| OCR Text |
Show Equation ( 20) forms a straight line in a diagram with ns. as abscissa and eL as ordinate. In Fig. 6, which was calculated for p = 525 torr, this is the line from the point O^ = 0° C, eL = 4.58 torr, which intersects the abscissa at eL = 0, rC^. = 13.5 C. Ablation which accentuates surface irregularities is thus possible only for those pairs of values oZ and eL which plot to the left of this line and below the curve ( concave upward) corresponding to the saturation vapor pressure of water. There is a further limitation, imposed by the fact that these values of / Oj" and e. must produce a melting surface. From Equation ( 8) for the boundary between melting and non- melting surfaces it follows that H + z^ lLeL = ^ + 2^^ E^ Q±£ ( 21) pCp pcp CLL A melting surface is produced only when the equivalent temperature of the air, o/ L + ( 0.6^ 3T^/ pCp)£, , determined using r£ , is greater than that for the melting surface, ^ + ( G.£ 2.3T-/ pCp) E0, reduced by Q+ B/ arL . For Q+ B = 0, Equation ( 21) plots in Fig. 6 as a straight ( dashed) line from a^ = o° C, eL = 4.58 torr, which intersects the abscissa at e, = 0 , a/ j_ = 15.3° C. This line lies to the right of the boundary line for ablation which accentuates surface irregularities, For Q+ B = 0 the promotion of irregularities is thus impossible, and still less so for Q+ B < 0 . Positive values of Q+ B are definitely necessary for ablation to promote irregularities, and this will occur the more strongly, the larger is the radiation balance, Q, for especially at the ridge crests B plays only a subordinate role. However, the values which Q can assume are not unlimited. For snow, Q+ B = 400 meal cm" 2 min" 1 would be a high value, though this does not exclude the possibility of higher values. Taking this together with the above 19 |
