Skier triggering, snow temperatures and the stability index for dry-slab avalanche initiation

For more than 30 years the quantitative method of evaluating stability (e.g . Roch, 1966; Fohn, 1987; Jamieson, 1995; Jamieson and Johnston, 1998a.) ha s been focused on calculation of a strength-to-load ratio (or stability ind ex): when the shear stress applied to the weak layer reaches the shear stre ngth, failure is imminent. However, field observations combined with experi ence and measurements indicate that snow-slab temperatures and slab hardnes s can have a strong influence on dry-snow slab stability. In this paper, we present a simple static analysis of the stability index, and discuss the i mportance of slab temperatures and hardness and macroscopic size effects (f actors not: contained in the stability index) on snow-slab stability. Our c onclusion is that the traditional method lacks some elements which are very important in snow-slab stability, particularly when skier triggering is in volved.