Snowpack characteristics associated with avalanche accidents

Abstract

Reports of 93 fatal avalanche accidents in Canada between 1972 and 1991 indicate that most of them involved dry snow slabs and were triggered by people. Slab thicknesses averaged 0.86 m and over half the slabs included snow deposited prior to the most recent storm. Two persistent microstructures of snow, namely surface hoar and faceted grains, were commonly reported in the weak layers believed to have released the overlying slabs. The people that had difficulty recognizing or evaluating instabilities involving surface hoar and faceted grains include amateur recreationists as well as professional forecasters and guides. This suggests that present field tests and forecasting techniques may be unsatis-factory for assessing the stability of slabs which overlie layers of surface hoar or faceted grains. Key words : avalanche accident, avalanche forecasting, critical weak layer, slab avalanche, snowpack microstructure, avalanche trigger.