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Show deep. The brims of such cells are straight- edged, often with a narrow edging of varicolored dust, giving the surface of the snow patch the appearance of a honeycomb. The formation of such cavities is sometimes attributed to the action of warm winds ( thawing cycles). It is not likely that wind alone is responsible for these formations inasmuch as depressions of the same type fill the lower edges of the elevated borders of snow patches where the wind does not reach. In passes ( gorges) where the wind can blow from only three directions, say, no special difference can be observed in the form and distribution of such cells as compared with those in open spaces. Crust and Skin Formation When a freeze follows a rain, heavy drizzle, wet snowfall, or thaw, ice crusts form over the snow. The " day" surface is slick, often rippled; the underneath surface of an ice crust is rough because of the snow frozen to it. When a wet hoarfrost layer has frozen and settled unevenly as bundles of needles, the crust surface may show protuberances. In this case the relief is underlined by color differences in the ice. The protuberances are transparent and the depressions are opaque. Ice crusts break into irregular pieces with ragged edges. Ice crusts may also appear inside the snowcover. Here both the upper and under surfaces are rough, unlike surface crusts which always present a smooth upper surface. Ice crusts inside snowcover are formed under the following conditions : a) when descending thaw water reaches a layer of subfreezing temperature and freezes over it; b) when thaw water descending through loose snow impregnates a lower dense layer which then freezes; c) when a layer of loose snow receives a coat of dense snow ( often with an admixture of dirt) moisturized by influences other than thawing and which later freezes; d) when a layer of wet snow is covered with dry snow and then freezes. The ice grains forming a crust can fuse so thoroughly that they form a layer of nearly clear ice, transparent even when several cm in thickness. In the absence of thawing, these ice crusts gradually break up to form granular snow. Disintegration takes place most rapidly in the lower layers. With thawing the ice crust breaks up into loose, rounded grains of wet neve. With favorable weather conditions crusts may form over large areas. During the falls of 1935 and 1936, the mountains and valleys encircling the town of Kirovsk were completely completely covered with a 5 cm ice crust overlying a layer of loose snow. During fog or light hoarfrosts a surface layer of loose snow is transformed into a dense, more or less elastic skin consisting almost entirely of crystals and preserving almost completely the structure of snow. The thickness of this skin is only 1- 3 cm, still enough to preserve the underlying snow from wind destruction. A skin on or not too deep beneath the surface may be the base for formation of an ice crust under the right conditions. During the second phase of winter, under the influence of direct radiation from an already high sun, the snowcover surface becomes shiny, almost as though - 16- |
