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Show There are several other avalanche factors worthy of mention but they cannot be analyzed without more study. lew storms are continuous. Observations indicate, as they logically should, that these breaks in the course of a storm are beneficial, even if quite short. Settlement takes place, minor avalanche cycles run their course and relieve the tensions in the snow pack. Not least in inrportance, the snow ranger catches a glimpse of his test areas. Whether or not they have sloughed out will influence all his decisions during the next onset of the storm. But on the basis of the data available it is not possible to assign any consistent value to this factor. Barometric pressure probably has some direct effect on snow conditions but we have not been able to identify it. Relative humidity undoubtedly plays a continuous part in snow development from the time it is formed until it becomes water again. However, accurate and consistent relative humidity readings are very difficult to obtain in cold weather. Our records, although complete toy a period of years, are not believed accurate enough for research purposes. In addition to the natural factors just discussed, there are a number of artificial or .accidental elements which may intervene with results difficult to predict. Palls of rock or ice and fractured cornices may start a local avalanche cycle either premature or of more violence than from natural release. Terrain analysis is the snow ranger's only guide in this matter. Either through ignorance or carelessness, skiers occasionally blunder onto unstable slopes and become the direct cause of their own injury. At the other extreme is the professional snow observer operating possibly on the same slopes, but according to plan and for the definite purpose of causing slough cycles. During the course of a winter,, Alta can expect one or two snowstorms accompanied by thunder and lightning. In one recorded case, a thunderclap released a large avalanche hours ahead of the general cycle. Sound is another item for the snow ranger to tack up on his mental bulletin board. When snow conditions are unstable, secondary avalanches are always a possibility. Either from ground or air vibrations, or by undercutting the slope, one avalanche may release another. Secondary avalanches during control work with explosives are an example of the same effect and snow rangers must be alert to them. At Alta slides of this type have occurred up to a quarter of a mile from the blasting point. See photograph page 17. IV. AVALANCHE HAZARD FORECASTING ' For reasons which should be fairly obvious by now, avalanche hazard forecasting is not an exact science either in time or location. Within limits, the hazard from slope to slope and exposure to exposure can be sub-classified. The limits are imposed less by the natural avalanche factors involved than by the protective measures available to the snow ranger. Slopes controlled by explosives or stabilized by skiing are safe long after the general danger level is extreme. -32- |