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Show Contributory Avalanche Factors Major storms are the principal direct cause of avalanches at Alta. This is just another way of stating the basic formula: Sufficient snow in a favorable location. A storm is divided for analysis into a number of factors of varying effect and importance. Of these natural factors, ten have been identified and evaluated to the point where they can be used as a basis for the study of snow behaviour and the forecasting of avalanche hazard at Alta. They are: 1. Old snow depth. 2. Old snow surface. New snow depth. k. New snow type. 5. New snow weight. 6. Sate of accumulation. 7- Wind force. 8. Wind direction. 9* Temperature developments. 10. Snow settlement. It can be argued that the terrain is also a variable and should be included in this list. If avalanche study had reached the point where forecasters could attempt to classify the hazard for each particular slope in the area, this would be true. Such classifications could probably be made on the basis of present knowledge, but gathering the necessary data a task beyond our resources. The terrain is therefore treated as a constant favorable to avalanches and is not included in calculations of the hazard* 1. The function of the old snow depth is to provide a smooth, even slide ~ path. It begins by covering up minor obstructions such as rocks and lowgrowing brush. As it deepens it reduces the effect and even eliminates entirely the effect of larger barriers like terraces, gullies, and basins, Frequently the old snow becomes involved to some extent and adds volume to the slide. Beyond the point where it covers minor ground obstructions, the old snow depth is always a factor favorable to avalanches. At Alta that point appears to be a 2k to 36 inch depth. The dividing line would undoubtedly be different in other areas. 2. The old snow surface is a good example of Dr. Jekyll-Mr. Hyde character which avalanche factors assume, and another reason why it is impossible to lay down arbitrary rules or formulae of forecasting. Fundamentally the problem is simple. A crusted surface makes a poor bond with later fallen snow and provides an excellent slide path. A loose snow surface offers a good bond and a high friction slide path. Unfortunately we can't stop there. A crusted s"UTface protects the underlying snow from getting involved in a slide* It also promotes an early start of the avalanche cycle so that the unstable snow will come out piecemeal instead of all at once* It should be stated early that these avalanche factors play a dual role most of the time, that they cannot be considered singly on a basis of addition and subtraction, and that we do not have enough data to evaluatethem exactly either singly or in combination. On the other hand we do have enough data to distinguish between dangerous situations and those which are merely unstable. No snow ranger can afford to shut the area down just because a little snow is moving; there is snow in motion on about half the days of a winter season at Alta. Neither can the snow ranger afford to take long chances with public safety. Unable to draw a hairline distinction among the overlapping, dual-performing factors which result in avalanches, he must act when the indicators are markedly favorable. He must also act in |
