| OCR Text |
Show - 5 - Several factors must be considered both in estimating and measuring the apparent diffusion coefficient, D. One factor is the presence of a solid network which, for many diffusion processes, obstructs diffusion and reduces D, Another factor is the " hand to hand " passage of vapor molecules, described by Yoslda, in which vapor is condensed on one side and released on the other. This Is usually thought to increase D. Both of the above factors are accounted for in the following analysis. If a straight line is extended through a homogeneous snow cover in the z direction, a fraction f of the line will intersect void space and a fraction 1- f will intersect solid material. The quantity f is the porosity of the snow. Since the solid ( ice) has a heat conductivity of the order of 102 larger than that of air, the temperature gradient along the line in the solid will be much smaller than that in the void space, ( although the ratio will be much less than 10^). To a fair approximation in high- porosity snow, the entire temperature drop along the line may be considered to take place within the void space. Consequently the average temperature gradient within the void space is related to the observed temperature gradient by a relationship obtained previously by Yoslda, This equation is obviously limited at very high densities ( low porosities). If one now imagines a plane of unit area normal to the z axis intersecting the snow and fixed to the solid network, the flux through the plane, from equation ( 1) is J * Dctp/ dZ--' - D ( dp/ d T) ( d T/ dz I The flux through the plane, however, is occurring entirely In the void |
