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Show Part III. Appendix A. AVALANCHE RESEARCH IN THE UNITED STATES Precipitation Intensity Studies ( Precipitation Intensity Studies have been conducted during the winters of 1950- 51 and 1951- 52 by Avalanche Forecaster M. M. Atwater, and Snow Ranger Hans Jungster at Alta, Utah, Wasatch National Forest. Alta is in the middle alpine zone.) Although the relationship of precipitation intensities to avalanche occurrence has been studied for only two seasons, the work has already yielded significant results. The objective of the project was to discover some simple and convenient method of tracking direct action avalanche hazard up the scale, hour by hour. P. I. studies are not the first attempt along this line but are definitely the most promising. They are original research and have not been attempted elsewhere. Precipitation intensity is defined for our purposes as the amount of water being deposited per hour in the form of snow. That it has a relationship to avalanche occurrence is not a new idea. Indirectly, we have been able to observe its effect by means of other factors such as new snow depth, average density and snowfall intensity. The obstacle was that P. I. could be calculated only as an average over a considerable period of time- at 24- hour intervals or the duration of a storm. This generalized data could only be obtained after the storm was over. P. I. was therefore of limited value as a hazard forecasting technique. P. I. appeared to combine elements of snow quantity, type, density, rate of fall and possibly wind action. There were definite clues that the combination had an important bearing on whether or not avalanches of dangerous size would occur. By the use of the operations recorder and special gages it became possible to pin down these clues and reveal a definite pattern: the P. I. Factor. On the basis of research to date the following conclusion appears to be valid: with P. I. at the rate of .10 inch per hour or more, continuously, with strong wind action but in the absence of premature sluff cycles, avalanche hazard is acute whenever the P. I. Factor is 1. The P. I. Factor is precipitation intensity times duration, or the total precipitation for the period. For example, if P. I. is .10 per hour, the P. I. Factor will be 1 in 10 hours. If P. I. is .15 per hour, the P. I. Factor will be 1 in a bit less than seven hours. Like everything else in avalanche hazard forecasting, the P. I. Factor is not a hard and fast dividing line but an indicator. In the course of our studies we have come upon some extreme departures. In one storm, there were large avalanches with a P. I. Factor of only .40. In another case with a P. I. Factor of 2.73 the outcome was continuous sluffing. The value of P. I. study as a hazard forecasting tool would be greatly reduced if these departures could not be interpreted on the basis of the related - 98 - |