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Show afford to guess. The only way to make sure of a slab formation is to break it, and thus force it to stabilize. The techniques of artificial stabilization are described in Chapter 6. Windslabs habitually form on lee slopes but this is a rule that we have learned to apply with caution ( Figure 15). Air currents in the mountains are so erratic that it is possible to find a lee slope on practically every exposure. The slab area may not be very large, but as we have pointed out before, it doesn't take a climax avalanche to kill a human being. Besides wind force and direction, snow types, temperature and surface contours are involved. The granular, high- density forms of snow are the most favorable to slab. Differences in contour, often insignificant, are the only ready explanation for the fact that we often find areas of slab, ordinary windpacking and loose snow all on the same slope. Some authorities assign an important role to relative humidity. Often enough to make us alter our view on the importance of wind direction, we have observed slab avalanche conditions developing universally, without regard to exposure. This phenomenon has always taken place during wind . action on falling snow and when all other factors were favorable. The appearance of a slab avalanche path after occurrence is unmistakable. In contrast to loose snow avalanches, the entire slab field - top, sides, and bottom - releases almost simultaneously. The fracture is a line instead of a point. The line is broken and angular and often assumes a rough dome shape. The fracture plane is perpendicular to the mountainside. Upon release, the slab layer shatters into square- edged blocks ( Figure 16). If composed of soft slab, these blocks are small, but if the slab is old and hard, they may retain large size throughout the slide. Cornices are a snow formation allied to slab. They build up on the lee side of mountain crests and ridges which lie at right angles to the wind. Occasionally they are straight- walled, but their characteristic shape is that of a breaking wave. They are another manifestation of windpacking, usually of a stable type. But it is not uncommon to find a slab stratum in a cornice which behaves just as it would elsewhere. The obvious hazard from cornices is due to fractures of the overhang from simple overloading ( Figure 17), weakening due to temperature or rain, or from sun erosion of the underside. These falling blocks are generally large enough to be dangerous themselves and may also release avalanches on the slopes below. Another less obvious danger is the lever action a cornice exerts on its anchorage as it builds higher and farther out over the slope. If the effect is not relieved by local fracturing, the entire mass may eventually capsize. No cornice which overhangs improvements or an area of heavy use should be allowed to develop either a marked overhang or the crack along the base which indicates lever action. - 35 - |