| Term: | Permafrost |
| Definition: |
Layer of soil or rock, at some depth beneath the surface, in which the temperature has been continuously below 0 NSIDCCryosphere
A layer of soil at varying depths below the surface in which the temperature has remained below freezing continuously from a few to several thousands of years. NOAA-NWS Ground (soil or rock and included ice and organic material) that remains at or below 0C for at least two consecutive years. IPCC2014 Layer of soil or rock in which the temperature has been continuously below 0 degrees Celsius for at least some years. WMOHydrology Ground (soil or rock and included ice and organic material) that remains at or below 0˚C for at least two consecutive years IPAPermafrost Ground (soil or rock and included Ice and organic material) that remains at or below 0C for at least two consecutive years. Permafrost is synonymous with perennially Cryotic Ground: it is defined on the basis of temperature. It is not necessarily frozen, because the Freezing Point of the included water may be depressed several degrees below 0C; moisture in the form of water or Ice may or may not be present. In other words, whereas all perennially Frozen Ground is Permafrost, not all Permafrost is perennially frozen. Permafrost should not be regarded as permanent, because natural or man-made changes in the climate or terrain may cause the temperature of the ground to rise above 0C. The following classification may be used for Permafrost distribution models: Very High Likelihood (> 90% underlain by Permafrost): Most of this area is expected to be underlain by Permafrost. _ High Likelihood (70-90% underlain by Permafrost): Large parts of this area are expected to be underlain by Permafrost. Locally Permafrost Thicknesses of >50 m are expected depending on the topographic conditions. _ Medium Likelihood (40-70% underlain by Permafrost): Depending on the local surface characteristics and topography Permafrost is expected. Large bodies of Permafrost as well as areas with only isolated patches are possible. However, no thick (> 20m) and Continuous Permafrost layers are expected. _ Low Likelihood (10-40% underlain by Permafrost): Little Permafrost is expected in these areas. However, patches of Permafrost are possible due to local microclimatic conditions. _ Very Low Likelihood (1-10% underlain by Permafrost): Generally, no Permafrost is expected in these areas. However, sporadic exceptions are possible due to local microclimatic conditions. _ Extremely Low Likelihood (<1% underlain by Permafrost): No Permafrost is expected within these areas. However, cold caves, local Dead Ice or relict Ground Ice cannot be completely ruled out even in these areas depending on local microclimate and paleoglacial conditions. TrombottoGeocryology 1. A layer of soil or bedrock at a variable depth beneath the surface of the earth in which the temperature has been below freezing continuously from a few to several thousands of years. (Also called perennially frozen ground, pergelisol, permanently frozen ground.) Permafrost exists where the summer heating fails to descend to the base of the layer of frozen ground. A continuous stratum of permafrost is found where the annual mean temperature is below about -5C (23F). 2. As limited in application by P. F. Svetsov, soil that is known to have been frozen for at least a century. AMSglossary 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. PhysicalGeography Layer of soil or rock, at some depth beneath the surface, in which the temperature has been continuously below 0 degrees C for at least some years. It exists where summer heating fails to reach the base of the layer of frozen ground. syn. Pergelisol WMOMeteoterm GCW |