OCR Text |
Show downward. Slab avalanches originate from a cliff- like fracture line. The distinction between soft and hard slab, however, is not well defined. The extremes are obvious, but guidelines are needed for the less obvious cases. If angular blocks of snow are found in the avalanche debris, the slide is usually called a hard slab avalanche, even though the blocks constitute a small percentage of the total debris. Where avalanche paths are long and steep the debris from even the hardest snow may contain no angular blocks. In this case, the density of the snow in the starting zone is the best guide. Snow densities of 250 to 300 kg/ m3 mark the transition from soft to hard slab. Trigger action is divided into natural ( N) and artificial ( A). The latter is further divided into artillery ( AA), ava-launcher ( AL) , hand- placed explosives ( AE) , and ski ( AS). Avalanche size is generally ranked from 1 to 5 in ascending order. Following the European practice, size 1 is reserved for sluffs. These are snow slides that move less than 150 feet slope di stance. Sizes 2 through 5 apply to small, medium, large, and very large avalanches for the avalanche path in question. This means that a medium ( size 3) avalanche on a very large path may be bigger than a large or very large ( size k or 5) avalanche on a smaller path. The best way out of this inconvenient situation is better data on the actual dimensions of avalanches. The surface over which the avalanche runs is the fourth element of the standard avalanche classification. The sliding layer is usually either one of the layers within the snowpack ( designated 0) or the ground ( G). Sometimes an avalanche starts out sliding over a snow layer then digs down to the ground. This is shown as 0G in the classification. Air blast is the final element of the standard classification If this relatively rare event is observed, or if there is good evidence it took place, a J is entered. Air blast should be reported only if damage or severe turbulence is observed well ahead or to the side of the moving snow. Turbulence immediately ahead of the rapidly moving avalanche front is to be expected and should not be entered as air blast. Broken trees or other damage well to the side of the path or beyond the most distant avalanche debris is good evidence of air blast. in addition to the standard avalanche classification, it is very helpful to have information on the type of motion, the size and location of the starting spots, the percentage of the total path that slid, the vertical distance traveled by the moving snow and notes on damage or injury caused by the moving snow. 31 |