PRECIPITATION ON MOUNTAIN SLOPES POCKELS I03 



(2) The method developed by me for computing the condensa- 

 tion that occurs on any given mountain slope cannot be applied to 

 computing the mean value of the precipitation for any given interval 

 of time, by introducing into the computation the mean values of 

 the temperature* and moisture for this interval. We should in this 

 way find too small a precipitation. Thus, for example, the altitude 

 of the mountains might not suffice to cause any condensation at all 

 for the average condition of the air, but could cause it on those 

 occasions when the moisture exceeds its average value, wherefore 

 the average value of the rainfall for the interval of time under con- 

 sideration would be different from zero. As the variation of the 

 moisture from its average value may cause rainfalls where otherwise 

 there would be none, so also, with the currents of air mechanically 

 forced to ascend mountain ranges, and whose effect is superposed 

 upon that of the general circulation of the air in cyclonic areas ; for 

 it can happen that neither one of these two causes may alone suffice 

 to form rain, but that both together do. This explains why eleva- 

 tions of the surface of the earth of from ioo to 200 meters increase 

 the annual mean value of the total precipitation, as for instance, as 

 shown by the charts in Assmann's memoir of 1886, " Einfluss, etc., " 

 "On the influence of mountains on the climate of central Germany." 



(3) The examples given in my article show that in so far as con- 

 densation in general takes place on the slopes of mountains, its 

 intensity (therefore also, the density of the precipitation when fall- 

 ing vertically) is in general greatest where the slope of the moun- 

 tain is steepest. If now we consider that in the course of all the 

 various conditions of the atmosphere that may occur in a long 

 interval of time, the first condensation occurs most frequently above 

 the upper portion of the slope, then it follows that the average den- 

 sity of precipitation computed for a long interval of time, must 

 increase, not only with the inclination of the slope, but also with 

 the absolute altitude of the locality under consideration. To this 

 case corresponds the formula for the annual quantity of precipita- 

 tion expressed in millimeters deduced by Dr. R. Huber in his 

 " Untersuchungen, etc. Investigation of the distribution of precipi- 

 tation in the canton of Basle," namely: 



N = 793 -f 0.414/1 -f 381.6 tan a 



where h is the altitude in meters, and a indicates the gradient angle. 

 (See A. Riggenbach, Verhandlungen der Naturforschenden Gesell- 

 schaft. Basel, 1895. Vol. X, p. 425). 



