| OCR Text |
Show by geometric rather than arithmetic progression as the wind velocity goes up. Second, it alters the form of the snow, grinding large particles into smaller ones, possibly changing them from crystalline to granular in the process and thus subjecting the snow to violent fluctuations in temperature and humidity. In the act of depositing it on the surface, wind leaves the snow crusted or packed, slabbed or drifted, altered in size, shape, weight and depth. Instead of a blanket of even depth and texture we have a chaotic mixture. To the forecaster, wind force can never be a factor unfavorable to avalanches. Even though building a safe wind crust in one location it is busily constructing a dangerous slab in another. If below critical levels, its value is zero. At Alta, the critical level is in the neighborhood of 10 miles per hour. S. Wind direction is the selector which distributes the effects of wind force throughout the area. It is responsible for the wide variation in the hazard, which may be critical on one set of exposures and moderate or non-existent on others. At Alta, for example, NW storms can endanger the highway when the major portion of the ski area is unaffected. SW storms find "Closed to Skiing11 signs up on many runs while highway traffic is undisturbed. Force is obviously the dominant member of this team. As the comparative analysis in the next section shows, critical wind velocity acting on enough snow can produce a general avalanche cycle regardless of wind direction. 9. Temperature changes before, during, and after a storm, play a rather subtle part in the development of avalanche cycles. Our data at Alta are neither extensive nor accurate enough to be of more than general value. The typical Alta storm begins at temperatures in the neighborhood of 32°. The temperatures fall gradually during the storm and rise gradually afterward. This is the forecaster1 s "normal11 indicated by observations to be favorable to snow stabilization. Temperatures which fluctuate rapidly up or down, especially the former, appear to favor unstable snow conditions. Storm temperatures which produce damp snow have demonstrated their bias in favor of large avalanche cycles and the same is true for markedly low temperatures. Prolonged cold after a storm, unusual at Alta, retards settlement and preserves any instabilities which may exist. Rapidly rising temperatures promote damp slides and a quick end to the avalanche cycle whatever its intensity. 10. Settlement is a stabilizing factor. It goes on continuously, fair weather or foul, and thus acts to keep the snow in place. Storm settlement rates fluctuate widely and their effect needs further study. To summarize, major storms are the principal direct source of dangerous avalanches at Alta. New snow depth, rate of accumulation and wind action are dominant and can produce large avalanche cycles regardless of the other factors. It is in borderline situations that the minor factors become important and cause the trained snow observer to apply restrictions which appear unnecessary to the layman. The reverse is also true, that a skillful administrator can keep the area open when rule of thumb indications v/ould encourage closure. -31- |
