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Show further thickened by snow transported from other places. With steady, stronger winds of up to 6- 8 m/ sec. velocity, snow is transferred in streams running over the snow cover surface, following the same channel for hours at a time, dividing upon meeting obstacles and reuniting later. This is the type of snow transfer characteristic of " ground blows". Snow is transported by separate ( individual) whirlwinds when the wind is gusty rather than steady. When the wind is strong, surface snow is lifted high, sometimes to tens of meters, and is transferred as a cloud of snow dust. Observations on snowstorms show that most snow is transported by the wind at low levels, up to 20 cm above the surface. Here the snow crystals collide, strike the snow surface, and break up. The more t, he platelets are broken up, the denser the snow layer laid down. This accounts for the density of windblown snow drifts, even where the wind does not exceed 8- 10 m/ sec. velocities. With low blowing and during the average snowstorm with the same wind velocity, when part of the snow is transported high over the ground and is not broken up, a softer layer of snow is deposited. The longer it takes for the snow transfer, the denser the arrangement of snow particles. The longer the snowstorm ( blow) lasts, the denser the drifts. Strong winds destroy even a strong, dense snow layer. During a blow January 23- 26, 1936, a wind of 25 m/ sec. velocity blew the whole of a snow cover 50- 80 cm high onto open ground. At the top were 35 cm of snow with density 0.25 - 0.30; under this was a coarse, grainy snow. Only moist and wet snow cannot be blown by wind. On March 2, 1935, a wind of 34 m/- sec. velocity, temperature + 3° C, transported no snow, but just carried away lumps from piled snow. This same wind ( 12 balls) destroyed brick chimneys, blew down telephone poles and carried off roofs. Dense, dry snow can be blown out in thin layers unevenly distributed, and at different times, over the whole surface. This action produces steps in the snow surface. When the wind is strong, the blown snow corrodes deep trenches and depressions in even dense snow cover. The heaps remaining from old dense snow are elongated in the direction of the wind. On the lateral surface of these heaps or ridges polished by blown snow, one can clearly distinguish the multiple layers difficult to observe otherwise. The higher parts of these elongated heaps headed upwind sometimes tower in peaks which in time bend over. Also, tracks in the snow surface become more prominent with time. In places where icy snow remains intact because of its density, the surface becomes polished like a mirror. Gusty winds of inconstant direction deposit snow in formless piles or in layers of varying thickness. When . wind direction is constant, the snow is laid down in parallel rows of flat ridges which may appear on valley floors or on mountain slopes. Less often the snow is piled in dunes. Like sand dunes, these snow dunes have their steep slopes on the leeward side and their more gentle slopes to windward; they are mostly of irregular shape, are 3- 5 m in diameter, and are 20- 30 cm high. They form in a disordered fashion over the snow surface and move gradually with the wind, forming in open valleys with steady winds of constant direction and long duration only when the driven snow is of coarse texture. - 5- |