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Show Barometric pressure is a possible avalanche factor, but we have never been able to establish any correlation. Solar radiation is a much better possibility. It is known that radiation can penetrate snow to a greater depth than insolation. It may be this factor rather than surface thawing which is active in the temperature release of delayed action slabs. Investigation would be desirable. DELAYED ACTION AVALANCHE FACTORS Though fewer in number, delayed action avalanches are characteristically greater in size and destructive ability. Slab and wet spring avalanches are typical. They are called delayed action because they run after a time interval and from some cause or causes in addition to the storm which created the hazard in the first place. Direct action hazard is soon over, one way or the other. It is produced by fairly obvious combinations of factors now that we have identified and analyzed them and the return to stability can be definitely established. Delayed action hazard is much more difficult to forecast. It can persist for an entire season and then materialize from rather minor causes. The forces at work within the snowpack are more obscure and difficult to investigate than those which operate on or near the surface. Slab is at the bottom of most delayed action avalanches, in more than one sense. Recognition of delayed action hazard depends upon investigation of the snowpack by means of excavation and instruments such as the penetrometer and shear frame. In an area under continuous observation and close control, the climate and weather factors do have application to delayed action hazard, for they point out the layers which may make trouble later. Their further development can then be traced by means of the instruments above. Actually, in an area of concentrated use, an administrator can seldom afford to let a slab avalanche hazard persist for very long. He fractures the slab artificially and forces it to give him an immediate answer. However, areas of continuous observation are few and it is frequently desirable or necessary to make an estimate of the stability of a snowpack. Examples are highways traversing alpine zones or week- end recreational areas. Our procedure in this country is to investigate the snowpack directly in the slidepath, or as close to it as we can get with reasonable safety to the observe. This method has produced some interesting results which are described in detail in Appendix A. The cohesion of a slab layer can be roughly measured in terms of its resistance to penetration. We have reached the following tentative conclusion. A layer of about 10 kg resistance to penetration, as measured by the penetrometer, offers little support to a slab above. - 54 - |