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Show The contributory avalanche factors are valuable primarily for recognition of direct- action hazard, the slides which a re the immediate outcome of a storm. They also offer definite clues to the probability of delayed action hazard conditions. 1. Old Snow Depth. This affects hazard in two ways. It alters the contours and in this connection might almost be called a terrain factor. First it covers up local obstructions such as brush, rocks and small gullies. As the season progresses it gradually obliterates more important features such as terraces, basins, larger gullies and outcrops, and stands of timber. The second function of the snow already in place is as a possible supply of ammunition for an avalanche starting on the surface. If the snowpack is harboring a slab, then the old snow may start the avalanche itself through the collapse of an unstable layer. In both aspects, old snow depth acts as a factor progressively more favorable to avalanches. About two feet is generally sufficient to cover minor ground obstructions and get the process started. 2. Old Snow Surface. This varies from the different crust types to loose snow in any degree of consolidation. The crusts offer a low friction base for new snow. They also resist penetration into the snowpack, so that we have here the first of the dual- acting factors. A crust may be promoting an avalanche in new fallen snow and at the same time limiting its proportions by confining it to the new snow. The lighter crusts have a tendency to disappear shortly after they are interred in the snowpack through metamorphosis. The heavy rain crusts, or an icy crust formed by a midwinter thaw, usually persist throughout the winter. They offer an ideal sliding base for a slab that lies above. On the other hand, they lock all the snow layers beneath in place. Even a heavy crust can be absorbed. At Alta a rain crust strong enough to walk on was completely absorbed by a fall of moist snow following a cold snap. No doubt heat exchange between the two layers was the cause. Whether or not a crust persists after it disappears into the snowpack can only be determined mechanically: by excavation, by sounding with a probe, or the penetrometer. A loose or partly consolidated surface has high friction and offers a good bond to new snow. On the other hand, it offers little resistance to penetration and may become involved in any slide action that occurs. This contradictory state of affairs with respect to surface conditions is not as baffling as it looks. A crusted surface means that avalanche action is more likely and will probably be confined to the new snow. An unconsolidated surface means that an avalanche is less likely but will probably involve some penetration in depth if it occurs. The chances are that so much new snow will be required to get such a process started that a hazardous condition would be forecast in any event. - 47 - |