Snow

Snow, the crystalline form of atmospheric moisture formed when the temperature is below the freezing-point. Snow falls in flakes, each of which consists of a number of symmetrically six-rayed, star-like crystals sometimes exceedingly complex in form. More than a thousand forms have been described. The opaque whiteness of snow, like that of table-salt, results from the numerous reflections from the faces of the minute crystals, which individually are transparent. Snowflakes contain about nine times as many volumes of air, entangled, so to speak, among their crystals, as they contain water; so that a fall of snow ten inches deep is about equivalent to an inch of rain. Snow is a bad conductor of heat, so that when on the ground it protects plants from frost. Snow never falls at the sea-level within the tropics, and seldom in the southern hemisphere north of 48° S. The snow-limit for sea-level passes through Buenos Ayres, Cape Town, Melbourne, and Sydney in the southern, and through Mexico, North Africa, Asia Minor, the south of the Caspian, the north of Hindostan, and Canton, in the northern hemisphere. In England, whilst it descends to sea-level in winter, it rises in summer several miles overhead, the fleecy cirrus-clouds then seen being composed of snow. Considerably higher than the isotherm of 32° is the snow-line, or line of perpetual snow, above which the snow never entirely melts. At Quito, near the equator, it is at 15,800 feet; in Mexico (19° N.) at 14,800; on the south side of the Himalayas, which is supplied with abundant moisture from the Indian Ocean, it is at 16,200 feet but on the north side, which is heated by the dry air from Thibet, at 17,400. In Granada (37° N.) it is at 11,200 feet, and on Mont Blanc (46° N.) 8,500 feet. Though there is generally some snow on Ben Nevis, no point in the British Isles actually reaches the snow-line. In Iceland (60° N.) it is at 3,100 feet; at the North Cape 2,000 feet, and at Spitzbergen at sea-level. Besides the protection of vegetation, the chief geological actions of snow are the formation of avalanches (q.v.), glaciers (q.v.), and summer floods, such as those of Mesopotamia.