The quick growth of depth-hoar crystals was observed at night-time just below the snow surface on a south-facing slope. This growth was due to a high temperature gradient (> 100 K m−1) near the snow surface under clear skies after a thin deposition of new snow on older and denser snow. The temperature gradient was greater when internal melting had taken place during daytime, keeping the sub-surface snow temperature at 0°C even after sunset until all liquid water had frozen. To understand the relationship between the crystal growth rate and the temperature gradient, a series of experiments was carried out in the laboratory. The snow sample was set under a constant temperature gradient between 100 and 300 K m− and sustained for about 50 h. The average crystal size increased linearly with time and the crystal growth rate increased as the given temperature gradient increased. The growth rates were in the order of 10−9 m s−1, which gave a good agreement with the results of the field observation.
Abstract.Field observations of surface hoax formation were carried out with the measurements of water vapor condensation rate, snow surface temperature, air temperature, humidity, wind speed, and net radiation. Large surface hoar crystals were observed to form under the breezy wind, I to 2 m s -i, at 0.1m high. The condensation rate increased linearly with the product of the vapor pressure gradient and the wind speed. The bulk transfer coefficient of water vapor Ce was roughly constant when the surface hoar crystals were small, whereas it showed some increase as the hoar crystals grew to several millimeters in height. A possible cause of this increase in Ce is that the developed surface hoar crystals modify the aerodynamic roughness and consequently increase the turbulent transfer of water vapor.
ABSTRACT. The quick growth of depth-hoar crystals was observed at nighttime just below the snow surface on a south-facing slope. This growth was due to a high temperature gradient (> 100 K m-I) near the snow surface under clear skies after a thin deposition of new snow on older and denser snow. The temperature gradient was greater when internal melting had taken place during daytime, keeping the sub-surface snow temperature at O°C even after sunset until all liquid water had frozen. To understand the relationship between the crystal growth rate and the temperature gradient, a series of experiments was carried out in the laboratory. The snow sample was set under a constant temperature gradient between 100 and 300 K m-I and sustained for about 50 h. The average crystal size increased linearly with time and the crystal growth rate increased as the given temperature gradient increased. The growth rates were in the order of 10-9 m S-I, which gave a good agreement with the results of the field observation. INTRODU CTIONThe seasonal snow cover has often been treated as a homogeneous medium, though it is highly stratified with many layers. Each layer has its own physical properties due to the initial snow condition at the time of deposition on the previous surface and the subsequent metamorphosis, which is mainly determined by the varying temperature field, load and arrangement of ice particles in the layer.A layer with relatively low strength plays an important role in a slab-avalanche release. The kinetic growth metamorphism produces a depth-hoar layer, which is typically weak because of thin bonds between particles. Accordingly, depth-hoar crystals have long been of interest in avalanche studies.Most studies on depth-hoar crystals have dealt with the basal part of a snow cover because the crystals have been frequently observed there. Therefore, previous experiments on kinetic growth metamorphism were performed for high-density snow (> 180 kg m-3 ), under low temperature gradients of < 100Km-1 on a time scale of several tens of days. Akitaya (1974) made a precise experiment in the cold laboratory on the morphology of depth-hoar crystals and constructed a diagram relating crystal shapes to temperature and temperature gradient. Marbouty (1980) determined the growth-rate dependency on temperature, temperature gradient and snow density. Both researchers executed their experiments under the conditions typical of basal layer. However, lower snow density and higher temperature gradients may be found near the snow surface, where the temperature field is highly fluctuating due to exposure to the ambient air temperature. These conditions may produce depth-hoar crystals in a short period. Actually, Akitaya and Shimizu (1987) found depth-hoar crystals in the layer beneath the snow surface. Colbeck (1989) simulated the high growth rate of faceted crystals near the surface.In this study, field observations were carried out to examine how long it takes to metamorphose new-snow crystals to depth-hoar crystals near the surface. Also, ...
ABSTRACT. Observations and measurements were made of high-speed avalanches at Shiai-dani, (he Kurobe canyon, (0 reveal their real features and properties, using strain-gauge-type load cells in measuring impact force and speed. A maximum impact pressure of 140 X 10 4 N m -' was obtained, as well as data indicating the existence of a number of wave fronts, ranging in speed from 9 to 60 m S-I inside an avalanche.Violent changes in atmospheric pressure ranging from -2 1 to + 5 mbar were observed near its path when a high-speed avalanche passed by. RESUME.
Surface hoar growing for several clear and humid days were observed. During daytime, air and snow-surface temperature increased and relative humidity decreased, hence evaporation (sublimation) occurred at the snow surface. The amount of evaporation calculated using a bulk-transfer method suggests that the surface-hoar crystals which grew during the previous night should have disappeared but they were observed to survive on the snow surface even during the daytime. During the following night, new surface-hoar crystals formed on top of the older ones and grew even larger. This result indicates that, although the surface-hoar crystals evaporated into the air during the daytime, snow grains beneath the surface were warmed by solar radiation and evaporated to the air. They may partially condense into the surface-hoar crystals and make up for the reduction in size. Depth-hoar crystals formed beneath the snow surface for several days and the surface layer, composed of both types of hoar crystal, showed a very weak shear strength.
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