OCR Text |
Show ice crystals are also caused by sublimation and by the nature of their occurrence are very much like cavern crystals. Moisture rising from the ground or carried in by wind coming through vent holes with cooling settles out the excess water vapor. Subsurface ice crystals can be made to appear artificially on small objects such as match boxes and bits of paper by placing them in spaces made under the snow for the purpose. The causes and conditions of subsurface crystal growth vary. Where there is a large internal snow surface, the temperatures of the snow grains and of the enclosed air are the same, and the water vapor pressure is the same as for vapor saturated over ice. The crystal growth here is not due to an excess of external water vapor entering the space, but is the result of a redistribution of material brought about by differences in surface energy ( tension) of the separate crystals. The vapor- ice system is in equilibrium and its temperature is steady and unchanging. Snow Density Snow density was measured with a weighing device in general use at meteorological stations. For thin layers of loose snow, where the error in measuring density can be considerable with such a device, a thin- walled calibrated cylinder with sharp edges and a 25 cm2 surface was used. In this case, snow height was measured to an accuracy of 1 mm. Test specimens of freshly fallen snow were taken on a special wooden plaque laid on the snow surface and cleared daily. For two successive winters flat dishes of 500 cm3 content were placed outside for the same purpose. The height and weight of the snow were measured immediately following a fall. This latter method, the most accurate where increase in the amount of ( new) snow is very slight, can be used only in still weather. Altogether, about 4000 density measurements were taken, most of them in different snowcovers. Snow which falls as large crystals during still weather has a density of 0.02- 0.05. The lowest density measured was 0.008 in a 20 cm layer of downy snow. The density of snow composed of small crystals can be as high as 0.10. When light winds of 1- 2 m/ sec. velocity pile snow, the density increases. In cases of such light wind action, no matter how large or small the snow crystals, the crystals are laid down more compactly, and density can never be less than 0.05. Snow which falls during still weather will have the same density over a large area, whereas snow subject to wind action will vary in density according to protection of the site. In country intersected by gullies, and beside obstacles, a layer laid down by one snowfall will vary in density at adjacent sites. Snow fallen in a protected place may have a density of 0.04, and nearby, on a site open to the wind, 0.40. With the beginning of recrystallization and sinking of the snow, the density of even the loosest snow increases. With transformation to fine granular snow, density increases to 0.08- 0.12. Subsequent increases, to 0.15- 0.20 with transformation to granular snow, are more gradual. Once increase in density has ceased, the snow remains unaltered until thawing sets in. There is a sharp increase in density with thaw, when even quite loose snow will increase in density to 0.20. - 18- |