262 EARTH FEATURES AND THEIR MEANING 



isotherm of 32 Fahrenheit. Within the tropics this line may 

 rise as high as 18,000 feet above the sea, whereas in polar lati- 

 tudes it descends to sea level. 



Importance of mountain barriers in initiating glaciers. The 

 precipitation within any district depends, however, not alone 

 upon the amount of moisture which is brought to it in the clouds, 

 but upon the amount which is abstracted before the clouds have 

 passed over it. The capacity of space to hold moisture increases 

 with its temperature, and hence any lowering of this temperature 

 will reduce the capacity. If lowered sufficiently, the point of 

 complete saturation will be reached and further cooling must 

 result in precipitation. Hence, anything which forces an air 

 current to rise into more rarefied zones above, will lower the pres- 

 sure upon it and so bring about a cooling effect in which no heat 

 is abstracted. This so-called adiabatic refrigeration of a gas 

 may be illustrated by the cool current which issues in a jet from 

 a warm expanded rubber tire after the cock has been opened ; or 

 even better, by the instant solidification at extreme low tempera- 

 tures of such normal gases as carbonic acid when they are allowed 

 to issue under heavy pressure from a small orifice. 



As applied to moisture-laden and near-surface winds, the 

 effective agents of adiabatic cooling are the upland areas upon 

 the continents, and especially the ranges of mountains. These 

 barriers force the moving clouds to rise, cool, and deposit their 

 moisture. It is, therefore, the highland barriers which face the 

 on-coming, moisture-laden winds that receive the heaviest pre- 

 cipitation. Within temperate regions, because of the prevalence 

 of westerly winds, those barriers which face the western shores 

 receive the heaviest fall. Within the tropics, on the other hand, 

 it is the barriers facing the eastern shores which, because of the 

 easterly " trades," are most favorable to precipitation. 



Thus it is in the Sierra Nevadas of California,' and not in the 

 Rockies or the Appalachians, that the glaciers of the United States 

 are found. The highland of the Swiss Alps lying likewise athwart 

 the " westerlies" of the temperate zone acquires the moisture 

 for nourishment of its glaciers from the western ocean here 

 the Atlantic (Fig. 291). Within the tropics the conditions are 

 reversed, and it is in general the ranges which lie nearer the eastern 

 coasts that are the more favored. If no barrier is found upon 



