| OCR Text |
Show Blown snow is perfectly loose right after its deposition, and only after several hours does it begin to freeze together. This freezing continues 2- 3 days; and when the snow is very dense and the freeze intense, even longer. In such case, the density of the layer increases markedly. This same increase in density is observed in every dry, mixed snow. Loose snow removed from railroad tracks remains unbound for only a short time, and subsequent removal may be difficult if it has remained untouched for any length of time. Snow easily moved with wooden shovels at first is difficult to handle later on with metal ones. This same phenomenon is observed in avalanche snow. The layers of a fresh avalanche can be so loose that a leg will sink in up to the knee. After several hours the snow is frozen together, but is still easy to break up with a wooden club. In 2- 3 days it will support a man and must be broken with a metal pick or bar. Snow density is not so much influenced by compression and decrease in volume as by the mixing and compactness of the particles packed. The sintering of snow particles is not produced by melting under pressure at points of contact, nor by freezing of the clinging water. The pressure of an overlying snow layer is not enough to produce snow melt even at mild freezing temperatures. The melting point of ice is lowered 0.0074 for every atmosphere of pressure; and to melt ice at the mild freezing temperature of - 4° C, a pressure of nearly 500 kg/ m^ would be required. If pressure were to be exerted on snow with sufficient force to melt ice at the points of particle contact, the result would be the destruction rather than the melting of the snow crystals, because the continuous pressure load would in time exceed the structural resistance limits for solid ice. Yet snow sinters at a temperature of - 50° C. Loose know composed of ice grains measuring 1- 2 mm remains quite friable when compressed in a cylinder at a pressure of 40 g/ cm^. This pressure is that of a snow cover with a density of 0.40 and a height of 10 m at - 5° C. This same snow will sinter in 2- 3 days when piled. If snow sintering were the result of ice melt under pressure, it would appear immediately following the pressure, not in 2- 3 days, and certainly not after the pressure is relieved. Most probably, snow sintering is a phenomenon occurring in loose hygroscopic material remaining undisturbed for a sufficient time. When snow is compressed into a ball, the increase in density is not the result of melting under pressure. The shape of the snow crystals is the important factor. The more ramiform these are, the more easily the snow can be compressed into a ball. Wind- driven snow is not homogeneous. Even slight variations in wind direction and velocity produce distinct layers. Often a layer laid down by the same snowstorm can be differentiated into several strata of different structure; observation of the lee side of a snow pile shows that such a layer is not homogeneous in composition. Here the pile is sorted and differentiated from top to bottom. Fraction zones vary according to variations in the wind gusts. With strong winds some whole ice grains, sand and even gravel are transported with the snow dust. In such a case the heaviest particles remain at the top and the lighter material is carried downward by the wind. When sand and dust are transported with the snow, a dirty line will show along the top of the snow accumula- - 7- |
