OCR Text |
Show not so much by the greater concentration ( water storage) gradient as by greater wind penetration. As it penetrates the snow, the wind changes the snow temperature and with it the water vapor density. It also transfers water vapor from one layer to another. The recrystallization process can be speeded up by the consequent suction or pumping action. Between January 18 and 20, 1939, there was a steady wind of 6- 7 ball force; the snowcover was 50 cm thick, the upper layers consisting of freshly fallen snow containing a small percentage of fine grains and the bottom layer consisting of grains measuring 1.0- 1.5 mm. After undergoing wind action, the top layer consisted entirely of fine granular snow; in the bottom layer grain size increased to 2.0- 2.5 mm; and in the intermediate layers considerable numbers of regularly shaped crystals appeared that did not exist before. Changes also occurred in layers underlying ice crusts 2- 3 mm thick. In a case described earlier in which a 2 m layer of loose, fine granular snow was observed on the south slope of Mount Aykuayventchorr, the changes which took place can be explained by the protection of the site from the wind and by the strength of the snow. Snow recrystallization is accelerated by abrupt temperature changes that result in either evaporation or sublimation. Thus, blown snow with a density of 0.12 still consisted entirely of crystals and crystal fragments the second day after deposition. At night the ( snow) temperature dropped from - 7° to - 20° C and to - 28° at the snow surface. In the morning the temperature rose to - 8° C. Ice grains now reappeared in the snow and the top 2- 3 cm of snow material became loose. Changes produced by temperature variations are marked only in the surface layers, however. As observation has shown, a temporary rise in snow temperature to 0° C affects subsequent snow transformation. If a snowfall is moist, or is later saturated by thawing, the following changes proceed rapidly. On November 14- 15, with air temperature at - 1° to - 5° C, there was a fall of moist snow consisting of stars and needles. An 11 cm layer accumulated; density was 0.11. In three days many small pellets appeared and the snow became loose. By December 2 the snow consisted of 1 mm grains. By December 7 coarse, regular crystals began to appear, and within a week these made up one third of the snow's bulk. Complete recrystallization had taken place at twice the normal rate. One could sometimes observe layers of coarse, granular hard snow, density 0.45, with fully developed crystals lying parallel, in which recrystallization was already past the second stage. It was nearly always possible to establish thawing as the cause here. The following is probably to be attributed to the same cause: The snow stratum of several mm thickness underlying an ice crust and in direct contact with it will change more rapidly than the layer immediately below because snow surface temperature rises to around 0° C when rains or thaws produce crusts. The ice crusts themselves, especially when snow crystal debris is observed in them, are also destroyed by thawing. It is possible that this accelerated change in ice crusts is like the phenomenon observed on glass when crystals form by thermic process. The changes in dry snow are sometimes explained as due to differences in water vapor pressure over uneven surfaces ( of varying curvature). Because of - 13- |