Permafrost, or perennially frozen ground, is defined as soil or rock having temperatures below 0 °C over at least two consecutive winters and the intervening summer. Much of the permafrost has been frozen since the Pleistocene. Permafrost occurs in the Arctic, Antarctic, and high alpine regions. About one-fifth of the total land area of the world is underlain by permafrost (Burdick et al. 1978).
The top layer of the ground in which the temperature fluctuates above or below 0 °C during the year is defined as the active layer (Andersland and Ladanyi 1994). Other terms such as seasonally frozen ground, seasonal frost, and annually thawed layer are synonyms for the active layer. The thickness of this layer varies spatially and temporally.
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