VIII 



THE THEORY OF THE FORMATION OF PRECIPITATION 

 ON MOUNTAIN SLOPES 



BY PROF. F. POCKELS 

 School of Technology, Dresden, Germany 

 [Translated from Ann. d. Physik, (^) Vol. IV, pp. ^.^g-^So. igoi] 

 Reprinted from the Monthly Weather Review for April, kjoi 



It is a Avell known principle of climatology that the side of a 

 mountain range which is turned toward the prevailing wind lias in 

 general a greater precipitation than the plain on the windward 

 side, and a still greater in comparison with the leeward side of the 

 mountain range. There has been no doubt as to the explanation 

 of this phenomenon since it has been recognized that the principal 

 cause of the condensation of the aqueous vapor is the adiabatic 

 cooling of the rising mass of air; for a current of air impinging against 

 rising ground must, in order to pass over it, necessarily rise. So 

 far as the author knows, however, no attempt has yet been made to 

 investigate this process quantitatively, except perhaps, for the 

 stratum of air immediately contiguous to the earth, whose ascension 

 being equal to that of the surface itself, is thereby known directly. 

 Such a quantitative treatment will be attempted in the following 

 article. Even although this is only possible under special assump- 

 tions, which represent nature with the closest approximation, it will, 

 however, always offer a practical basis for estimating the purely 

 mechanical influence exerted by the configuration of the surface 

 of the earth on the formation of rain. 



In order to find the standard vertical components of the velocity 

 of the air currents that determine the condensation, Ave must, first 

 of all, solve the hydrodynamic problem of the movement of the air 

 over a rigid surface of a given shape. In this connection Ave must 

 make a series of simplifying assumptions, as follows: 



i. The current must be steady; 2, it must be continuous and 

 free from whirls; 3, it must Aoav everywhere parallel to a definite 



80 



