Near Surface Faceted Crystals: Conditions Necessary for Growth and Contribution to Avalanche Formation, Southwest Montana, U.S.A.

In the winter of 1995-96 we investigated the formation of faceted crystals that develop in the upper levels of the snowpack. We used an array of six thermocouples connected to a datalogger to measure hourly diurnal temperature changes in the region from 0.005 m above the snow surface to 0.20 m below the snow surface. Measurements during clear sky conditions in March showed temperature gradients in excess of 2000 C/m at night in the top 0.05 m of the snowpack, with the temperature gradient shifting direction and exceeding 1000 C/m through this layer during the day. A significant weak layer of faceted snow formed within 36 hours with a grain size of about 1 mm in the upper snowpack. Widespread avalanche activity occurred for up to nine days after this layer was buried