Snow cover is a good insulator since it has low thermal conductivity. The structure of the seasonal snow cover usually consists of higher-density layers at the top with coarse, lower-density depth hoar layers at the base. Because of its much smaller thermal conductivity, changes in depth hoar fraction can have a significant impact on the insulating effect of the seasonal snow cover. A one-dimensional finite difference conductive heat transfer model with phase change was applied to investigate the effect of variations in the depth hoar fraction of the seasonal snow cover on the ground thermal regime. The snow cover was treated as a single layer with the effective thermal properties determined with considerations of the effect of the wind slab and depth hoar layer. The model was applied to investigate the thermal regime of the active layer and permafrost from October 5, 1986, through July 13, 1991, at West Dock near Prudhoe Bay, Alaska. Results indicate that the calculated temperatures on the ground surface, in the active layer, and in deep permafrost are in excellent agreement with the measured temperatures. A sensitivity study shows that in permafrost regions, variations in the depth hoar fraction from 0.0 to 0.6 can increase the daily ground surface temperature 12.8¿C and the mean annual ground surface temperature by 5.5¿C and delay the active layer freeze-up by several months. In nonpermafrost regions an increase in the depth hoar fraction from 0.0 to 0.6 can increase the daily ground surface temperature by 8.4¿C and the mean annual ground surface temperature by up to 2.4¿C and reduce the seasonal freezing depth by up to 80%. ¿ American Geophysical Union 1996