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Show Soft slab on the surface is another matter. It too offers a high friction base but a small amount of new snow may be enough to destroy its cohesion. Explosives are the answer to this situation. 3. New Snow Depth. Strictly speaking, any amount of new snow is favorable to avalanches. But it is the avalanche large enough to endanger life and property that concerns us. The criterion is a quantity of new snow sufficient to produce a dangerous avalanche. Observations over a long period suggest 12 inches as a convenient guide. Less than that amount is unlikely to produce anything spectacular, unless the old snow gets into the act. Beyond 12 inches the forecaster should be paying close attention to the situation as a whole. This must not be taken to mean that with less than 12 inches of snowfall the avalanche patrolman has nothing to worry about, or that with more he should herd everyone into the storm cellars. Our records include avalanches of dangerous proportions occurring with as little as 2 inches of new snow ( delayed action release of slab) and no slide activity of importance with as much as 72 inches. Twelve inches of snowfall is a handy signboard along the upward curve of avalanche hazard, no more and no less. 4. New Snow Type. ( Includes form crystalline or powdery, granular, pellet and mixtures; size fine, medium and coarse; and free moisture condition dry, damp or wet.) See the Standard Snow Terminology, Appendix B. This is another contradictory factor. Very dry snow, for instance, has little cohesion of any sort and avalanches so readily that it seldom builds up to dangerous volume except under true blizzard conditions. " Dryness" is a function of temperature and very dry snow falls only at temperatures far below freezing, from 10 degrees downward. The crystalline snows falling at ordinary temperatures- around 25 degrees- have good internal cohesion due to the interlocking of the rays. As mentioned earlier, these rays deteriorate with age and their cohesion is normally replaced by packing and cementing. Granular and pellet snows have fewer points of contact and therefore less natural cohesion. Damp snow, containing enough free moisture so that it will pack if squeezed lightly in the glove, has increased cohesion due to cementing. This is not necessaryly an advantage for the avalanche patrolman, since it may lead to greater build- ups of mass before sliding begins. Wet snow is saturated and the water lubricates rather than cements. One snow type needs special mention. We have labeled it " transition" because it is on the dividing line between dry and damp. With a little practice it can be detected by the field test. When squeezed in the glove it neither packs firmly like damp snow nor refuses to pack at all like dry snow. It is just in between. Granular or a mixture containing granular is the most common form of this type of snow. But whatever its form, it is peculiarly susceptible to the formation of slab under wind action. - 48 - |