OCR Text |
Show greater pressure over points and lesser pressure over depressions, it is stated, vaporization takes place at sharp points or angles and condensation occurs in hollows. As noted above, one can readily observe the disappearance of fine details of crystal structure in snow within a few hours of its fall. But differences in water vapor pressure are a factor only in the case of short- radius curves. For a radius of 0.01 mm this difference amounts to only 0.1%; and since such small particles make up only a minimal part of snowcover bulk, it is impossible to attribute further changes in snow to the difference in water vapor pressure over surfaces of varying curvature. If the redistribution of snow substance depended solely on the curvature of particle surfaces, snowflakes would grow together at points of high relief and refill depressions at points of crystal contact upon evaporation, thus transforming the snow into a bound mass of ever increasing firmness. To the last, such snow would constitute a porous mass of ovoid particles, lacking points; and a dense layer would have the structure of hardened foam with no trace of crystals.* Actually^ the salient points do not evaporate away, but rather, after being broken off, restore themselves due to the capacity of crystals to reconstruct their facets. Growing crystals take shape with the minimum expenditure of surface energy ( tension) and reconstruct any destroyed parts, especially summits and points. The crystals gradually acquire a more regular shape ( flat sides, straight ribs, and sharp points), and the bond between crystals decreases rather than increases, to the point where formerly bound snow will become completely friable. Any snowcover, then, is a polycrystal consisting of countless small crystals. Since the small crystals possess a certain amount of free energy and therefore are less resistant to damage than larger crystals, these latter grow at the expense of the small crystals when conditions are right. The rate of growth ( of the large crystals) increases with rise in temperature, expecially with rises to near the melting point. It is still not clear why grain growth stops, or why large, perfect crystals grow upon these grains. Snow Thaw When thawing begins, the snowcover settles markedly and changes from loose snow to a bound mass. No actual water droplets are observed in moist snow; probably only a water film is present. With prolonged thawing droplets of water begin to appear. The snow becomes wet, containing capillary water. It loses its firmness and breaks or yields easily under a weight it would support when dry. This is due to the decrease in surface energy ( tension) and thus the firmness of the snow, brought about by wetting of the solid mass. Sharp snow grains and crystals of regular shape lose their edges and become rounded. Later, the amount of water increases and flows downward through the snow as gravitational water. When this water reaches dense snow layers, it impregnates them and spreads horizontally, both inside the layers and over layer surfaces. Lenses of loose snow in dense layers may remain dry. The more compact the snow, the more rapid the horizontal impregnation of its layers. Where ice crusts are present in the upper layers, they may begin to melt, leaving the snow covering them dry. * Editor's Note: The author is essentially describing here the common process of destructive metamorphism, which apparently is uncommon in his research area. - 14- |