OCR Text |
Show Much progress has been made in forecasting direct action soft slab avalanches from meterological observations. Factors such as wind, temperature and precipitation intensity are weighed against preset criteria and the hazard is judged accordingly. However, the interplay of meterological factors is complex and standard meterological observations cannot be exclusively relied on for soft slab prediction. Ambient air temperature for example, cannot be correlated with crystal structure which is determined by upper atmospheric conditions. Also, because of radiation, air temperature may not be indicative of the metamorphism of freshly deposited snow. With present instrumentation the forecaster is hard- pressed to keep track of the various crystal structure and property changes taking place during a storm. Besides basic changes in dendritic structure, crystal layers will vary in such important modifying features as degree of riming , impurity concentration, wind fragmentation, wetness, and clustering. Thus, a set of meterological conditions measured at a convenient location with convenient instruments could apply equally well to a variety of slabs which differ from one another in many mechanical properties. Measurement of the mechanical properties of newly deposited snow are needed to supplement meterological data and improve upon the predictability of soft slab release. There is a scarcity of experimental data pertaining to properties of newly deposited snow in the density range of 0.05 to 0.20 gm/ cnK Most of the information which covers this low range is extrapolated from relationships derived from experiments on snow with densities greater than 0.2 gm/ cirP. Soft slab avalanches usually occur during or immediately after a storm and involve snow in this low |