NEAR-SURFACE FACETED CRYSTALS FORMED BY DIURNAL RECRYSTALLIZATION - ACASE-STUDY OF WEAK LAYER FORMATION IN THE MOUNTAIN SNOWPACK AND ITS CONTRIBUTION TO SNOW AVALANCHES

In the winter of 1995-96 we investigated the temperature and vapor pre ssure gradient conditions associated with the formation of faceted cry stals that develop in the upper levels of the snowpack due to diurnal recrystallization. We used an array of six thermocouples connected to a datalogger to continuously measure snow temperatures in the region f rom 0.005 m above the snow surface to 0.20 m below the snow surface. M easurements during clear sky conditions in March showed temperature gr adients in excess of 200 degrees C m(-1) at night in the top 0.05 m of the snowpack, with the temperature gradient shifting direction and ex ceeding 100 degrees C m(-1) through this layer during the day. These t emperature gradients resulted in vapor pressure gradients which exceed ed 25 mb m(-1) during the day and at night. During this time, a signif icant weak layer of 1 mm faceted snow formed within 36 h. Widespread a valanche activity occurred for up to 9 d after this layer was buried b y 0.50 m of snow.