370 Scientific Proceedings, Royal Dublin Society. 



rainfall at the higher levels around it. It is difficult, however, to extend the 

 results obtained in other places to help to decide in such a matter as this. 

 The position of the maximum zone depends on the absolute heights and 

 details of the mountain system, on the prevailing winds, and on the average 

 conditions of temperature and humidity.^ The maximum zone lies lower in 

 winter than in summer by reason of the lower temperature and larger relative 

 humidity, and in Wioklow the average rainfall for the summer is probably 

 greater than for the winter. The fact that a rainfall of 53 inches was found 

 for Glenasmole Lodge is, with the other considerations, a justification for 

 considering a rainfall over 60 inches probable in the centre of Wicklow. 



With reference to the values found for the frequency of rainfall as repre- 

 sented by the average number of rain days per year, it is remarkable that the 

 difference generally is very small, and in cases quite negligible even for 

 stations the rainfalls of which show the greatest contrast. Thus the fre- 

 quency of rain at Phoenix Park is 215, the same as at Gleudalough, although 

 the rainfall for the latter (59) is more than double the rainfall of the former 

 (28"2). Again, there is observed a considerable difference in the rain-fre- 

 quency for the neighbouring stations, Phoenix Park (215) and Straffan (180), 

 whereas the difference in rainfall is small and in the opposite direction. 



It is perhaps possible to draw only questionable deductions from the 

 results just mentioned as to the rain-frequency. It would seem, however, 

 that the great differences observed in annual rainfall over a limited area are 

 generally not due to rain days being more frequent in the mountains than in 

 the low-lying districts, but rather to the rain being more intense, or possibly 

 more prolonged in the course of each rain day. Further facts on these points 

 are urgently needed. The question of rain-frequency, so far as the author 

 knows, has not been previously considered in relation to rainfall-distribution ; 

 and no confirmation of the present results can be given. 



Thkoretical Considerations. 

 When we proceed to seek the causes of the influence of relief on rainfall, 

 we find the subject has not received very satisfactory treatment, and that 

 existing views are vague and scarcely adequate to explain the several facts 

 established. Older writers rested content with tlie suggestion that moist air 

 was condensed by its contact with the cold mountain side. It is now proved, 

 however, that any condensation produced by such a process or by the mixing 

 of warm with cold air-masses is insignificant. It is recognized that the most 



■ Lehrbuoh der Meteorologie," J. Hann, 1901, pp. 350-352. 



