| OCR Text |
Show cohesion. All snow that is more or less firm, with a density of at least 0.2, sometimes called " layed" or " settled" snow, is formed by wind transfer and thawing rather than by lying following a fall. Factors Influencing Recrystallization Rate The ice surfaces inside snow are very great, and for this reason the water vapor pressure in the pores between crystals is nearly always the same as that of a saturated vapor over ice. The changes in snow take place with upward water vapor transfer by sublimation. Thus, the greater the water vapor transfer, the more rapid the rate of transformation taking place in the snow. Snow porosity is an important factor in this connection. Diffusion as well as wind transfer of water vapor is more extensive in loose than in dense snow, causing loose snow to change at a faster rate and to a greater degree than dense snow. This difference can best be observed when a stratum of dense snow lies between two loose layers. Here the two loose layers will show large crystals, while the dense layer will have only begun to acquire a fine- grained structure. This type of fine- grained hard snow is nearly as firm as the snow from which it came. Due to the presence of ice grains, the surface of such snow, graded and polished by wind action, is shiny and gives the impression of pavement. Surface patches opened by blows differ markedly in sheen from patches of windblown deposition. The density of this snow is 0.25- 0.50. Recrystallization in dense snow is usually observed when it has reached the second or hard, granular stage and when density is much less. Snow with a density of 0.5 or more usually undergoes no change, remaining essentially the same as it falls. The nature of the underlying ground surface has an effect on the looseness ( friability) of the bottom snow layer and on the changes taking place in it. On ground covered with grass or lichen the snow is loose and undergoes a greater degree of recrystallization than on even, hard ( trampled) ground. The height of vegetation also affects recrystallization. Snow crystals are larger on tall grass than on low grass, as confirmed by the following: Fifty gr of granular snow taken from tall grass in the bottom layer of loose snow and 50 gr of granular snow taken from low grass 1 m away in the same bottom layer were screened separately at the same time. The 1.4 mm mesh screens retained 54% of the snow crystals from the tall grass and 46% of those from the short grass. A second screening gave 50% and 40%, respectively. The constant circulation of water vapor in snow is due to the vapor density differential between the top and bottom layers. In Chibin the air temperature at ground surface, beneath the snow, varies between 0° and - 2° C for most of the winter. The temperature at the snow surface varies with atmospheric conditions and may be considerably lower than that next the ground. The variation ( differential) in temperature distribution may be reversed only by a heavy thaw following a freeze, in which case the water vapor density is less next the ground, gradually increasing toward the " day" surface of the snowcover, and thus creating a constant upward flow of water vapor. Still, water vapor diffusion does not cause loosening and decrease in density of the warmest bottom layer through loss of material ( bulk), with consequent formation of empty spaces. In a normal winter temperature distribution in the snowcover, each layer - 11- |
