| OCR Text |
Show snow avalanches, especially the late ones, slip away right to the ground, carrying down rocks and surface soil, breaking off trees, and producing a great noise. Where the avalanche moves down over snow, it takes off the upper layers, leaving rupture steps behind. By this late time in the season, the underlying snow is wet to a certain extent; no case was observed of a wet snow avalanche descending over dry snow. On unbroken slopes the dumps are flattened, elongated cones with the forefront covering the snowcover a short distance ahead; but against obstacles and at breaks in grade, the dumps are chaotic accumulations of compressed and knurled masses with empty interspaces. The dumps are crossed in all directions by crevices and often show sizeable troughs gouged out by successive slides and individual snow blocks. An avalanche trough, not large, was observed in only one case, cutting through the dump of a previous avalanche. It is quite possible, however, for wet snow avalanche troughs to be as large as those of " lump ( ball) snow" type. We have noted no more possibly because no large- scale avalanche of wet snow was observed to descend from a cirque through a gully during the period of our observations. Moving at less speed than dry snow avalanches, sometimes no faster than a running man, wet snow avalanches conform to the slope relief to a greater degree than other types, as is clearly shown by avalanche tracks. The dependance of the dump form on relief features is more apparent than with other types. Density of wet avalanche snow is 0.45- 0.70; avalanche volume is measured in several hundred or thousand m3, the largest volume observed being 50,000 m3. Avalanches of wet snow originate in gullies, not deep for the most part, and in cirques, on rupture slopes of 25- 45°. They break away on sharper grades than any other type, mainly on southern slopes during the latter part of the day. However, one of the largest of this type observed, 10,000 m3, broke away at 1: 00 A. M. on the northwest slope of Mount Ukspor, a circumstance that can be explained as the result of a foehn ( chinook) raising the temperature from 1° to 17° C. Even night frosts do not freeze the lower layers of melting snowcover and thus prevent its avalanching. Rain does not precipitate wet snow avalanches. The effect of rains is negligible except when these occur during thaws. Observed only during April and May, wet snow avalanches come second in frequency to those of pulverized snow in Chibin. Freguency ( Recurrence) of Avalanches Depending on the amount of snow and on weather conditions during the winter, avalanche volume and frequency vary between wide limits. Table 9 gives figures for eight avalanche sites along a railroad branch line under constant observation by our station. - 51- |
