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Show SNOW AND SNOWSLIDES Snow Cover, Its Formation and Physical Properties In winter snow falls frequently; two days out of three is the average, but most of the time the precipitation is negligible and a 10 cm per day increase in snow cover is comparatively rare. The snow residue ( accumulation) for the six winter months, November through April, varies between 250 and 400 mm. Snowfalls are composed of crystals varying in form and size and in the amount of liquid water carried. These are the features that define the nature and structure of snow cover formation. Since variations in the structure of soft ( loose) snow which are of considerable significance in snow storms and in avalanche production are negligible at first glance and rapidly disappear, we shall describe the most essential variations in fresh snow cover. Freshly fallen snow is transported by winds with forces of 5 m/ sec. and up. ( Type) A: This is a soft but bound snow layer composed of crystals of different sizes and shapes; it does not fall apart ( scatter) or fill in tracks, and it separates into lumps of indefinite shape and will take a clear print ( of the foot or other object). This type of layer is formed during low temperature frosts and is the most common variety of snowfall. Density is 0.05 - 0.10. Type A does not produce avalanches but provides much of the material for snowdrifts. ( Type) B: This is a fluffy unbound snow composed of large but fine and well- formed stars measuring up to 10 mm and of platelettes measuring up to 6 mm across. This snow covers the ground with a soft, light, downy layer that settles fast but which remains mobile and will cover a ski track. A dark object can easily be seen through a layer 10 cm thick; and a 20 cm layer is still so transparent to sunlight that even when the sun is in low position a clear outline of the hand shows through. This type of snow falls during still weather with freezes varying from - 20° to - 10°, rarely - 5° C, and may increase the snow cover by as much as 30- 40 cm a day. Density is 0.02 - 0.05. Due to its fluffiness, even a layer of considerable thickness does not provide the material for snowdrifts, but it can produce numerous slides. ( Type) C: This is an unbound snow consisting of platelettes and stars measuring about 5 mm in diameter. In spite of its tendency to scatter ( come apart) easily, it covers tracks much less than fluffy snow. It falls often but does not form layers of any considerable thickness. Density is 0.04 - 0.10. This snow sometimes produces avalanches. ( Type) D: This snow is composed of amorphous grains, columns, needles, and very small stars. It falls often, but even during a big snow the layer thickness does not exceed a few mm. It is of no significance in snowstorms and slides. The literature often states that such powdery snow prevails in the polar regions and that the snow cover there is formed essentially by this type of snow. It is frequent in the far north and often falls from a cloudless sky or from clouds unproductive of feathery, high- piling flakes, never laying down a cover of any significant thickness. During thirteen years of observation above the Arctic Circle, on the Kolski Peninsula and at Tymyr, the writer only once observed an - 3- |