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Show few seconds or even minutes after being released. For example, if a party leader crosses a hazardous slope safely and the avalanche picks up the third or fourth person following, it is of the hangfire type. For the unwary traveler, one slope is about as dangerous as another, but conformation does yield clues to the safest routes. A crossing at the top is always first choice, for regardless of where or how the slide breaks, there will be a minimum amount of snow above the skier. A crossing anywhere within the main body of the potential slide is last choice. At the toe, the traveler must give a slidepath the widest possible berth so as to avoid cutting the anchorage. The dimensions of a slope play an obvious part in hazard. They govern two of the measurements of any avalanche the slidepath can produce: length and width. Climate determines the third dimension: depth. All of the terrain features mentioned above are less important than grade. The higher the slope angle, the more likely it is to slide regardless of any other condition. Authorities generally agree that the minimum angle favorable to avalanches is 25 degrees. Snow must be extremely unstable to move on slopes between 25 and 30 degrees. Slopes from 25- 35 degrees may avalanche, especially if stimulated by the cutting action of a pair of skis or some other outside disturbance. The critical zone lies above 35 degrees. From this angle to the point where snow can no longer cling except by windpacking, slopes are apt to slide in some fashion with every storm. Vegetation and Timber Vegetation of any kind except grass has a restraining effect on avalanches. Tree cover is the most important. Alta, for instance, was quite heavily forested when the miners arrived. As they removed the trees for fuel and mine timbers, avalanches grew progressively worse. With the passing of the mine era, natural reforestation has made good headway and many slidepaths are becoming inactive. Perhaps the classic example of the effects of deforestation is the slide cycle of 1910 in Washington. The most destructive avalanches ran on slopes where the timber had been destroyed by fire. Logging operations may have the same effect on occasion. Heavy timber is an avalanche preventive provided the cover extends to the tops of the potential slidepaths, as it did in the Cascades before fires and logging opened them up. ( Figure 10). Heavy timber is an effective avalanche barrier but not invulnerable. There are many instances where even mature forests have been destroyed by avalanches of unusual power, starting from treeless land above. In general, the existence of heavy forest cover is an indicator that slides in that location are rare or of minor importance. Conversely the characteristic debris- strewn paths cut through forests by snow action are a valuable guide to the observer in unfamiliar terrain. - 24 - |