| OCR Text |
Show - 7 - water vapor, a conclusion the present authors find untenable in view of the above arguments. Several explanations of this increase can be proposed. It is very possible that a narrow air gap developed over part of the cross section between the wire gauze and the snow in the can below. The thermal conductivity in such a gap would be greatly reduced because of the absence of the highly conducting solid network. Consequently the temperature gradient would be greatly increased in the gap. As shown by equation ( 11), the flux through a given cross section is proportional to the temperature gradient and would thus be greatly increased. Since the weight change of the separate cans depends on the mass flux across the boundaries, this phenomenon could well account for anomolous diffusion coefficients. An estimate of the increase in the observed D due to the presence of an air gap can be made by assuming that the ratio of the temperature gradient fn the gap compared to that in the neighboring snow is equal to the inverse ratio of the respective thermal conductivities, A/ AV • The expression of Jansson ( 6), 0.00005 + 0.0019^=*+ 0.006^> 2 { P - density), can be substituted for } \ . Thermal conductivity in air and the diffusion of latent heat as water vapor contribute almost equally to the conductivity in the gap, /\\/ This situation will, of course, change at higher altitudes since the latter quantity is inversely proportional to pressure. Ignoring this for the moment, / i\/ may be approximated as twice the thermal conductivity ofairat0° C; V ? ^ 5 ^ ^ 1 0 " 5 C ^ W " ' ° C ' ' $ CC "' ^ / } v = 0,454 Ilp+ 54p* ( 13) This ratio must be multiplied by the fraction of the total cross section in which a gap occurs to calculate the expected increase in D. If this |
