Lyons — The Distribution of Mean Annual Rainfall. 371 



frequent and most effective process in condensation is the adiabatic cooling 

 produced by the expansion of ascending masses of air. The condensation 

 will result at first in the formation of necessarily minute cloud-particles ; and 

 a second phase or process is involved in the development of rain-drops from 

 these minute cloud-drops. The factors and conditions necessary for this 

 second development are very obscure. The continued growth of a drop by 

 further condensation is difficult to understand in most cases, owing to tlie 

 latent heat involved in the process.^ Such continued condensation could 

 probably occur only if the expansion was rapid' or sustained.^ The develop- 

 ment of rain-drops by the coalescence of the cloud -particles is a process also 

 presenting physical difficulties ;* and it is well recognized that an important 

 factor in this process of coalescence is the electrical conditions in the air.^ 



Confining attention to the condensation phenomena, it is seen that the 

 ascensional movement of air may be brought about in two ways, giving two 

 distinct classes of rain. The first will be produced in the upper currents, 

 associated with such barometric systems as cyclones and secondaries, and 

 with such disturbances as thunder- squalls. In the second case, wind blow- 

 ing towards mountains will be forced upwards ; and under suitable conditions 

 condensation will take place. The latter effect will be local ; and rains thus 

 produced are called orographical rains, to distinguish them from the meteoro- 

 logical rains associated with such atmospheric disturbances as cyclones. In 

 considering the influence of relief on the rainfall, it is most important to 

 distinguish between these two kinds of rain. It would seem reasonable to 

 suppose that tlie relief-features would exercise very little, if any, influence 

 on the condensation occurring in the case of the meteorological rains, the 

 causes of which must be sought in the middle or higher layers of the 

 atmosphere. This supposition should hold especially in the case of small 

 areas, over which the conditions in such layers would probably be uniform. 

 In this connexion it is important to note that Mill has recently stated 

 that, in a study of tlie distribution of such rains as occur in cyclones and 

 thunder-squalls, he finds no relation to the configuration of the country.* 

 It is thus natural to look to the orographical rains as an explanation of the 

 increased rainfall on elevated ground. The theory of the formation of con- 

 densation on mountain slopes was mathematically worked out in some detail, 



' Osborne Keynolds, Scientific Papers, vol. i.,pp. 214-230. 

 ^ Angot, "Annales du Bureau Central Meteorologique," 1895. 

 3 Aitken, Proc. Eoy. Soc, li., 1892, p. 408. 



* Aitken, ibiil. ; Lord Kayleigh, Phil. Mag., xlviii., 1899, pp. 321-337. 

 ^ Lord Eayleigh, Proc. Eoy. Soc, vol. xxviii., 1879, p. 406. 



^ Quart. Journ., Eoy. Meteor. Soc, vol. xxxiv.. No. 146, April, 1908, pp. 72, 73. He regards 

 it, however, as too soon to speak with certainty on this point. 



