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Show - 11 - When the temperature gradient is removed, and especially when the snow temperature rises to the freezing point, destructive metamorphism quickly reverses the depth hoar formation, and the snow crystals once more tend to approach isometric ice grains. If depth hoar formation has been strong, these grains often are larger than those produced purely by destructive metamorphism. Even where they are indistinguishable, the. altered gross mechanical character of the snow persists, and old depth hoar layers can often be recognized by their lower strength properties long after they have lost their crystallographic identity [ see LaChapelle ( 13J It is not necessary that the mass divergence be large for depth hoar to form; it may, in fact, be very small or absent throughout most of the snow cover. Moreover, the mass flux through a given plane need not be great. The actual occurrence may be termed a large mass translation, A small amount of ice is lost at the bottom of the snow cover and a corresponding gain appears higher up through a displacement process in which the intervening layers may in their entirety pass through the vapor stage and be reformed as depth hoar. As noted above, this effect is most pronounced in the lower layers of the snow cover, but in the case when steep temperature gradients are persistent, the entire snow cover from top to bottom may be turned to depth hoar, ( Snow conditions in south- east Switzerland prior to the catastrophic avalanche cycle of January 1951 were an excellent example of this latter occurrence.) There remains considerable scope for investigation of the vapor transfer phenomenon in snow. From the standpoint of basic physics, it can provide additional understanding of snow behavior, and from the practical standpoint there is much to be learned about prediction and artificial modification of the depth hoar formation which is so important mechanically |