STRENGTH CHANGES OF LAYERS OF FACETED SNOW CRYSTALS IN THE COLUMBIA AND ROCKY MOUNTAIN SNOWPACK CLIMATES IN SOUTHWESTERN CANADA KEYWORDS snow crystals, faceted crystals, snow strength, snow climate, avalanche, avalanche forecasting Greg Johnson 8 and Bruce Jamieson 8, b Department of Civil Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada Department of Geology and Geophysics, University of Calgary, Calgary, Albterta T2N 1N4, Canada 2. LITERATURE REVIEW 3. SNOWPACK CLIMATES In a recent study, Stock et al. (1998) observed numerous layers of faceted crystals at Red Mountain Pass, Colorado from December to March. He measured stuffblock scores (Johnson and Birkeland, 1994), and hand hardnesses on valley bottoms and north and south-facing slopes. The study found faceted crystals were larger on north-facing slopes and slower to gain stability and strength. Jamieson and Johnston (1999) used Kendall- Tau correlations and ranked variables associated with the rate of shear strength change for surface hoar layers. They found the predictor variables that most significantly affect shear strength are height of the snowpack, the maximum crystal size, and slab thickness. In southwestern Canada there are three snowpack climate zones: coastal, intermountain, and continental (Fitzharris, 1981; Mock, 1995). Each of these climate zones (Figure 1) has different weather and snowpack conditions. The coastal climate of the Coast Mountains produces relatively warm temperatures and heavy snowfall. The Rocky Mountains have a continental climate associated with cold temperatures and shallow snowpacks. The intermountain climate of the Columbia Mountains is due to an overlap between the coastal and continental weather systems. As a result the intermountain climate has less snowfall 1. INTRODUCTION In southwest Montana 59 % of investigated avalanches between 1990 and 1996 (Birkeland et. aI., 1998) and in Canada 26 % of the fatal avalanche accidents between 1972 and 1992 (Jamieson and Johnston, 1992) were a resultof failures of layers of faceted crystals. Statistics for Canada show the number of avalanche fatalities is greatest in the continental snowpack climate of the Rocky Mountains and least in the coastal snowpack climate of the Coastal Mountains (Jamieson and Geldsetzer, 1996). To prevent avalanche accidents, avalanche forecasters rely on snowpack data, weather forecasts, and previous avalanche activity. Often weather, snowpack conditions, and time restrict data collection, increasing reliance on snowpack evolution models. However, the shear strength of layers of faceted crystals is often poorly predicted by such models (Fierz, 1998). Various snowpack variables influence the strength of layers of faceted crystals, but these variables have not been widely studied. In this study, snowpack variables and shear strength of layers of faceted crystals were measured in the continental climate of the Rocky Mountains and the intermountain climate of the Columbia Mountains of southwestern Canada. Relationships between shear strength and snowpack variables are established by using physical arguments and Spearman rank correlations. The strength of layers of faceted crystals is important for forecasting snow stability. During the winters of 1993-2000, in the intermountain snowpack climate of the Columbia Mountains and the continental snowpack climate of the Rocky Mountains of southwestern Canada, over 100 strength measurements of 16 layers of faceted crystals were made. Rank correlations are used to relate the strength of layers of faceted crystals with measured snowpack properties and calculated snowpack variables. Additionally, the contrast in snowpack properties between snowpack climates allows comparison between shear strength and snowpack factors. Factors showing the greatest potential for predicting shear strength include load and slab thickness. a Corresponding author Tel: 403-220-7479; Fax 403-282-7026; e-mail: gjohnson72@hotmail.com 86