Permanently frozen ground exists in circumpolar and high-altitude regions as a balance between average air temperature and the geothermal gradient (the increase in soil temperature with depth that arises from the transfer of heat from the Earth's core). To a lesser extent, whether or not the ground remains permanently frozen also depends on seasonal temperature variation and on the physical and thermal properties of the soil. The graphic shows the depth extent of frozen (blue) and thawed (brown) ground over a time span of two years, beginning in April. The soil region near the surface that undergoes seasonal freeze/thaw cycles is called the active layer. The frozen soil region below the active layer, if it exists, is called permafrost.
Snapshot 1: Frost extent resulting from very low average air temperature.
Snapshot 2: Frost extent resulting from higher soil thermal diffusivity.
Snapshot 3: Frost extent resulting from high average air temperature.