OCR Text |
Show 7 may have. a soft, felt- like texture of very low bearing capacity and little apparent mechanical strength. Minor sluffing may occur. Avalanches in such snow still release by characteristic slab fractures, but resemble loose snow avalanches once they are In motion. As the amount of rime on stellar crystals decreases ( Figure lb), so does the probability of large soft slab avalanches. Snow layers formed from such crystals appear much less able to sustain tensile stresses than the types described above. Sluffing is common. Occasionally, shallow and very soft slabs may form, but these run as generally harmless small avalanches. In respect to layers formed from heavily rimed crystals or graupel, such snow is unstable and very seldom builds up to dangerous thicknesses on steep slopes; it runs off as sluffs first. Direct- action avalanches, especially soft slabs, exhibit a contradictory behavior of newly- deposited snow. The eventual formation of a slab avalanche hazard becomes more probable as the initial instability of the new snow decreases. Highly unstable snow does not form a serious hazard by virtue of the very fact that it slides so easily. The avalanches, small and mostly harmless, run long before the snow accumulates to a depth where they might be dangerous. As the degree of cohesion among crystals increases, the snow initially is more stable and has the opportunity to build up thick layers capable of temporarily sustaining stresses but eventually fracturing as slabs. Field evidence in the form of observed soft slab behavior strongly suggests that this degree of cohesion is directly related to the amount of rime on the falling crystals. In the absence of rime, the crystals cohere |