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Show 0 21 43 - 10 8 27 - 20 - 4 2 - 31 - 17 - 3 In cases where air temperature rises above 0° C and there is a dry wind, the snow will remain dry and not melt because the lowering of snow temperature due to evaporation compensates for the heat transfer from the air. Table 1 shows the amount of snow in grams that can be melted at normal atmospheric pressure by cooling of 1 m^ of air of varying humidity and temperature to 0° C with condensation of water vapor. The minus sign indicates that the snow does not melt and that freezing takes place because of cooling from the evaporation state to complete air saturation. The following values were used for the calculations tabulated: Specific heat capacity of air 0.24 Specific heat capacity of water vapor 0.48 Latent heat of fusion ( ice) 80 cals. Latent heat of vaporization ( ice) 676 cals. Table 1 Amount of Snow ( in grams) Melted with Cooling of 1 m^ of Air to 0° C T Relative Humidity in % 100 75 50 25 0° + 3 + 6 When moist snow freezes, it forms a fragile, nonsolid, easily destroyed layer; but when wet snow freezes, it forms a layer of solid ice containing air bubbles. Wet snow freezes slowly. At the beginning of cold weather, wet snow may remain a long time under a frozen layer. In the spring, with frequent temperature variations between plus and minus, portions of the loose surface snow are transformed into rounded ice grains measuring up to 6 mm. Dry granular and coarse granular snow of the lower layers is transformed into this same kind of snow as its particles lose their sharp edges with thawing. We shall call this type of snow " wet granular snow." Its density is 0.25- 0.50. Dense snow that has not undergone recrystallization does not become granular on melting. Certain snow patches, especially those formed by great avalanches, also snow bridges over streams, are often composed entirely of such snow. In summer the thawed ground around such patches gives off heat from below, resulting in elevation of the snow border to heights of 1- 2 m above the ground. These elevated narrow snow borders do not curve downward; they may be solid to a depth of 15 cm and can support a man. The density of this wet solid snow is 0.50- 0.70. The surface of thawing snow patches, particularly dense snow, is filled with depressions of various shapes, very often quadrilaterals of regular form measuring 20- 50 cm across, with sloping sides, forming saucer- like hollows 3- 10 cm - 15- |
