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Though the winter snows upon the plains, and the slopes of mountains of medium height, disappear during the warm season; yet, in all latitudes, the tops of high mountains are covered with a layer of permanent snow, which the summer heat of these great altitudes is not sufficient to melt.

The lower limit of perpetual snow, called the snow line, is found, within the tropics, about three miles above the level of the sea. In temperate latitudes it occurs at the height of a little less than two miles; and at the northern limit of the continents, it is about half a mile above the level of the sea, or, perhaps, even less than this.

On the Arctic Islands, vast fields of snow remain permanently, at a few hundred feet above the sea level.

The winter snows, falling into the icy waters of the polar oceans, are but partially dissolved; and, remaining upon the freezing surface, they help to form those vast ice floes which encumber the polar seas at all times.

The following table gives the observed height of the snow line in the different latitudes:—

HEIGHT OF THE SNOW LINE

Lat. N.	New World	Feet
75°	North Greenland	2,300
54°	Unalaska	3,500
48°	Mt. Baker, Oregon, about	8,000
43°	Rocky Mountains	12,500
39°	Rocky Mountains	14,500
38°	Sierra Nevada	11,000
19°	Popocatepetl, Mexico	14,900
5°	Tolima, Columbia	15,300
Lat. S. 1°	Andes of Ecuador	15,800
17°	Andes of Bolivia, west side	18,500
17°	Andes of Bolivia, east side	15,700
33°	Andes of central Chili	14,700
42°	Andes of Patagonia	6,000
54°	Andes of Straits of Magellan	3,700
75°	Bear Island	600
71°	Mageroe, Cape North	2,300
67°	Sulitelma, Lapland	3,800
61°	Scandinavian Alps	5,300
50°	Altai Mountains	7,000
46°	Alps, north side	8,800
46°	Alps, south side	9,200
43°	Caucasus	11,000
35°	Hindu Kush	13,000
31°	Himalaya, south side	16,200
31°	Himalaya, north side	17,400
12°	Abyssinian Mountains	14,000
Lat. S. 3°	Kilimanjaro	16,000
44°	New Zealand Alps	7,500