Extrapolating the skier stability of buried surface hoar layers from study plot measurements

Abstract

Buried layers of surface hoar pose a challenge to avalanche forecasters in many areas, partly because some layers stabilise quickly and others remain unstable for a month or more. This paper relates the measurements of two surface hoar layers in a study plot, one buried 30 December 1999 and the other buried 21 February 2000, to skier-triggered slab avalanches within 100 km of the study plot in the Columbia Mountains of western Canada. The two surface hoar layers were monitored at a tree-line study slope at Rogers Pass every 4 to 8 days until the end of March 2000. Physical properties of the slab (load, thickness, hardness profile), and weak layer (shear strength, temperature, temperature gradient, crystal size, crystal form) were observed. Approximately once every 2 weeks, the weak layers were photographed in the pit wall to document their texture. On the same days, disaggregated crystals from the weak layers were photographed on a crystal screen. The February 21 layer, which initially consisted of 4–6-mm crystals, was loaded more slowly by snowfall, gained strength and stability more slowly, yielded initially lower stability indices and released many more skier-triggered avalanches than the December 30 layer, which initially consisted of larger, 10–20-mm crystals. Critical study plot values of load, shear strength, and stability are compared with critical values measured adjacent to over 50 skier-triggered slab avalanches. The shear strength of the weak layer, calculated skier stability index Sk 38, layer thickness, and load on the weak layer show potential predictive value for the stabilisation of buried surface hoar layers. While the time series of photographs of separated crystals shows distinct changes, the time series of photographs of the buried surface hoar layers in situ reveals little useful information on textural changes other than thinning of the layers.