| OCR Text |
Show - 6 - by a distance, d, which is large compared with their thickness, ( This corresponds to the extreme case where conduction through the ice framework is zero.) A temperature difference exists between top and bottom of the snow cover, imposing a uniform temperature gradient within the snow. Assume this gradient to be l° C./ cm, a large value. The ice laminae will have an emissivity for heat radiation, e, less than 1.0, but the latter value will be assumed, giving the maximum radiation transfer. The radiation transfer between two adjacent laminae at absolute temperatures T]_ and T2 ( and, since the temperature gradient is uniform, between top and bottom of the snow layer) is then given by2 Qr. ^ T4- 0 where Q = radiant heat flux r < 5~ = 0.% l7JO~'° c&{//£ vn'x/ rC} ir)/ cJ< q4 Let d = 0.5 mm and T± = 273° K ( 0° C); then T2 = 273.05° and Q^ = 0,5 cal/ cm2/ day. For a given temperature difference, the radiation transfer diminishes with absolute temperatures Let d « 0,5 mm and T± = 2* f3° K (- 30° C); then T2 = 243- 05° and Qr = 0.3 cal/ cm2/ day. These figures probably represent a low value, for the mean intercrystal-ine distance can well be greater. On the other hand, it is highly unlikely that it exceeds more than a very few millimeters. Let d * 3.0 mm and T]_ = 273° K? then T2 = 273.3° and Q^ = 2.9 cal/ cm2/ day, This latter figure probably represents an extreme; ordinarily the radiation transfer might fall below 3 cal/ day. Certainly under the temperature regine discussed by Murcray and Echols (- 10 to - 30° C.) it must be |
