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Show INTRODUCTION This paper concerns the problem of forecasting direct- action avalanches in new snow. These run during or immediately after a snowfall; they are most commonly, but not necessarily, soft slabs ( de Quervain, I965). The forecasting of climax avalanches, which depends on investigation of structural weaknesses in the snow cover, will not be discussed. Criteria in the form of contributory factors have been used as a basis for anticipating direction- action avalanche hazard from observable snow and weather conditions. ( U. S. Forest Service, 1952, 1961; LaChapelle, 1965) A number of factors were identified as significant ( see below) at an early date. Experience in the United States during the past two decades has confirmed their validity. Many of these factors are interrelated, although new snow depth, precipitation, wind velocity and air temperature are probably the dominant ones. All were chosen empirically, though there are reasons to believe that some would be significant from a theoretical standpoint. Each has survived on the pragmatic basis of demonstrated usefulness. From the start it has been explicitly recognized that other factors may also be significant, or that the present ones are inadequately evaluated. Specific notice has been taken of the unexplored possibilities in new snow type ( U. S. Forest Service, 1961). Importance of this factor has recently been restated, in general if not in specific terms, by Zingg ( 1965)- Lack of specifics, in fact, has persistently characterized consideration of this factor. Broad and somewhat loose terminology like " pellet11, " powder", or " granular" is used to classify the types of new snow. An accumulating body |