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Show gauge the hazard in time to do something about it. For this 'reason if no other, no snow observer can ever hope to make a perfect score. He cannot guarantee an avalanche every time he closes a slope or a general cycle every time he closes the area. In two hours the picture can change entirely. And even a superficial glance at the charts will convince anyone that the dividing line between avalanche conditions of moderate or high intensity is often too shadowy to distinguish until after the shovr is over. Delayed Action and Fair Weather Avalanches Delayed action and fair weather avalanches are not frequent at Alta. The case can very well be different in other areas with other climatic conditions. But whatever their number, they are a continual source of worry for the administrator. When prolonged cold follows a storm, the enow does not pack and settle and may remain unstable indefinitely. Dry* snow avalanches may occur long after the skiers have forgotten thQ storm that made them possible. A much more dangerous condition is windslab which remains unstable even after all other slopes have settled. The Swiss have made exhaustive Studies of slab in the field and in the laboratory. They have developed a technique known as the ram profile of testing for slab. This consists of driving a rod into the snowpack and determining its structure from the variable amount of resistance encountered. This process is interesting, but has obvious limitations for the practical observer. In the first place, he has neither the time nor the facilities to make ram profiles of an entire ski area. In the second place no ram profile can tell him when or tinder just what conditions a slab is going to break off. Unless they fracture early, during a general $torm avalanche cycle, slabs are apt to sTiow a persistent if false stability. They stay in place under great weights and then break at the touch of a ski or with the addition of an insignificant amount of new snow. (See the major storm comparison chart, #6S.) In the fracture plane of a major slab avalanche from Mt. Baldy at Alta at least four slab layers were visible* In another from Peruvian Gulch, the fracture was approximately 8 feet deep* For delayed action avalanches of any type, the snow ranger has several answers. One is to keep the public out of dangerous locations. It is his job through familiarity with his area and constant field testing to know where the danger areas are« He can then act to get the slide down and out of the way as soon as feasible. The only good avalanche is one that has happened. On most slopes the experienced observer, knowing the release points, can ski them down with reasonable safety. On the big slide paths he must use explosives. Overhanging cornices are a delayed action hazard similar to slab. |
