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avalanche,rapidly descending large mass of snow, ice, soil, rock, or mixtures of these materials, sliding or falling in response to the force of gravity. Avalanches, which are natural forms of erosion and often seasonal, are usually classified by their content such as a debris or snow avalanche. Speeds can reach over 200 mi per hr (300 km per hr). They are triggered by such events as earthquake tremors, human-caused disturbances, or excessive rainfall on high gradient slopes, often where materials are loosely consolidated or weathered. Avalanches of snow result when weak layers within a snowpack fail to support the weight of the snow above it and collapse, causing the overlying snow to break free and flow downhill. A high proportion of people who die in snow avalanches trigger the event, typically when skiing or snowmobiling in backcountry areas where the risk of an avalanche is higher. Destruction from avalanches results both from the avalanche wind (the air pushed ahead of the mass) and from the actual impact of the avalanche material.
avalancheA process such as that in which a single ionization leads to a large number of ions. The electrons and ions produced ionize more atoms, so that the number of ions multiplies quickly. See Geiger counter.
(Russian lavina, from the German Lawine, from the Late Latin labina, “landslide”), masses of snow on mountain slopes that begin moving and slide downward.
Avalanches may occur in all mountain regions where there are stable snow covers. Snow accumulated on mountain slopes is freed when (1) the slopes are overloaded with snow during a storm or as a result of low cohesive force between new snow and the underlying surface in the first two days after a snowfall ends (dry snow avalanches); (2) a water lubricant is formed between the bottom surface of the snow and the underlying surface of the slope during thaws or rains (wet snow avalanches); or (3) a loose layer consisting of disconnected deep-lying frost crystals forms in the lower parts of the snow stratum (sublimation diaphthore-sis avalanches). The reason for the loosening is that temperatures are higher in the lower snow horizons, and water vapor moves from there to the higher (and colder) horizons. This causes evaporation of snow in the warm horizon, and it becomes the slide horizon.
Three types of avalanches may be distinguished, depending on the nature of snow movement along the slopes: osovy (snowslide) avalanches, which slide along the entire surface of the slope without channels; chute avalanches, which move in hollows, ravines, and eroded furrows; and leaping avalanches, which fall freely from ledges. The average speed of avalanches is 20–30 m per sec. The fall of an avalanche is usually accompanied by a low whistle (if dry snow is falling), by a scraping sound (if the snow is wet), or by a deafening roar (if an avalanche wind occurs). The frequency of avalanches and their size depend on the morphology of the avalanche. Chute avalanches from deep hollows occur frequently but are not large; avalanches rarely originate by disruption of cirques, but when they do, such avalanches reach huge proportions. The remains of avalanches usually form névé basins.
Avalanches have enormous destructive force. They cause major disasters and prevent the normal operation of roads, industrial installations, and sports facilities. Preventive measures include maintaining a mountain avalanche service and mountain engineering surveillance, forecasting the time when avalanches will come down, and artificially setting off avalanches by firing guns and by setting off explosions. Engineering protective measures consist in preventing snow from sliding in avalanche channels by planting trees on the slopes and reinforcing them with support structures, using guide dikes to divert avalanches from the objects being protected, and running the avalanches over the object by means of suspended covers and tunnels. In areas subject to avalanches special maps are compiled indicating regions with significant, medium, and slight danger and also regions with potential danger—areas that are safe at the present time but could become dangerous if forests were cut, earth were removed from the slopes, and so on.
REFERENCESTushinskii, G. K. Ledniki, snezhniki, laviny Sovetskogo Soiuza. Moscow, 1963.
Losev, K. S. Laviny SSSR. Leningrad, 1966.
Lavinoopasnye raiony Sovetskogo Soiuza. Moscow, 1970.
Inzhenernaia gliatsiologiia. Moscow, 1971.
Karta lavinoopasnykh raionov Sovetskogo Soiuza. Moscow, 1971.
G. K. TUSHINSKII
What does it mean when you dream about an avalanche?
An avalanche signifies being overwhelmed, especially by emotions that could not be experienced or previously expressed owing to the “frozen” nature of the individual.