(Also called perennially frozen ground, pergelisol, permanently frozen
ground.) 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. 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).
