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Show The windslab, or snowslab. avalanche ( Figure 14) is unquestionably the worst killer of all and equal to the wet spring avalanche as a destroyer of property. It combines great mass with high speed to produce maximum energy. Slab is a type of windpacked snow. The mechanics of its formation are not thoroughly understood and the characteristics that distinguish it from the stable types of windpacked snow are obscure. No matter how produced, it has the ability to retain its unstable character for days, weeks and even months, thus leading to the release of delayed-action avalanches unexpectedly and often from minor causes such as the addition of a small amount of new snow, a skier cutting across a hillside, or sun action. Records show instances of slabs so delicately poised that they were released by sound. The secondary avalanche during control operations with explosives is commonplace. Windslab may be soft or hard. It is generally soft when formed during snowfall, hard when formed of snow picked up from the surface or after aging within the snowpack. Hard or soft, it has certain distinguishing traits: low rate of shrinkage in the layer itself, low rate of settlement in comparison with the snowpack as a whole, a brittle quality which causes it to fracture violently and totally when loaded beyond its strength. Slow motion pictures of slab avalanche releases show the entire hillside in action within a matter of moments. The low shrinkage rate is undoubtedly an element which causes the original slab to retain its brittleness. The low settlement rate prevents the layer from becoming homogeneous with the rest of the snowpack and creates zones of low support, even actual air pockets. These air pockets promote rapid sublimation of the snow beneath, further deterioration of the slab's anchorage, and a low friction surface when the slab does collapse. Even where these conditions do not exist and the windslab layer is indistinguishable to the eye from the rest of the snowpack, our colored thread profile studies indicate that it can retain its identify throughout a winter. Some slabs do lose their essential brittle quality and become homogeneous. Exactly why one does and another does not is a question still to be answered. If found on the surface, snowslab has a dull, chalky, nonreflecting appearance. It has a hollow sound underfoot, if the slab is hard, and often settles with a crunching sound which every experienced mountaineer recognizes as a danger signal. Forecasting the hazard of windslab avalanches is difficult, especially for the delayed- action variety. We may know that the slab exists. There is as yet no accurate method of comparing the factors holding it in place load- bearing capacity, resistance to temperature changes, anchorage against those working for release the pull of gravity and the angle of the slope. As mentioned earlier, the Swiss have devoted many years of research to this problem and some excellent work is also being done at Berthoud Pass in this country. In the meantime, until research gives us better methods of gauging the relative stability of slab, the avalanche patrolman in the field cannot - 33 - |