OCR Text |
Show grain size. The increase in cross area of these empty spaces creates a disproportionate idea of great volume ( bulk), especially when the snow material is unevenly distributed as in fibrous snow. Such snow is comparable to a fog of constant water content, which will appear thicker the smaller the component droplets Where the volume of the individual spaces is markedly greater than crystal volume, the crystals will pack more densely on redistribution; and as a result, a layer of coarse, granular, fibrous snow with a density of 0.15- 0.18, crumbling easily with slight pressure, will occupy from one and a half to two times less space. The transformation of fresh snow into fine granular snow continues for nearly two weeks. The growth of the snow grains ( to 2- 3 mm) goes on for another month. By this time discrete crystals of regular form begin to appear in the snow; and in another month these crystals show in considerable number, giving the snow a coarse granular texture. Thus, the three stages of the recrystallization process take 2- 3 months. It is of interest to note that changes in snow proceed at the same rate under different climatic conditions. In Chibin, with its comparatively mild winters, on the Tymyr Peninsula with its - 50° C temperature and permanent frost, in the steppe- forest region around Krasnoyarsk, in the black- soil zone south of Moscow, we have observed coarse crystals in the bottom layer of loose snow at the beginning of January. Factors which strongly affect recrystallization rate are present at the same time, factors capable of either accelerating or completely halting the process. These factors will be mentioned later. Following a snowfall, the snow does not fuse at first, but later on, during the recrystallization process, when particle cohesion and solidity decrease. Fusion takes place at first as a result of the destruction of the crystals of discrete snow particles. Following this, increase in grain size begins. The number of grains and thus the number of points of intergranular contact now decrease, causing the formerly bound snow to become friable. The term " free-flowing" can be applied to such snow. Weakening of the bonds between particles takes place in spite of settling and the consequent increase in density. As an example of the fragility of snow lying in a thick layer, the following is a case in point: During February, 1935, in a forest tract on the south side of Mount Aykuayventchorr, the entire snowcover, 2 m thick, was too weak to support a man, who would plunge straight through to the ground. One could even trace the un-evenness of the ground by feeling through the snow with the feet. To make progress through this snow, one had to brace oneself progressively backwards. The upper half of this snow consisted of small- grained pellets. Density was 0.12. We could not measure the density of the lower layers because we could not prevent the top snow from refilling the test hole. One seldom encounters such a heavy snow cover, and then only in forest tracts where the wind does not penetrate. As recrystallization proceeds, the snow becomes more resistant to wind transfer. The possibility of wind transfer is reduced after the snow has lain on the ground for even a few days. Granular snow can be transported only by a strong wind. This is due to the compaction by settling of the crystals which have become fragmented and have thus lost part of their flight capacity. As mentioned above, the recrystallization of snow does not increase particle - 10- |