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Show INTRODUCTION Once snow has been deposited from the atmosphere, its subsequent evolution is strongly influenced by the internal diffusion of water vapor. In the absence of strong temperature gradients, vapor diffusion from one part of a snow crystal to another along pressure gradients established by differences in surface curvature results in the reduction of complex crystals to isometric snow grains. This process is termed destructive metamorphism. On the other hand, a strong temperature gradient in the snow cover establishes a gross vapor gradient which overrides those due to surface tension differences. This causes snow crystals to sublime and redeposit around new centers of crystallization. The latter process is termed constructive metamorphism, and leads to mechanically weak snow layers consisting of cup-shaped crystals, or depth hoar. Precipitated snow thus represents an unstable form of ice crystals under the temperature regime normal to a temperate snow cover. It has been suggested jTBader, ( 1 fj that depth hoar is an equilibrium form of crystalline ice, and the observed stability of these crystals, once formed, appears to bear this out. Mature depth hoar structure persists for extended periods of time with very little physical change as long as snow temperature remains below freezing and superimposed compressive loads are light. The existence of depth hoar layers in the snow cover apparently was recognized in the polar regions over a centure ago L_ Seligman, ( 2) J . Its presence in Alpine snow covers was first described by Paulcke ( 3), who pointed out its significance in avalanche formation. Since then, the peculiarly weak mechanical structure of depth hoar has come to be recognized as an important factor in snow stability. While the effect of temperature gradient on depth hoar formation has been clearly recognized, and also demonstrated |