| OCR Text |
Show As already pointed out, the highly variable effects of heat supply on the ablation can be recognized from Fig. 2. If a heat exchange coefficient of 10 meal cm"^ min" 1 deg" 1 is assumed, it is seen that for a melting surface an increase of 1 degree in / i/ j or of 10 meal cm"' min" 1 in Q+ B raises the ablation, - M, by 0.075 mm h" 1 or 1.8 mm d" 1. An increase in e, of 0.49 torr has the same effect ( at 760 torr), as shown from the known terms in the psychrometric theory, p Cp/ ObW Tw = 0.49 torr deg . For a non- melting surface the change in - M is highly dependent on the surface temperature, so that a change in nj^ , Q+ B, or e. also alters the surface temperature n/~ and thus at the same time compensates in part for the original change. For the above increases in s\ rL or Q+ B at a surface temperature slightly below 0° C there is an increase of - M around 0.0041 mm h" 1 or 0.10 mm d" 1, while at - 30° C, - M increases by 0.00056 mm h" 1 or 0.013 mm d" 1- These values are only 5.5% and 0.8% respectively of the changes in ablation for the same changes in n \ or Q+ B at a melting surface. An increase in e. even results in a decrease in - M. For the same increase in eL as above and at a surface temperature just below 0° C, M increases (- M decreases) about 0.0053 mm h" 1 or 0.13 mm d" 1. Here, too, this effect is only 7.1% of that for a melting surface. It thus is important to distinguish whether the changes in ablation - M resulting from changes in the meteorological parameters Q+ B, "> J£ and e involve a melting or a non- melting surface. An increase in vapor pressure causes an increase in ablation for a melting surface, but a ( considerably smaller) decrease in ablation for a non- melting surface. An increase of heat flux from radiation and by conduction from the ground, as well as an increase in air temperature causes an increase of ablation in both cases. This ablation for a non- melting surface is always only a few percent of that for a melting surface, 13 |
