. 



ON THE DISTEIBUTION OF MEAN ANNUAL RAINFALL AND 

 AVERAGE NUMBER OF RAIN-DAYS PER YEAR OVER 

 AN AREA INCLUDING THE COUNTIES OF DUBLIN, 

 WICKLOW, KILDARE, AND MEATH : A STUDY IN LOCAL 

 VARIATION OF RAINFALL. 



By WILLIAM J. LYONS, B.A., A.R.C.Sc, (Lond.), 

 Royal College of Science for Ireland. 



[communicated by phofkssor w. brown, b.sc] 



(Plate XXL: Map.) 



[Read February 22. Ordered for Publication March 8. Published May 19, 1910.] 



Introduction. 

 The distribution of rainfall in space presents very marked variation, not 

 only when considered for short periods of time, but even in the case of the 

 annual and mean annual precipitation. The distribution, particularly that 

 of the mean annual rainfall, depends on the relief or orographieal features 

 of the country ; and this relation to the orography is found even in the 

 local variation within very limited areas. In preparing a map to show 

 the distribution of rainfall over a large area such as the British Isles, 

 only the general features of tlie distribution can be represented ; and in such 

 cases, in assigning a value to the mean annual rainfall of a small part of the 

 area, the local differences peculiar to that part are necessarily levelled 

 down so as to represent the average for the district. The study of these 

 local differences is, however, often of more interest than that of the general 

 distribution over large areas, and presents a problem in physical, rather than 

 in mere statistical meteorology. 



The immediate causes of the formation of rain are very obscure, and 

 existing theories rest on physical speculations rather than on meteorological 

 data. No satisfactory theory is likely to be developed as the result of an 

 increase in our knowledge of the physics of condensation and allied 

 phenomena, unless there is a corresponding extension in our meteoro- 

 logical study of the subject. The local variation of rainfall seems an 



