102 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 5 I 



SUPPLEMENTARY REMARKS BY PROFESSOR POCKELS 

 ON THE THEORY OF THE FORMATION OF RAIN ON 

 MOUNTAIN SLOPES 



{Reprinted from Monthly Weather Review for July, igoi) 



(1) Assuming the average vertical distribution of temperature 

 and moisture for each of the four seasons of the year as it is deduced 

 by von Bezold from the scientific balloon ascensions published by 

 Berson and Assmann in their " Ergebnissen. " "The results of 

 scientific balloon voyages," there result the following minimum 

 elevations required in order that condensation may begin in a mass 

 1 »f air that was originally at the absolute altitude H above sea level. 



The smallest number in each column is also the smallest altitude 

 that a mountain ridge must possess in order to cause the formation 

 of clouds under the assumed conditions, but it is only in the case of 

 a very broad mountain ridge that such small altitude will suffice. 

 We see that in the autumn and winter a mountain of about 400 

 meters in height will suffice to produce a formation of cloud in con- 

 tact with the summit of the mountain whereas, in spring and summer 

 the mountain must be higher (namely about 500 or 570 meters 

 respectively), and when the air passes over this mountain the forma- 

 tion of cloud will begin in the layer lying at 500 or 1000 meters above 

 its summit. These numbers at present serve only as examples; in 

 practice, however, they suggest that as soon as we observe the for- 

 mation of cloud above a mountain of less altitude than the above 

 given tabular minimum altitude, we may conclude somewhat as to 

 the average moisture at that altitude at that time. We may also 

 remark that on account of the increasing flatness of the lines of flow 

 as the altitude increases, the above given minimum altitudes must be 

 exceeded by so much the more in proption as the width of the sum- 

 mit ridge is smaller, and the altitude of the layer in which the con- 

 densation begins is higher. 



