OCR Text |
Show radiation balance. This, on the other hand, is largely determined, disregarding snow surface albedo and global radiation, by direct solar radiation. Thus it is that strong solar radiation, relatively high air temperatures and vapor pressure values less than 4.58 torr, form the most favorable conditions for the occurrence of selective ablation. As shown by the table presented at the beginning of this section, low air pressure is also a contributing factor. These results from the present discussion are in good agreement with the " favorable factors for forming penitentes" which C. Troll C6] deduced from extensive observations. It must be admitted, however, that his penetrating study of sun cups and similar formations showed that many other influences were involved, discussion of which lies quite outside the framework of the present discourse. Here it should be remarked in conclusion about the treatment of the general question of energy transfer associated with snow cover ablation, that the significant characteristics of selective ablation can be readily deduced from the laws of heat balance at a surface and the associated laws of heat and mass transfer 27 |