The distance from the ground surface downward to the level beneath which there is no annual fluctuation in ground temperature. A change of no more than 0.1C throughout the year is arbitrarily consider
ed as 'practically no annual fluctuation'. In the Northern hemisphere the temperature at the Depth of Zero Annual Amplitude ranges from about -0.1C at the southern limit of the Permafrost region to about -15C in the extreme polar reaches of the zone of Continuous Permafrost. The Depth of Zero Annual Amplitude varies widely but generally lies between 10 and 20 m below the ground surface, depending on climatic and terrain conditions such as amplitude of annual surface temperature variation, vegetation, Snow Cover and characteristics of the soils and rocks including thermal diffusivity.