Refinements of empirical models to forecast the shear strength of persistent weak snow layers PART B: Layers of surface hoar crystals Antonia Zeidler a , Bruce Jamieson a,b, ⁎ a Department of Civil Engineering, University of Calgary, Calgary, Alberta, Canada T2N 1N4 b Department of Geology and Geophysics, University of Calgary, Calgary, Alberta, Canada T2N 1N4 Received 20 June 2005; accepted 19 November 2005 Abstract Buried layers of surface hoar often release skier-triggered avalanches in the Columbia Mountains of Canada and their shear strength can be used to assess the stability of a slab overlaying these layers. In 2001 Chalmers introduced an Interval Model to calculate the shear strength of layers of surface hoar based on manual snowprofile observations. We refined his model by adjusting the measured shear strength for the normal load and included only data points where the weak layer depth did not exceed 100 cm to better account for skier triggering. Further, we used average and daily loading rates as well as a regression analysis to determine the best estimate of the shear strength change. Our final Forecasting Model used a multivariate regression to calculate the shear strength on days with snowprofile observations and as well as average and daily loading rates to forecast the shear strength on days without manual snowprofile observations. The performance of the model (r 2 ) was 0.71 and 0.63 using average and daily loading rates, respectively. A companion paper, Part A, develops a forecasting model for weak layers of faceted crystals. © 2005 Elsevier B.V. All rights reserved. Keywords: Avalanche forecasting; Snow stratigraphy; Surface hoar; Snow cover stability 1. Introduction The stability of a slab overlaying a weak layer depends partly on the shear strength of the weak layer, which can be measured with a shear frame. Unfortu- nately shear strength measurements are time consum- ing and are not done on a regular basis in most forecasting operations. Therefore it is desirable to be able to calculate the shear strength without shear frame testing so that this information can be used in daily avalanche forecasting. Although the shear strength of snow is influenced by the number, size, shape and orientation of intergranular bonds (Yoshida, 1963; Keeler, 1969; Colbeck, 1997), shear strength changes are hard to quantify and consistent measurements are rare. Chalmers (2001) and Zeidler and Jamieson (2002) have shown that the calculation of the shear strength based on snowprofile observations is prom- ising, although the estimation of the shear strength change on days when no snow profiles are recorded is more difficult. Hägeli and McClung (2003) analyzed the character- istics of natural avalanches and list three types of Cold Regions Science and Technology 44 (2006) 184 – 193 www.elsevier.com/locate/coldregions ⁎ Corresponding author. Department of Civil Engineering, University of Calgary, Calgary, Alberta, Canada T2N 1N4. Tel.: +1 403 220 7479; fax: +1 403 282 7026. E-mail address: bruce.jamieson@ucalgary.ca (B. Jamieson). 0165-232X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.coldregions.2005.11.004