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SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 51 



indifferent whether we consider the condensed water to be floating 

 in the air or to fall away from the air, on the other hand it makes 

 an important difference in the case of the compression that accom- 

 panies descending air. 



If, for instance, the mass of air again sinks after it has passed 

 over the ridge of a mountain range, while the total quantity of 

 moisture £ grams remains unchanged so that no precipitation falls 

 away from the air, then in the course of the descent of the air 

 exactly the reverse process takes place, so that graphically repre- 

 sented the air may be said to pass backward along the sameadiabats, 

 first the adiabat of the condensation stage until all the water is 

 again evaporated or until the saturation curve, £ grams, is attained 

 and then down the adiabat of the dry stage. 



But it is otherwise if the precipitation has fallen away from the 

 air. In this case during the descent of the air no water can be 

 evaporated and the air follows backward along the adiabat of the 

 dry stage only. 



4000m 



o 



-JO O* ZO° 20° 



FIG. IO. FOEHN WIND 



An example for the foehn wind is given in fig. 10. 



At the initial level the temperature was 14 C. and the relative 

 humidity was 60 per cent. For this we find the saturation curve 

 for 10 grams and hence 



10 X 60 -f- 1 00 = 6 grams 



is the mixing ratio. If now the air expands adiabatically then as 

 shown by the intersection of the diagonal for the dry adiabat with 

 the 6-gram line of saturation the air will attain a temperature of 

 5 at 900 meters altitude. If further expansion takes place the air 

 follows the condensation adiabat, it attains the freezing point at 

 1750 meters altitude where we find the quantity for saturation 4.6 



