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Show Snowfall Intensity Studies Snowfall intensity has been recognized for some time as one of the more important direct action avalanche factors. Prior to the winter of 1951- 52 observation was possible only in terms of average rate of snowfall over a period of hours. l( iore accurate determination was desirable, especially in connection with P. I. studies. During the winter of 1951- 52, we developed and tested a device for measuring snowfall intensity continuously ( Figure No. 47). The instrument is still in the modification stage, but enough data were gathered to suggest that we may eventually have to reconsider our ideas on how rapidly snow can fall. The highest S. l.' s recorded by previous methods of observation were in the neighborhood of two inches per hour- In March, 1952, the S. I. gage in an 8- hour period recorded S. I.' s up to four inches per hour. The device consists of a slowly revolving turntable within an open housing. The turntable is scanned by a photo- electric eye. When one inch of snow gathers on the turntable, the electric eye beam is interrupted. This actuates a wiper which cleans off the turntable, and a relay which sends a pulse to the operations recorder. Then the cycle starts over again. Avalanche Timing One of the big gaps in our investigation of the complex forces which produce avalanches has always been lack of accurate information on avalanche occurrence. The direct action avalanche generally runs during periods of zero visibility. Even the delayed action slides which occur in full daylight are observed less often than might be expected. Yet the ultimate refinement of hazard forecasting techniques cannot be attained without accurate records of the time of avalanche occurrence. With the installation of an operations recorder at Alta in the winter of 1950- 51, experiments began in an attempt to obtain this essential information. Electrical circuits were run to slopes in the vicinity and switches were placed directly in the avalanche paths. A slide would actuate the switch and send a pulse to the operations recorder ( Figure 48). The mechanical difficulties of keeping this system in working order proved to be great. For this reason we have not yet been able to obtain avalanche occurrence records for an entire winter. However, from this system we have obtained our first accurate and coherent data on the exact time of avalanche occurrence in relation to the contributory factors. The project returned one unespected dividend. The switches proved capable of recording sluff cycles as well as large avalanches. Thus, on Storm Plot No. 3, we knew about the early sluff cycles which cancelled out the high P. I. Factor. The avalanche occurrence timing system therefore becomes of immediate practical value to the hazard forecaster. - 119 - |