Permafrost

subarctic and Arctic regions

permafrost, perennially frozen ground, a naturally occurring material with a temperature colder than 0 °C (32 °F) continuously for two or more years. Such a layer of frozen ground is designated exclusively on the basis of temperature. Part or all of its moisture may be unfrozen, depending on the chemical composition of the water or the depression of the freezing point by capillary forces. Permafrost with saline soil moisture, for example, may be colder than 0 °C for several years but contain no ice and thus not be firmly cemented. Most permafrost, however, is consolidated by ice.

Permafrost with no water, and thus no ice, is termed dry permafrost. The upper surface of permafrost is called the permafrost table. In permafrost areas the surface layer of ground that freezes in the winter (seasonally frozen ground) and thaws in summer is called the active layer. The thickness of the active layer depends mainly on the moisture content, varying from less than a foot in thickness in wet, organic sediments to several feet in well-drained gravels.

Permafrost forms and exists in a climate where the mean annual air temperature is 0 °C or colder. Such a climate is generally characterized by long cold winters with little snow and short, relatively dry, cool summers. Permafrost, therefore, is widespread in the Arctic, subarctic, and Antarctica. It is estimated to underlie 24 percent of the land surface in the Northern Hemisphere.