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Show Wind affects the snow in several ways. It transports snow from one locality to another, thus overloading one slope at the expense of another. In the same process it levels off the contours of the land, thus producing steeper and smoother slidepaths. It can even accomplish a major feat of plastic surgery and change a slope profile from concave to convex. It alters the form and size of the snow particles, grinding them into smaller and less complicated shapes. It builds snow structures, cornices, drifts, snow cushions and the like. It compresses S D W into stable windpack and windcrust, or into highly unstable slab. In connection with temperature changes, it promotes metamorphosis in the snow. An example is the chinook wind, a much more effective thawing agent than sun action. Wind is effective on snow in direct proportion to its velocity. For significant action the lower limit is about 15 miles per hours. This is a sample velocity, of course, from a single location. Significant velocities will vary with altitude and exposure. In fact, the technique of hazard forecasting on the basis of the contributory factors is a sampling process. The better the background of observations over a period of time and the more familiar the snow ranger is with his area, the more accurately he can interpret his samples. Direction is the selective part of the wind factor. It governs the uneven distribution of snow throughout the area: which way the cornices lean, which slopes are commonly overloaded, where the slabs habitually form. Locally, wind direction can be a factor of major importance. Because of it, the hazard varies from one particular exposure or part of the area to another. In general, however, it plays a negative part. Velocity is the dominant characteristic since it produces hazard somewhere, regardless of direction. In alpine terrain, wind currents at ground level, where they are most effective, are so modified by exposure and contour that it is impossible for an observer to calculate the influence of direction except for restricted areas. Finally, the frequent spectacle of general avalanche cycles, with, slides running from every slope, regardless of exposure, has convinced us that wind direction is important only as a local factor on certain individual exposures. 10. Temperature plays a rather subtle part in the development of direct action avalanche hazard. Warnings to the hazard forecaster generally consist of departures from the normal pattern. The typical alpine storm begins at moderate temperature, between 25 and 32 degrees, which falls gradually during the storm. The temperature spread is apt to be narrow. When the storm ends there is likely to be a further drop in temperature and a " cold snap". This post- storm cold period is apt to be prolonged for several days in the high alpine zone and much shorter in the other two zones. In the latter case, the time of day when the storm ends has influence. Following the cold snap, temperatures rise again. This is the normal pattern. In the middle and coastal alpine zones, it leads to a rapid termination of the avalanche cycle, whatever its proportions, and to this extent the factor is - 51 - |