Extrapolating the skier stability of buried surface hoar layers from studyplot measurements

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 su rface hoar layers in a study plot, one buried 30 December 1999 and the othe r buried 21 February 2000, to skier-triggered slab avalanches within 100 ki n 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 Ro gers Pass every 4 to 8 days until the end of March 2000. Physical propertie s of the slab (load, thickness, hardness profile), and weak layer (shear st rength, temperature, temperature gradient, crystal size, crystal form) were observed. Approximately once every 2 weeks, the weak layers were photograp hed in the pit wall to document their texture. On the same days, disaggrega ted crystals from the weak layers were photographed on a crystal screen. The February 21 layer, which initially consisted of 4-6-mm crystals, was lo aded more slowly by snowfall, gained strength and stability more slowly, yi elded initially lower stability indices and released many more skier-trigge red avalanches than the December 30 layer, which initially consisted of lar ger, 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, c alculated skier stability index Sk(38), layer thickness, and load on the we ak layer show potential predictive value for the stabilisation of buried su rface hoar layers. While the time series of photographs of separated crysta ls shows distinct changes, the time series of photographs of the buried sur face hoar layers in situ reveals little useful information on textural chan ges other than thinning of the layers. (C) 2001 Elsevier Science B.V. All r ights reserved.