66 A PRIMER OF FORESTRY. 



the headwaters of streams, especially in the Rock}- 

 Mountain regions of the United States. Evaporation 

 is there so active that great banks of snow lying in the 

 full glare of the sun often disappear without melting 

 even enough to moisten the ground on the hillsides 

 below them. Vast quantities of water evaporate in 

 this way without ever reaching the streams. Measure- 

 ments made by the Bureau of Forestr}^ show that 

 evaporation from snow may be four oi- live times as 

 great as from water under like circumstances. 



RAINFALL. 



The causes of rain are for the most part wholly 

 be3^ond the reach of influence from the forest. Such 

 are the great currents of warm and cold water in the 

 ocean, the direction of the prevailing winds, and the 

 presence or absence of mountain ranges. But there 

 are two reasons which lead us to believe that forests 

 do affect the rainfall. These are their colder and 

 moister air, and the resistance which the}^ offer to the 

 motion of the winds. A great number of observations 

 has been made in different parts of the world to dis- 

 cover how much the rainfall really is affected by the 

 forest, but for several reasons no generally accepted 

 result has yet been reached. In the first place, accu- 

 rate observations on rainfall are not easy to make. 

 The height above the ground at which a rain gauge 

 is placed affects it very seriously. A variation of 10 

 feet in height will often m.ake more difference in the 

 amount of rain caught tlian most observers claim for 

 the whole action of the forest. The rainfall of two 

 stations at unequal heights above sea level is some- 



