| OCR Text |
Show 5 an erratic and sometimes poor sampling of storms in the normal course of a winter. ( 3) Habitual field observations by the author of falling snow crystal types ( hand lense examination) for more than a decade. The Formation of Soft Slab Avalanches. This avalanche type forms from snow of relatively low cohesion which still exhibits the formation of tension cracks and slides away from a fracture line as a distinct layer characteristic of slab avalanches. Adhesion of a new snowfall to the previous snow surface is one of the determining variables. Precipitation intensity, the rate at which the snow load is deposited, often plays a dominant role. Some wind drifting is required, but soft slab formation is not strictly confined to lee deposition areas. The minimum critical wind velocity is around 6 to 7 m sec~ . The probable occurrence of a maximum critical velocity, above which avalanching is rare, has also been observed at around 30 m sec . Temperature trends have a subtler effect, often exerted indirectly by influencing new snow density. The latter factor is important, soft slab formation being strongly related to the higher new snow densities. A significant negative influence is exerted by " sluffing", the natural release of many small, loose- snow avalanches which tend to stabilize the steeper slopes against the formation of thick slabs. Sluffing is most common at new snow densities below about 0.07 g cm~ 3. Soft slab avalanche hazard seldom persists for long periods, for metamorphism and settlement rapidly alter the snow conditions prevailing at the time of deposition. Constructive metamorphism in a soft slab may occasionally precipitate a delayed avalanche, but more frequently these slabs are stabilized by settlement. |
