372 Scientific Proceedings, Royal Dublin Society. 



by Prof . F. Pockels, Dresden, in 1901.' Making certain assumptions, and 

 considering only simple relief forms at right angles to the wind, Pockels 

 showed that, in the ease of liigli mountains at least, a maximum rain-zone 

 would be found on the windward slope, and that tlie precipitation would 

 depend on the slope rather than on the actual height of the mountain.^ The 

 conclusion was also given that mountains less than 500 or 600 metres high 

 would not occasion rain under ordinary atmospheric conditions, either in 

 summer or in winter. 



It is apparent that considerable difficulties arise in attempting to 

 explain by reference to orographical or local rains only tlie influence of 

 relief on rainfall. The influence as already noted is observed even in 

 the case of small elevations; and in the case of moderate altitudes, such 

 as were studied in the present paper, the effect is very marked. Over 

 the present area and the British Isles generally a very considerable pro- 

 portion of the total annual precipitation must be associated with meteoro- 

 logical rains ; and the difficulties of explaining the local variation by the 

 orographical rains is increased. It would be necessary to suppose that such 

 orographical rains were very frequent, or very intense, or long continued. The 

 results for the number of rain days per year, given in the present analysis, 

 do not appear to justify the assumption that rain is much more frequent 

 in the mountains than in the plains. Apart from this, however, it is difficult 

 to understand, on Pockels' theory, or from general considerations, how the 

 intensity and duration of the local rains could be such as to account suffi- 

 ciently for the great differences observed in the rainfall. It must also be 

 pointed out that local rains are more frequent in summer than in winter ; 

 but in summer, when the relative humidity is low, and the temperature high, 

 tlie influence of mountains in causing precipitation is less than in winter.' 



It is not desirable to go further into the discussion of this important 

 matter in the present paper. A considerable amount of existing evidence has 

 to be carefully balanced, and further information on several points obtained, 

 before any finality of opinion can be reached. It may be suggested, however, 

 with all the reserve necessary under the conditions of our present knowledge, 

 that, apart from inducing condensation under favourable conditions, mountains 

 increase the rainfall bj' their action on already condensed cloud-masses. It is not 

 difficult to indicate liow their action might probably help in the second process of 



' Aim. d. Physik, 1901 (4), vol. iv., p. 459, and Monthly 'Weather Review, U. S. Weather Bureau, 

 April, 1901. 



- Hann's " Klimatologie, " vol. i., p. 298. 



' Prof. J. Hann, "Lehrbuch der Meteorologie," p. 350 ; Supan, Memoir " On the Distribution 

 of Rainfall over Islands and Continents of the Globe," pp. 40-43. 



