When the lower air is warm enough partially to melt the crystals, they form minute balls. When raindrops, formed in the upper air, fall through a cold current, they are often frozen, producing sleet instead of snow.
WHERE PERMANENT
SNOW EXISTS
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 |