Effect of mountain permafrost on snowpack stability

摘要

Statistics on recreational avalanche accidents suggest that a substantial part of all accidents occur in areas where mountain permafrost exists. Accordingly, it has been proposed that the presence of sub-zero ground temperatures favours snowpack instability. In the mainly transitional climate of the Swiss Alps, permafrost prevails in shady slopes above about 2500 m a.s.l. In order to investigate the effect of mountain permafrost on snowpack stability, over 400 snow profiles with stability tests were analysed, taken in both permafrost-free and permafrost terrain, i.e., at different elevations. Samples were collected in January, February and March, when the snowpack was cold. Locations on glaciers were excluded from the analysis. To compare profiles from the two areas they were rated into five classes of stability. Also, the temperature at the base of the snowpack, the temperature gradient and the maximum grain size in the lowermost 50 cm were compared. No significant difference in snowpack stability between permafrost and permafrost-free profile locations could be found. Basal snow temperatures were statistically significantly lower for the permafrost locations. Snowpack depth had a significant effect on the ground surface temperature. With the slightly lower basal temperatures, temperature gradients were accordingly slightly lower as well. The effect on the maximal grain size, supposedly an index for the past temperature gradient, and on snowpack stability was minor. Overall, no indication was found that permafrost terrain causes the development of an unstable snowpack. However, a shallow snow depth favoured below freezing ground temperatures as well as snowpack instability. Snow depth was significantly positively related to snowpack stability. In conclusion, the presence of many avalanche accidents on permafrost terrain rather reflects skiing preference - due to better snow conditions on shady slopes - than a causation and is therefore merely a coincidence.