OCR Text |
Show But tho snow observer's chief aim is to distinguish between the three stages of general avalanche hazard: Insignificant, requiring no action on his part; moderate, requiring restrictions in high hazard areas only; and major, requiring maximum protective measures. Basis of Avalanche Hazard Forecasting The principal basis of avalanche hazard forecasting is the observer1s snow experience in general and his familiarity with the area in particular. Because of the variables involved, many of them pulling in different directions at the same time, nothing could be more misleading than arbitrary standards or formulae of avalanche hazard. Insignificant and major hazard conditions are not difficult to recognize. It is on the borderline between moderate and critical danger that the snow observer can demonstrate his skill. The ability usually called intuition is nothing of the sort. It is good training in snow craft plus personal acquaintance with the individual character of the slope®. The most dangerous slide path is not always the most impressive to look at. Terrain analysis would not, for example, reveal that the part of Peruvian Ridge at Alta lying close to the lift is safe 95 percent of the winter, while another part, almost exactly the same in grade, contour, and exposure, is dangerous 95 percent. Only familiarity can convince the snow ranger that the Hellgate slides are mor^ spectacular than dangerous while a very modest appearing abutment half way down Flagstaff Mountain is a slab avalanche breeder of the worst kind. Detailed and systematic records of past avalanche performance are a type of area familiarity which can be passed on from one administrator to another. They give the observer, new or old, a coherent, annually increasing body of information and reference material. Under comparative analysis, these records reveal definite standards of maximum and minimum hazard. They are the basis of avalanche hazard forecasting and safety procedure at Alta. Not all avalanches are the direct outcome of storms, but the majority at Alta are. Delayed action avalanches are discussed separately. On the following charts, over UO major Alta storms are reduced to their common denominators. Wherever possible, the avalanche factors are expressed as ratios suitable for direct comparison. The information is highly concentrated. Comments and explanatory notes follow but the serious student of snow behaviour will find opportunity for his own analyses. Some of the obvious comparisons are indicated. Fact or values favorable, unfavorable, and negative to avalanches are assigned according to the following table. The dividing lines are arbitrary from necessity. They represent the observer's best judgment on the basis of experience and information to date. See Page |