Ground Freezing, Permafrost and the Active Layer

In many periglacial environments, a consequence of the long period of winter cold and the relatively short period of summer thaw is the formation of ground that remains frozen throughout the year. As soil freezes the water present within the soil either freezes in-situ, to form pore ice, or moves towards the freezing plane, to form lenses of segregated ice. Permafrost is defined on the basis of temperature. The permafrost table is the upper surface of the permafrost, and the ground above the permafrost table is called the supra-permafrost layer. Thermal conductivity is a major control over permafrost temperature gradient and active-layer thickness. The active layer refers to the layer of ground in areas underlain by permafrost which thaws during summer. The thickness of the active layer varies from year to year depending on controls such as ambient air temperature, slope orientation and angle, vegetation, drainage, snow cover, soil and/or rock type, and water content.