THE WORK OF SNOW AND ICE 103 



(by night) which are injurious to many plants, and it keeps the dust 

 and sand beneath from being blown about by the wind. 



Where snow endures from year to year over any considerable 

 area, it constitutes a snow-field. Snow-fields occur in mountains 

 in nearly all latitudes; but the altitude which is necessary in the 

 equatorial region is great (15,000 to 18,000 feet), that in the tem- 

 perate region less, and that in the polar regions slight. In polar 

 regions, indeed, snow-fields occur even down to sea-level. 



Snow-fields are by no means rare even in the United States. 

 They occur in the high mountains of California, Colorado, and Utah 

 (rare), and in the high mountains of all the states farther north. 

 The snow-fields of the more northerly states are more numerous 

 and larger than those farther south. In the mountains north of 

 the United States they are still larger, and in Alaska some of them 

 attain great size. They occur also in the high mountains of most 

 other countries. In Africa, they are found very near the equator, 

 but they are small, and limited to very high mountains. 



Besides these and other small fields of snow and ice, there are 

 two great snow- (or snow-and-ice) fields in Greenland and Ant- 

 arctica. The snow-and-ice field of Greenland contains much more 

 frozen water than all the mountain snow-fields mentioned above, 

 while that of Antarctica contains probably several times as much 

 as that of all other fields together. 



The snow-line. The snow-line in the mountains is the line above 

 which the snow is not all melted in summer. Its position is in- 

 fluenced by several conditions, one of which is temperature. The 

 snow-line is higher in lower (warmer) latitudes, and lower in higher 

 (colder) latitudes. Temperature is, however, not the only thing 

 which fixes the position of the snow-line, for in various mountains, 

 for example the Himalayas, it is higher on the north side than on 

 the south, although the temperature on the south side is much 

 higher than on the north. 



Another factor is the amount of snowfall. The snowfall is much 

 heavier on the south side of the Himalayas than on the north, and 

 the thin cover of snow on the colder north slope is melted farther 

 up the side of the mountain in summer, than the heavier cover on 

 the warmer south slope. 



