Item: A between-storm indicator of avalanche activity
Title: A between-storm indicator of avalanche activity
Proceedings: Proceedings of the 2000 International Snow Science Workshop, October 1-6, Big Sky, Montana
Authors: Robert E. Davis, US Army Engineer Research and Development Center, Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, Kelly Elder Rocky, Mountain Research Station, USDA Forest Service, Fort Collins, Colorado
Abstract: Earlier studies examining relationships between weather factors and avalanche activity showed that the important between-storm factors include an index related to the minimum and maximum daily air temperatures. The index helped explain the differences in avalanche activity with similar storms, but with different circumstances affecting the snow between storms. This study used the results of the previous investigation to guide case studies taking a closer look at the temperature index, which has the name vapor gradient index (VGI). The VGI accumulates each day between storms by adding the current day's value VGI' with the accumulated value from the previous day. During storms the value remains constant, and is reset to zero when the snow stops falling. To calculate the VGI' for the current day, one requires two temperatures, the daily minimum temperature and the value midway between the daily minimum and maximum. For each temperature the one then uses Clausius-Clapeyron equation to calculate the saturation vapor pressure at each temperature. The difference between the two vapor pressures becomes the VGI', the value of the current day. We used the SNTHERM model of snow processes to calculate grain growth during a 17-day period between storms in the early winter,1994, at Mammoth Mountain, California. During this period the VGI showed a high correlation to grain growth.
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Keywords: snow, metamorphism, avalanche activity
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