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Show CHAPTER 5. AVALANCHE HAZARD FORECASTING The purpose of avalanche study in this country is to provide the administrator in the field with reliable methods of estimating avalanche hazard. There is no thought in this objective of predicting avalanche occurrence exactly either in time or location. Actual avalanche occurrence is governed by a set of variables which cannot be reduced to any such mathematical formula as x plus y minus z equals avalanche. Even if it were technically feasible, prediction of the exact time and location of an avalanche would be of secondary importance to the administrator in the field. His problem is to recognize the development of a hazardous situation in time to do something about it: Clear the traffic from a highway; get the skiers out of an area; artificially stabilize a slope before the hazard climbs to menacing proportions. Since the safety margin he must maintain is generally measured in hours and an avalanche hazard can disappear in minutes, a hazard forecast is what the term implies and no more. It means that if a given set of conditions persists, avalanches of dangerous size are either likely or unlikely within the time margin of safety. Fortunately for the avalanche patrolman, hazard does not appear all of a sudden like a reef in a fog. It is more like a storm which heralds its approach with a falling barometer, wind and temperature changes and a bank of clouds. The storm may never do anything but threaten, but the prudent farmer gets his hay into the barn. THE TEN CONTRIBUTORY AVALANCHE FACTORS Terrain and climate are the basic causes of avalanches: Terrain is a constant, always prepared to do its part. Climate is the variable, from season to season and at shorter range, in which case we have agreed to call it weather. The contributory avalanche factors are subdivisions of climate and weather. Though he never heard of hazard forecasting, the principle of using these avalanche factors as guides was summed up in 1910 by Beals: " Not the quantity of snow alone, but also the manner in which it fell." In order to discuss these contributory factors coherently, it is necessary to identify and discuss each as if it were a separate entity. We remind the reader that in nature they are not separated, but function as a combination. We have identified, recorded and analyzed ten climate or weather factors which influence avalanche hazard. They are all variables: their relationships are complex. Not infrequently they even play dual roles, favoring avalanches in one way or in a particular location while inhibiting them in another. Nevertheless, observation has revealed certain very definite patterns which can be recognized and employed as guides by the man in the field. - 46 - |