A region of permanently frozen ground where summer heating is insufficient to counteract the low temperatures attained during winter. This occurs where the mean annual surface temperature is below approximately −5 °C, thus primarily in the polar regions, but also at high altitudes at lower latitudes. Permafrost is estimated to underlie approximately 20 per cent of the Earth's land surface.
Freezing may penetrate to considerable depths (reaching over 1 500 m in parts of Siberia). Ground ice is normally (but not always) present. The surface layer is usually subject to thawing in summer to a depth that depends upon prevailing conditions. A layer that may thaw in certain years is known as a supra-permafrost layer or pereletok. A layer of permanently unfrozen ground that lies above permafrost but beneath a seasonally frozen surface layer is known as talik, as are areas surrounded by permafrost that do not themselves freeze.
Permafrost regions exhibit characteristic surface features, including ice wedges (ice-filled cracks that taper downwards), polygonal ground (regular patterns of stones caused by localized frost heave, solifluction, and cracking), ice mounds, pingoes, and frost blisters.
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