OCR Text |
Show poorly, the snow layer they form avalanches easily and a serious hazard from large avalanches seldom occurs. ( Examples in the form of storm and avalanche case histories will be presented in the final version of this paper to be submitted in August 1966). Snow crystal characteristics do not, of course, hold the entire key to occurrence of such avalanches. The pre- requisite of suitable terrain is assumed. Other determining contributory factors must be favorable. A certain minimum new snow depth must be atta ined; 25- 30 cm is the critical minimum for dangerous soft slabs. Precipitation intensity must be high enough ( above 2.5 mm of water per hour, roughly) to surmount the stabilizing influences of metamorphism and settlement. But when these and other conditions are met, there still remains a highly variable behavior of newly- deposited snow*. At times large avalanches may occur widely, while at others, with very similar contributory factors, only extensive sluffing is observed. Evidence accumulated in the climate of northern Utah indicates that this difference is dictated by the degree of riming on falling snow crystals In the manner outlined above. * Another aspect of this variable behavior is introduced when new sncw falls on an unstable base such as depth hoar. All forms of snow will avalanche from a sufficiently unstable depth hoar layer. But this represents a climax avalanche situation reached by internal metamorphism of already- deposited snow, which is beyond the scope of this paper. |