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Show Free moisture in the snow, for example, promotes internal cohesion up to a point by making the particles sticky and actually welding them together by freezing. This effect reverses itself when the snow is wet rather than damp. In this case free moisture acts as a lubricant and a thawing agent. Alternate periods of freezing and thawing encourage metamorphosis and settlement. Extremes of temperature either retard or speed up the process. Wind action can affect the snow like settlement, by packing the grains more closely together. On the other hand, warm wind- the chinook of this country and the foehn of Central Europe- is a more effective thawing agent than sunshine. All of these factors, and a number of others play a part in the cohesion of a snow layer. They are variables, difficult to observe and to interpret. They also affect external cohesion. External cohesion is the bond between a snow layer and its surroundings, top, bottom and sides. When new snow falls on an uncrusted surface the external bond is strong. In fact the layers will amalgamate to such an extent that they cannot be distinguished except by mechanical means. The bond between layers which came together under different temperature conditions are apt to be strong because of the effects of heat exchange. It has been observed that crust layers are often absorbed in this process. Between two layers of radically different snow type external cohesion is apt to be weak at the start of the storm and to grow progressively weaker. The best example is slab which lacks resilience and cannot settle at the same rate as the other layers. The internal cohesion of a slab is very strong as shown by its resistance to penetration. Slab avalanche releases are almost always the result of external cohesion failure. Slow motion picture sequences show the original fracture running around the boundaries of the slab, followed by interior shattering. To argue the relative importance of internal and external cohesion would be unprofitable. Internal cohesion appears to be more directly concerned in loose snow avalanches and external cohesion for those of slab type. But there is plenty of conflicting evidence. The fact is that both are involved in any avalanche and their relative importance would be different in every case. The problem is one for basic research. Avalanche Triggers There are at least four ways in which the trigger can be pulled to cause the release of an avalanche: weight, shearing, temperature and vibration ( Page 44). Weight is probably the most frequent. New snow piles up until the structure collapses of its own weight and begins to slide. Shearing can take place in various ways: the slicing action of a pair of skis or any exterior force that cuts the bond. A slide in motion has a shearing effect on the snow beneath, coupled with vibration. Vibration, like shearing, can be applied in different ways. Sound is one. We have on record at Alta an avalanche which was released by a thunderclap. We suspect that the vibration of heavy machinery has caused others, as well as sudden pressure changes from gusts of wind. In our experience. - 43 - |