A layer in the ground that remains frozen throughout the year that generally occurs where the annual mean air temperature is below -1 C. Most permafrost lies below an active layer that thaws for periods of weeks to months in the summer. Permafrost is continuous over most land north of 65 N and discontinuous south to about 50 N in Canada and to about 47 N in eastern Asia. The total area covered is about 7.6 million km2 of continuous and 17.3 million km2 of discontinuous permafrost, with the total volume equivalent to a sea level change of 0.08-0.18 m.
Land that remains at or below zero degrees Celsius continuously for at least 2 consecutive years. Permafrost is not defined by soil moisture content, overlying snow cover, or location; but is defined solely by temperature. Permafrost may contain over 30 percent ice, or essentially no ice at all. It can be covered by several meters of snow, or be completely bare of snow. Underground, permafrost consists of frozen soils ranging from gravel to silt. Silty soil is composed of fine, powdery sedimentary particles. They possess great "wicking" capabilities that enable water to migrate and accumulate as large bodies of ice in the permanently frozen silt. Two main areas of permafrost are recognized. Permafrost areas which do not thaw at all during the year and permafrost areas whose upper layers thaw briefly during the warm season underlain by thicker layers which do not thaw even at mid-summer.
a layer of soil which remains frozen from year-to-year; permafrost exists in many northern soils and may be a few inches to 1000 feet thick; continuous permafrost occurs everywhere in a given area, and discontinuous permafrost occurs in some places but not in others in a given area.
Zone of permanently frozen water found in high latitude soils and sediments. Five types of permafrost have been recognized: continuous permafrost, discontinuous permafrost, sporadic permafrost, alpine permafrost, and subsea permafrost.
layer of soil or rock, at some depth beneath the surface, in which the temperature has been continuously below 0°C for at least several years; it exists where summer heating fails to reach the base of the layer of frozen ground.