OCR Text |
Show compression. The usually high density and strength of avalanche snow in its region of damming and the frequently demonstrated " freezing in" of the victims of avalanches as soon as they come to a standstill are explainable by the development of heat due to flow resistances and compression, which produces first a surface melting and a freezing together of the snow crystals after the compression. From the estimates of Section 2 it can be shown that the total frictional and compression work converted to heat per cubic meter of avalanche snow reaches approximately the value ( Y - Y i) • H if the avalanche comes to rest after the height of fall H measured vertically. The heat from friction and compression approaches approximately W = ( Y - Y ) H/ 427 keal/ m3 L If the air temperature lies in the neighborhood of the freezing point, the heat from friction and compression can melt W/ 80 kilograms of ice per cubic meter of avalanche snow. Which, at any given time, will occur on the surface of the snow crystals. As soon as a film of water forms on the surface of the particle the coefficient of moving friction [ i- is reduced to a fraction of the coefficient of static friction, as has been confirmed frequently by investigations of various materials. If the initial temperature of the snow \ A \ ^ s *** - 7 ^ M7r % '"& S j j ^ s" mir - iS A ~_ >• - , 3.7 F' 8 ->.. A- 00 f -- • gin, 7 f- L , B- 13° C> I 11° C • ^ ** • - - • :_-. • ^ ^ "^ -^. ""- "^ h - ""** » 1,6 2,0 2,4 p in kg/ cm2 2,8 3.2 3,6 U, 0 t. t tfi Fig. 27. Compression of snow at various temperatures. Ordinate - Density in kg/ m3; Abcissa - p in kg/ cm2; mit - with. lies below the freezing point, its average heating by friction and compression will reach about A t = 2W/ y , if the specific heat of snow is taken as 1/ 2 kcal/ kg/° C. The consequences of these secondary thermal effects are not considered further here, since the investigation of a few examples has shown that this does not impair the fundamental usefulness of the laws of compression derived in this section. 37 |