SNOW TO ICE 



y.erland. Numerous islands to the west of North Greenland arc ;tl><> 

 partly covered with snow. In Antarctica there is a still larger 

 field, the largest of the earth. Its area is not known, but its ex- 

 tent is at least 6 or 8 times as great as that of Greenland. 



The only condition necessary for 

 a snow-field is an excess of snow-fall 

 over snow-waste. The lower edge of 

 a snow-field, the snow-line, is de- 

 pendent chiefly on temperature and 

 snow-fall. It does not depart much 

 from the summer isotherm of 32, 

 though where the snow-fall is light, 

 it may be above this isotherm. That 

 the snow-line is not a function of 

 temperature only, is shown by its 

 position in various places. Thus in 

 the equatorial portion of the Andes, 

 the snow-line has an altitude of about 

 16,000 feet on the east side of the 

 mountains, where the precipitation 

 is heavier, and of about 18,500 feet 

 on the west side, where it is lighter. 

 Yor the same reason the snow-line 

 in the Himalayas is lower on the 

 south side than on the north. Though 

 temperature and snow-fall are the 

 most important factors controlling 

 the position of the snow-line, both 

 humidity and movements of air are of some importance, since both 

 affect the rate of evaporation of snow and ice. 



Change of snow to ice. Snow does not lie on the surface long 

 before it undergoes obvious change. The light flakes are trans- 

 formed into granules, and the snow becomes "coarse-grained." 

 The granular character, so pronounced in the last banks of snow in 

 the spring, is even more distinct in perennial snow-fields. This 

 granular snow is called neve. Where the thickness of the snow is 

 great, the neve becomes compact below, and grades into porous ice. 

 Ice is found in some snow-fields at no great depth from the surface. 



Structure of the ice. The ice of a snow-field is in some sense 

 stratified. It is made up of successive falls of snow which tend to 



. 128. Map showing the 

 ice-cap of Greenland. Only the 

 borders (shaded parts) of the island 

 are free from ice. 



