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Show There are, of course, other hazardous combinations, but this one comes about as close to being a constant as it is possible to get in such a jungle of variables. P. I. has a further advantage, not possessed by many of the other factors. With proper equipment, it can be observed continuously, hour by hour. 8. Settlement ( often expressed as a percentage). With one exception, settlement in the snow is a stabilizing factor. The exception is the loose snow layer lying beneath a slab. The difference in settlement rates may cause the loose snow layer to shrink away from the slab, thus weakening its external cohesion. Settlement goes on continuously in old snow as well as new. Its over- all effect is to promote both external and internal cohesion. Shrinkage rates vary widely and our methods of observing them are still rather crude. However, we have some useful guideposts. Very low settlement rates during a storm, 15% and less, indicate that very little improvement in cohesion due to this factor is taking place. High settlement rates 40$ and above-- indicate that cohesion is improving rapidly. In borderline hazard situations, settlement has often proved to be a valuable guide to the severity of the hazard. A high settlement rate is no guarantee of safety, however, if the majority of the other indicators are red. As an element in hazard forecasting, settlement will become much more important as soon as we can develop better methods of observation. The problem is one of great technical difficulty. Examples of computing settlement: Example 1. Old snow depth - - - 100" ( from the cumulative stake) New snow depth 12" ( from the 24- hour stake) Total 112" Total snow depth 110" ( from the cumulative stake) Settlement 2" Example 2. New snow depth 10" Settlement 5" Settlement ratio or percentage: 5 divided by 10 equals 50$. If only the 24- hour stake is used to measure the new snow, settlement figures are a combination of settlement in both old and new snow. Separation can be obtained by the use of short interval stakes. 9. Wind is the most important of the contributory factors. In the absence of wind there is no slab formation, either from falling snow or snow transported from the surface. In fact, critical direct action avalanche hazard conditions seldom develop, regardless of other factors, without wind. Since strong wind action is characteristic of climate and weather in the alpine zones, this is not much comfort to the avalanche forecaster. - 50 - |