[Illustration: +Fig.+ 221.—An alpine snow-field.] [Illustration: +Fig.+ 222.—Map of Greenland. The borders only are free from ice. (Stieler.)] The only condition necessary for a snow-field is an excess of snowfall over snow waste. The lower edge of a snow-field, the _snow-line_, is dependent chiefly on temperature and snowfall. In general it does not depart much from the summer isotherm of 32°, though it may be well above this isotherm where the snowfall is light. That the snow-line is not a function of temperature only is shown by its position in various places. In the equatorial portion of the Andes, for example, 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. For the same reason the snow-line in the Himalayas is 3000 or 4000 feet lower on the _south_ side than on the north. While in equatorial regions the snow-line has an altitude of 15,000 to 18,000 feet, it approaches or even reaches sea-level in the latitude of Antarctica and North Greenland. In intermediate latitudes it has an intermediate position. While temperature and snowfall are the most important factors controlling the position of the snow-line, both humidity and the movements of the air are of some importance, since both affect the rate of evaporation of snow and ice. =The passage of snow into névé and ice.=—The snow does not lie on the surface long before it undergoes obvious change. The light flakes soon begin to be transformed into granules, and the snow becomes “coarse-grained.” The granular character, so pronounced in the snow of the last banks which remain in the spring in temperate latitudes, is even more distinct in perennial snow-fields, either at the surface or just beneath it. This granular snow is called _névé_, or _firn_. Still deeper beneath the surface, where the thickness of the snow is great, the névé becomes more compact and finally coherent, and grades into porous ice. This gradation is accomplished at no great depth, though the thicknesses of snow and névé are by no means constant.