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snow line,altitude above which or latitude beyond which snow does not melt in summer (usually called the permanent snow line), or, in winter, the line to which snow extends at a given point in time. Factors affecting the location of the snow line are the quantity of snowfall, the steepness of the slope on which snow rests, the exposure of an area to the sun and prevailing winds, the type and velocity of the winds, and the presence or absence of large bodies of water. The level of the snow line is much lower in winter than in summer. It is also affected by distance from the equator, along which it is found at an altitude of c.3 mi (5 km); in polar regions it is at sea level.
the line on the earth’s surface above which the accumulation of solid atmospheric precipitation is greater than the melting and evaporation of the precipitation. A distinction is made between the climatic snow line (the highest position at the end of the summer) and the seasonal, or temporary, line.
The snow line is lower in cold and wet regions and higher in warm and dry ones. In the antarctic it descends to sea level, and in the arctic it is several hundred meters above sea level. It reaches its greatest elevation in the dry tropical and subtropical regions (up to 7 km on the Tibetan Highlands), dropping to 4.4 km at the equator. The elevation of the snow line depends on local conditions; for example, some relief forms may give protection against the wind and promote the accumulation of snow or protect the surface against solar radiation and diminish thawing. The level of the climatic snow line corresponds to the position of the line on a horizontal, unshaded surface. The lower boundary of perennial snowfields is called the orographic snow line; in some places it is significantly lower than the climatic snow line. The Urals, the Taimyr Peninsula, Labrador, and certain other regions of mountain glaciation are entirely below the climatic snow line.
The elevation of the climatic snow line for a given year is determined from observations of the accumulation and melting of snow on glaciers, where the term “firn line” is used. The average elevation of the snow line over a period of years can be defined as (1) on mountain glaciers, the morphological boundary between the accumulation area, which is usually concave in profile, and the ablation area, which is usually convex; (2) the structural boundary between the area of conformably bedded snow and firn in the upper formations and the area where structured ice is dissected by the surface of melting in the lower formations (or, the structural boundary between the bergschrund area in the upper formations and the area of surfacial moraines in the lower formations); or (3) the average elevation of the surface of mountain glaciers, which approximates the elevation of the snow line.
REFERENCESTronov, M. V. Voprosy gornoi gliatsiologii. Moscow, 1954.
Tronov, M. V. Faktory oledeneniia i razvitie lednikov. Tomsk, 1972.
Tushinskii. G. K. Ledniki, snezhniki, laviny Sovetskogo Soiuza. Moscow, 1963.
P. A. SHUMSKII [23–1879–]