Snow Formation and Avalanches : Page 207

be borne in mind that a snow slab may be formed and then covered by a later fall of snow which completely conceals it. The presence of a snow slab which is broken by the weight of a skier as he travels can be detected by the dull thud occasioned by the settling of the slab, and when the slab is not covered, the lines of fracture can be seen to run off some distance from the point of breakage. This extension of the fracture is one of the visible points of distinction between wind slab and wind crust. The effect of breakage of wind crust is quite local, with no tendency to break up the crust to any distance from the point of fracture. Hoarfrost is sometimes formed between a wind slab and the underlying snow. Since the internal cohesion of hoarfrost is very low, this increases the likelihood that the wind slab, lubricated by the hoarfrost, will avalanche after breakage. The avalanche consists of large blocks of wind slab, which may subsequently break up during the fall, and is especially dangerous if the blocks are large. Skiers caught in such avalanches are usually crushed by the blocks.