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Show of recrystallized fine granular snow rather than of freshly fallen snow. 4) " Lump snow": As it decreases in stability ( solidity), one of the bottom snow layers can be destroyed or give way under the weight of the overlying layers or by some external agency; and the upper layers, by losing their support, begin to avalanche. It is mostly layers of coarse granular snow that crumble in this way, but layers of fine granular snow give way also, especially those of fibrous texture. Such avalanches break away only in steps, and the same layers detach along the whole rupture. Since the bottom layer is the least stable, the usual case is for the entire snowcover thickness to detach and avalanche. Where the slide relief is even, the dump of an avalanche of this type of snow is a humpy elongated cone, in the snow of which are interspersed broken slabs, some of considerable size, torn from place in the snowcover above. Where the avalanche track is long and the relief is broken, the dump is a chaotic accumulation of milled and compressed snow interspersed with lumps and balls, sometimes showing slide troughs and " flatirons." Ahead of the forefront of the dump, one often observes " shoulders" or evidence of " kneading" in the snow, and sometimes removal of the snowcover for a short distance in advance. Density of the avalanche ( debris) snow is 0.40- 0.55. " Lump ( or ball) snow" avalanches often take place in cirques, wide gullies and on broad, unbroken slopes. Their break- away slope is 30- 45°. Such avalanches are the largest in the Chibin area, their volume being measured in thousands of m3. Most avalanches of 20,000 m3 and over are of this type. The greatest volume recorded, 285,000 m3f was for an avalanche of December 22, 1936, which descended from a cirque on the north side of Mount Aykuayventchorr; 600 m3t the smallest volume observed, was for an avalanche in which an entire snowcover 1 m in thickness slid down the unbroken side of a railroad bed. Despite their great volume, " lump snow" avalanches follow the slope relief more than those of pulverized snow; also, their speed is less. They cause great destruction because of the powerful air wave produced. Because " lump snow" avalanches are ultimately due to recrystallization of one of the lower layers in the snowcover, they occur only when internal snow changes have proceeded to this point, i. e., around mid- December. For reasons not yet clear, they are infrequent after the beginning of March, although melting has not yet begun by this time. Measurements of snow strength at the rupture steps have shown that the tensile strength of the snow bed or section was often sufficient to have held the snow on the slope. Break- down or collapse ( mechanical load) of the bed, rather than lack of cohesion, is the direct cause of avalanching here. In three out of four cases of " lump snow" avalanches precipitated by hikers, the witnesses noted a loud collapse or cave- in of the snow bed as the first sign of the avalanche. Not until after this was the first motion of the slide noted, slow at first and then increasing in speed. In the fourth instance the slide circumstances were not entirely clear. Most " lump snow" avalanches occur with windstorms. This does not mean that in every case the weight of the drifted snow collapses the weak lower snow layers Penetration of the snowcover by the wind speeds recrystallization, and the - 48- |