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. More: While the top layer of soil heats up in the summer, permafrost exists in deeper layers where the summer heating fails to melt the frozen ground. A continuous layer of permafrost is found in areas where the annual mean temperature is below about −5°C (23°F).
