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Show others, a fluffy or downy snow. They show no periodicity during the 24 hours, but they increase in number with increase in sunlight. In such cases they originated mostly under cornices, sometimes as long as three days after a snowfall. Where the snowfall is accompanied by weak winds, a " top snow" avalanche takes on some of the features of a slide made up of loose windblown snow and break away at the same type of site, but from gullies rather than cirques. According to foreign sources, avalanches in the ( Swiss) Alps, where the increase in snowcover thickness from one snowfall may be 1 mf are very frequent and disastrous. At the same time, many observers identify these with avalanches of windblown snow. Only Hess makes a distinction between these two avalanche types and points out the difference in their track and type of movement. These avalanches are often accompanied by powerful air waves ( according to some data). In Chibin air waves were observed only with extremely large avalanches dumping " shouldered" ( ridged) deposits. 2) Pulverized snow: Avalanches of loose drifted snow occur during or within one or two days following a windstorm. They break away with a step or line rupture. The snow descends as a bound unit section, undergoes milling and then flows as a loosened mass, forming a cloud of snowdust. Only small avalanches of this type follow the relief and are deflected accordingly. Large avalanches, because of their high velocity, follow their primary heading and overrun obstacles such as log ore- loading platforms, and even stream beds and breaks 10- 15 m wide in the relief. Dense snow or an ice crust serves as the undersurface for this type of slide; sometimes the entire sub- slide cover is taken off right to the ground. The deposit has the form of a flat ( low) elongated cone with " shoulders", sometimes showing flattened waves concentric with the forefront of the deposit. The avalanche snow is a relatively loose homogeneous mass of density 0.20- 0.50, which quickly hardens. The debris is sometimes strewn with fragile balls the size of paving stones. Pulverized snow avalanches, like those of " top snow," do not form troughs or roll up snow masses. One frequently observes tree branches and rocks in the dump snow. Pulverized snow avalanches come mostly from gullies, both shallow and deep; the large ones of this type, from cirques and gullies. The break- away site is usually the head of a shallow gully, below precipitous rocks where snow has drifted. The break- away slope is 30- 45°. The volume of this type of avalanche ranges from several tens of m3 to several thousand; the greatest observed was 80,000 m3. in five cases the avalanches were set off by hikers crossing slopes. Pulverized snow avalanches are the most prevalent type around Chibin. On windy days they can be observed in several places, less frequently in just one. Their season begins a month after settling of the snowcover, when snow thickness is 30- 50 cm in the valleys, and ends with the last windstorms of the winter. It was exceptional to observe pulverized snow avalanches following the first windstorms in October. They show no particular diurnal schedule. Orientation of the avalanche slope is of significance only with respect to wind direction, since this type of avalanche is characteristic of lee slopes; but because of wind deflection by features of the relief, such avalanches sometimes descend slopes facing the general wind direction. Because of their speed, pulverized snow avalanches are very destructive, being capable of demolishing not only frame - 46- |