Influence of Snow Cover on the Lower Limit of Permafrost in Altai Mountains
Zhang Tingjun, Tong Boliang and Li Shude (Lanzhou Institute of Glaciology and Geocryology, Academia Sinica)
This paper mainly deals with the influence of snow cover on the lower limit of permafrost in Altai Mountains. Investigation reveals that the mean annual air temperature at the lower limit of permafrost in Altai Mountains is -5.4℃, 2 to 4℃ lower than that at the lower limit of alpine permafrost in North Hemisphere. Through investigations of snow cover it was observed that the heat preserving effect of snow decreases the seasonally frozen depth in winter; conversely, in summer snow has little effect on seasonally thawed depth, and even promotes thawing. Thus seasonally thawed depth is greater than seasonally frozen depth. The difference between these two is the latent seasonally thawed depth (△H) which results from seasonal snow accumulation. The appearence of △H shows that, in the course of the air-ground heat exchange,the energy absorption of ground surface is greater than its release. The internal energy of permafrost increases. It makes the temperature (tcp)of the permafrost at the depth of zero annual amplitude rise, and eventually the permafrost degenerate.