OCR Text |
Show - 10 - Qy - latent heat transfer T = temperature difference, ° C p ss vapor pressure difference, mm P a atmospheric pressure, mm ( There is some question about the applicability of this expression to internal processes in a snow cover, but for the sake of the present discussion it, or a similar relation, is assumed valid,) The ratio of Qfo to Qv is" seen to be linearly dependent on pressure, and the corresponding non- linear dependency on altitude is graphed in Figure 2, It must be recognized that there are two distinct processes of crystal metamorphism which may take place within a snow cover. Both depend on vapor transfer. The first of these, occurring when temperature gradients >^- are small or absent, is termed destructive metamorphism. Water vapor is transferred from one part of single snow crystal to another along the vapor pressure gradients set up by differences in curvature of different parts of the crystal. The ultimate result is to reduce the complex deposited snow crystals ( such as stars or dendrites) to rounded, isometric ice grains, A process of homogenization takes place whereby crystals originally of widely different form all approach this common end condition, and the snow cover shrinks and becomes mechanically stronger. The second type of change is termed constructive metamorphism, and occurs when the temperature gradient becomes large enough to over- ride the intracrystalline surface energy gradient and cause water vapor transport throughthe snow cover. The vapor so transported is redeposited around new centers of crystallization, forming entirely new crystals which are large, angular, layered and mechanically fragile ( depth hoar). Shrinkage in this case is small. |