Permafrost - Continuous and Discontinuous Permafrost

Continuous and Discontinuous Permafrost

Permafrost typically forms in any climate where the mean annual air temperature is less than the freezing point of water. Exceptions are found in moist-wintered forest climates, such as in Northern Scandinavia and the North-Eastern part of European Russia west of the Urals, where snow acts as an insulating blanket. The bottoms of many glaciers can also be free of permafrost.

Typically, the below-ground temperature varies less from season to season than the air temperature, with temperatures tending to increase with depth. Thus, if the mean annual air temperature is only slightly below 0 °C (32 °F), permafrost will form only in spots that are sheltered—usually with a northerly aspect. This creates what is known as discontinuous permafrost. Usually, permafrost will remain discontinuous in a climate where the mean annual soil surface temperature is between -5 and 0 °C (23 and 32 °F). In the moist-wintered areas mentioned before, there may not be even discontinuous permafrost down to −2 °C (28 °F). Discontinuous permafrost is often further divided into extensive discontinuous permafrost, where permafrost covers between 50 and 90 percent of the landscape and is usually found in areas with mean annual temperatures between -2 and -4 °C (28 and 25 °F), and sporadic permafrost, where permafrost cover is less than 50 percent of the landscape and typically occurs at mean annual temperatures between 0 and -2 °C (32 and 28 °F).

In soil science, the sporadic permafrost zone is abbreviated SPZ and the extensive discontinuous permafrost zone DPZ.

Exceptions occur in un-glaciated Siberia and Alaska where the present depth of permafrost is a relic of climatic conditions during glacial ages where winters were up to 11 °C (19.8 °F) colder than those of today. At mean annual soil surface temperatures below −5 °C (23 °F) the influence of aspect can never be sufficient to thaw permafrost and a zone of continuous permafrost (abbreviated to CPZ) forms. "Fossil" cold anomalies in the Geothermal gradient in areas where deep permafrost developed during the Pleistocene persist down to several hundred metres. The Suwałki cold anomaly in Poland led to the recognition that similar thermal disturbances related to Pleistocene-Holocene climatic changes are recorded in boreholes throughout Poland.

A line of continuous permafrost in the Northern Hemisphere represents the most southerly border where land is covered by continuous permafrost or glacial ice. The line of continuous permafrost varies around the world northward or southward due to regional climatic changes. In the southern hemisphere, most of the equivalent line would fall within the Southern Ocean if there were land there. Most of the Antarctic continent is overlain by glaciers.

In the Andes along the Atacama Desert permafrost extends down to an altitude of 4,400 metres (14,400 ft) and is continuous above 5,600 metres (18,400 ft).