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Show Introduction Depth hoar, or Schwimmschnee, is the end- product of constructive metamorphism in the snow cover resulting from the internal sublimation and fliffusion of water vapor under the influence of strong temperature gradients. The decline in mechanical strength accompanying this recrys-tallization is an important source of snow cover instability which finds expression in deterioration of load- bearing capacity and in avalanche formation. According to Seligman ( 1936), depth hoar crystals were noticed in polar snow over a century ago, and recognition of their importance to avalanche formation is attributed to Paulcke ( 1932), Depth hoar has been reproduced in the laboratory, for example, by de Quervain ( 1958), and the dominant role of the temperature gradient clearly demonstrated. Recently an attempt has been made to formulate a quantitative expression for the snow cover's internal water vapor flux in terms of the basic diffusion equation, and to relate this calculated flux to observed depth hoar formation rates ( Giddings and LaChapelle, 1961). Practical problems, such as avalanche formation, which are introduced by the presence of mechanically weak depth hoar crystals have led to various experiments for their elimination. Compaction of depth hoar layers on avalanche paths, for instance, has been found a practical method of increasing snow cover stability. Concentrated foot- packing in avalanche release zones is now regular practice following early- season depth hoar development in some ski areas under Forest Service administration. Recently Giddings ( i960) has suggested that depth hoar formation might be inhibited or eliminated by chemical poisoning, a crystallographic phenomenon observed for other substances than ice. Such poisoning, or alteration of crystal habit by minute traces of foreign substances, has also been |