CLIMATIC FACTORS : THE AIR 



91 



the same category. Glaze may be so heavy as to cause great dam- 

 age to forest trees through breakage. Conifers are particularly sus- 

 ceptible to such damage because of the load of ice that can accu- 

 mulate on their many needles. In young stands, the trees may be 

 broken down so that they die, or they may be so bent and twisted 

 that, should they grow to maturity, they form badly distorted 

 trees. 



FlG. 38. Average snow pack as it appears in March in the Sierra Nevada. 

 Echo Summit, Calif —Courtesy of W. D. Billings. 



Snow is an important source of soil moisture and, in addition, 

 may serve to modify the effects of low temperatures. Roughly 

 ten inches of snow are equivalent to an inch of rain although the 

 moisture content of snow is highly variable. Under average tem- 

 perature conditions, water derived from melting snow might 

 make up from 5 to 25 percent or more of the total precipitation, 

 but its importance is not determined entirely by amount. Since 

 conditions in the spring may be such that a heavy blanket of 

 snow disappears in a few hours, the water may run off rapidly, 



