OCR Text |
Show floor ( horizon) will have a higher temperature and a greater water vapor density than the layer itself, since it receives more water vapor than it gives off. Thus, the density of any snow layer increases rather than decreases as a result of water vapor diffusion. The layer next the ground is no exception, differing from those above it only in that its lower surface is in direct contact with the ground. Releasing vapor upward, it receives little or none from below especially if the underlying ground is dry or stony. But even here, decrease in volume ( bulk) as a result of water vapor transfer does not take place because of any internal relationship in the layer but rather, and solely, because the layer floor is in contact with the ground. The upflow of vapor from the lower surface of the snowcover cannot be great. Measurements of water storage ( concentration) were made on a light snow fallen on even, hard- packed ground. After the test layer was lightly sprinkled with colored powder, it was covered with snow. Discrepancies in the water storage measured were always within the normal limits of error for the 2- 3 months of measurement. The observations were confirmed by our calculations. The amount of diffused vapor is expressed by the formula: H= D al4 "- a^ 2 ST Q) where M is the amount of water vapor, D the diffusion coefficient, ai and a2 the concentration ( storage) of water vapor at two different layer floors, L the distance between floors, S the diffusion surface area, and T the time. Using the cgs system and taking the humidity figures given in the chapter " Snowcover Temperature," we get M the amount of water ( in grams), D the coefficient for a temperature of 0° C and normal atmospheric pressure of 0.22, ai absolute humidity under the snow at temperature - 1.3° C = 4.37 g/ m3 or 4.37 x 10- 6 g/ cm3; likewise, a2 at 10 cm above the ground at temperature - 2.2° C = 4.07 x 10"° cm^ ( L = 10 cm, S = 1cm2, T = 200 24- hour periods of 86,400 sec). Whence: ( 4.37 - 4.07) x 10_ o M = 0.22 Yo 1 x 86,400 x 200 = 0.114 Considering the space to be partly filled with snowgrains, and taking snow density to be 0.25, the sectional area reduces to 0.75 and distance L increases about one and a half times. Thus, to produce and to diffuse water vapor in winter, each cm2 of surface must evaporate 0.06 g or 2- 4 mm of snow. During the daytime in spring, the " day" surface may be warmer than the underlying layers. In such case, one of the middle layers may be the warmest, although diffusion to it may be slight. If we take the specific heat of ice as 0.5 and the ( latent) heat of sublimation as 680 cals., we find that to equalize the temperatures of two neighboring layers differing by 10° it is sufficient that each gram of relatively warmer snow evaporate and each gram of colder' snow sublimate 0.004 g of ice. This means that more or less appreciable changes in snow density due to water vapor diffusion can release considerable amounts of heat. When snow increases in density from 0.15 to 0.20, every gram of snow produces 227 cals., an amount of heat capable of melting the snow and bringing it to a boil with a number of calories to spare. The fact that recrystallization proceeds faster in a thin layer is explained - 12- |