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snow cover[′snō ‚kəv·ər]
the layer of snow on the earth’s surface resulting from snowfalls. Each year the snow cover extends over an area of 115 to 126 million sq km; approximately two-thirds of this area is land, and one-third is sea ice, primarily in the arctic and antarctic basins.
A snow cover that remains for more than one month is called stable; if it lasts less than one month it is called temporary. On land, a permanent snow cover forms on the glaciers of Antarctica, Greenland, and certain islands in the Arctic Ocean and in high mountain regions with intensive glaciation, such as the Andes, Cordilleras, Himalayas, and Karakoram. A stable snow cover forms over most of the USSR, with the exception of the southern regions of the Ukraine, Moldavia, the Caspian Lowland, the plains regions of Transcaucasia, Middle Asia, and southern Kazakhstan. Central Asia, Northeast China, the northern parts of Korea and Japan, North America north of 40° N lat., and the Atlas Mountains of Africa also have a stable snow cover. The snow cover is temporary in most of Western Europe, the plateaus of the Middle East, East China, the southern United States, and many mountain regions.
On the plains of the USSR, the average depth of the snow cover ranges from 30 to 70 cm. On the windward slopes of major mountain ranges and in the wettest regions in the west and east of the country, the depth reaches 200-400 cm. The snow cover in the USSR remains on the ground longer as one moves from the south and southwest to the north and northeast, ranging from 20 days, in the plains regions of the Crimea, Transcaucasia, and Middle Asia, to 240-280 days, in the country’s northern regions. At elevations above 2,000 m, snow remains on the ground more than 200 days a year in all mountain regions of the southern USSR; above the snow line it remains throughout the year.
The formation of the snow cover on the earth is determined by the overall atmospheric circulation. The amount of frozen precipitation particularly increases when air currents encounter mountain ranges; the distribution of the snow cover in mountains is extremely irregular because of frequent changes in the steepness and exposure of slopes and the characteristics of snow movement caused by wind. On plains the snow cover is most uniform under a forest cover. In forest-steppe and steppe conditions a great deal of snow is blown into ravines and gullies. In the USSR the regions of intense blizzard activity include the arctic islands, the coast of the Arctic Ocean, and all the mountain regions of the subarctic belt. In the high mountain regions of the temperate and subtropical zones, including the Carpathians, Caucasus, Pamir, Tien-Shan, Altai, Saian, and the mountains of the Baikal region and Transbaikalia, intense snow movement caused by blizzards occurs in the glacial-nival belt.
The surface of the snow cover is shaped in large part by the effects of solar radiation and wind. Forms of snow microrelief caused by wind may result from accumulation, such as drifts, and dunelike and barchan-like formations, or deflation, such as sastrugi and hollows.
The snow cover is characterized by several physical properties. The reflectivity ranges from 80-90 percent for recently fallen snow to 30-40 percent for old snow during the thawing period. The snow cover has a low heat conductivity as a result of its low density (0.05–0.1 g/cm3 for new snow, 0.3-0.4 g/cm3 for dry snow at the end of winter, and 0.5-0.6 g/cm3 for snow several years old on glaciers). The snow cover is layered and granular. The layers form as a result of interruptions in the deposit of snow when the surface becomes dirty and a crust forms. The granularity results from the processes of recrystallization of the snow layer, in which plate and column snowflakes are converted into shapeless grains of various sizes. The snow cover settles and becomes compacted during the course of the winter. Cross sections of the snow cover at the end of winter show the history of preceding snowfalls and the accompanying weather conditions, as well as heat reserves in the underlying ground. When there are sharp fluctuations in temperature within the snow cover, the individual layers are loosened and the bonds between the layers are weakened.
The snow cover has an enormous influence on climate, relief, hydrological and soil-forming processes, and the life of plants and animals. It prevents deep freezing of the soil and thus preserves winter crops, absorbs nitrogen compounds, thus fertilizing the soil, adsorbs atmospheric dust, and cools the layers of air near the surface of the earth. It feeds all glaciers and many rivers during thaws. Meltwater is the chief source of water for the rivers of the plains of Eastern Europe, Siberia, the northern part of North America, and most of the mountain rivers of the temperate zones. In mountainous regions significant amounts of snow are moved in the form of avalanches. When blizzards cause a deep snow cover to accumulate in the forest-steppe and steppe zones, gully erosion in the spring is more intense.
The snow cover serves important functions in agriculture. It contains significant moisture reserves, which ensure good harvest in many regions, and consequently integrated snow retention methods are implemented to preserve the snow cover on the fields. The amount of snow accumulated over the winter and the nature of the thawing period determine the volume of spring high water. During the winter, roads made of snow and ice for motor vehicles are built in swampy and relatively inaccessible taiga and tundra regions, storehouses are built from snow and ice, and airfields are laid out on the snow. Much effort is needed to protect railroads and highways from snowdrifts. Regular observations of the depth, density, longevity, and albedo of the surface of the snow cover are conducted at meteorological stations in the USSR. Snow reserves are measured by surveys at the stations and along set routes. Surveys from helicopters, aerial photographs, observations from spacecraft, and measurements of the radioactivity of the ground underneath the snow are important in the study of the snow cover.
Snow science, a part of glaciology, studies the scientific and applied aspects of the snow cover.
REFERENCESVoeikov, A. I. “Snezhnyi pokrov, ego vliianie na pochvu, klimat i po-godu i sposoby issledovaniia.” Zap. Russkogo geograficheskogo ob-va, 1889, vol. 18, issue 2.
Rikhter, G. D. Snezhnyi pokrov, ego formirovanie i svoitstva. Moscow-Leningrad, 1945.
Rikhter, G. D. “Rol’ snezhnogo pokrova ν fiziko-geograficheskom protsesse.” Tr. In-ta geografii AN SSSR, 1948, vol. 40.
Kuz’min, P. P. Formirovanie snezhnogo pokrova i melody opredeleniia snegozapasov. Leningrad, 1960.
Kuz’min, P. P. Protsess taianiia snezhnogo pokrova. Leningrad, 1961.
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Kotliakov, V. M. Snezhnyi pokrov Zemli i ledniki. Leningrad, 1968.
G. K. TUSHINSKII [23–1887–]