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Show steep grades on the slope. As the volume of the snowballs increases, as the slide moves down a gully where there is no space for lateral spread, the balls collide with and block one another, preventing free flow, and start to move as a unit mass, smoothing the underlying snow. Such avalanches " flow" as long narrow streams, at the speed of a running man, and stop on steep grades. The dumps of " knurled snow" avalanches are fans; and in the case of large avalanches descending gullies, tongue- shaped, nearly always showing discrete snow masses or balls projected ahead of the forefront. The dumps are deposited as a thin layer composed of small knurled balls and lumps, a few of which reach 0.5 m in diameter. " Knurled snow" avalanches are of small volume, only a few m3, or in a few isolated cases, a few hundred m3. Since " knurled snow" avalanches are due to warming by the sun, their diurnal and seasonal regimes are clear. As a function of solar position, they occur on different parts of the slope at different times of day, depending on exposure. It is a rare sunny day in March and April when several such avalanches do not occur, often in such number that many deposit fans overlap one another, forming networks over great stretches of mountainside. This type of avalanche is rare during cloudy weather, taking place only with thaws. Like " top snow" avalanches, those of " knurled snow" take place mostly on steep, precipitous slopes, at grades of 45- 60°. A peculiarity of " knurled snow" avalanches is their capacity to continue or recur for a relatively long period. For example, on March 9, 1935, " knurled snow" flowed slowly down a gully on Mount Ukspor without interruption, in a layer nearly 80 cm thick, and by the end of the day had built up to the height of the snow shields ( fences) on the slope. In popular literature such an avalanche is sometimes described as a snowball descending a mountainside, gradually growing in volume and tumbling into the valley as a tremendous mass. In Chibin this type of avalanche is the most insignificant, i. e., the least destructive. 7) " Rolypoly snow": With spring thawing the upper layers of wet snow begin to slip down slope. Pillows of loose snow that have formed under cornices are first to descend. Slipping away from denser snow, often still dry. they break up into smaller " rolypoly" balls and move down slope in a thin, even layer. The breakaway of such avalanches is line- type; their terminal dump, tongue- shaped. Avalanche volume is not great, a few tens of m3, less often, a few hundred. More " rolypoly snow" avalanches are caused by falling snow cornices than other types. More and more of these avalanches let go as snow melt proceeds; and by the middle of spring the tongue- shaped dumps of this type of avalanche form a continuous border along the upper mountainsides. They increase in volume also; the large ones break away in not too high a step and descend the slope to points of still sharp grade. April- May is the period for the avalanching of " rolypoly snow." 8) Wet snow: The deeper layers of wet granular snow are the last to melt. Their break- away is step- type. Wet snow descends as a unit section or bed, sometimes for its entire thickness. A wet snow avalanche, unlike those of " flow" type (" rolypoly snow" type, for example) with which it has much in common, does not scatter or disperse in small balls or lumps, but descends as a unit mass of large chunks that pile up in wave- like ridges. More frequently than other types, wet - 50- |
