Permafrost is defined on the basis of temperature, as soil or rock that remains below 0°C throughout the year, and forms when the ground cools sufficiently in winter to produce a frozen layer that persists throughout the following summer.
A typical example of ground temperatures within permafrost in the Interior Alaska region is shown in the left figure.
The annual range in ground temperatures is shown by the warmest and coolest temperatures occurring at depth. With increasing depth in the ground, the seasonal difference in temperature decreases. The point at which there is no discernable change in temperature is termed the "depth of zero annual amplitude".
In Fairbanks, this depth occurs at about 3-15 m depending on moisture content and surface materials. Each year a portion of the ground at the surface rises above 0°C for part of the year. This part of the ground, termed the active layer, freezes and thaws with the changing seasons.
Both the thickness of permafrost and the active layer depend on local climatic conditions, vegetation cover and soil properties as well as geothermal gradient .
An illustration of the range in temperatures experienced at different depths in the ground during the year. The active layer (shown in grey) thaws each summer and freezes each winter, while the permafrost layer remains below 0°C.