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⭐ In the context of permafrost, the active layer is considered…

The active layer is the surface layer of permafrost regions that thaws during warmer months and refreezes during colder months, creating a dynamic zone above the permanently frozen ground.

⭐ Core Definition: Active layer

In environments containing permafrost, the active layer is the top layer of soil that thaws during the summer and freezes again during the autumn. In all climates, whether they contain permafrost or not, the temperature in the lower levels of the soil will remain more stable than that at the surface, where the influence of the ambient temperature is greatest. This means that, over many years, the influence of cooling in winter and heating in summer (in temperate climates) will decrease as depth increases.

If the winter temperature is below the freezing point of water, a frost front will form in the soil. This "frost front" is the boundary between frozen and unfrozen soil, and with the coming of spring and summer, the soil is thawed, always from the top down. If the heating during summer exceeds the cooling during winter, the soil will be completely thawed during the summer and there will be no permafrost. This occurs when the mean annual temperature is above 0 °C (32 °F), but also occurs when the mean annual temperature is slightly below 0 °C on sites exposed to the sun with coarse-textured parent materials (vegetation).

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👉 Active layer in the context of Permafrost

Permafrost (from perma- 'permanent' and frost) is soil or underwater sediment which continuously remains below 0 °C (32 °F) for two years or more; the oldest permafrost has been continuously frozen for around 700,000 years. Whilst the shallowest permafrost has a vertical extent of below a meter (3 ft), the deepest is greater than 1,500 m (4,900 ft). Similarly, the area of individual permafrost zones may be limited to narrow mountain summits or extend across vast Arctic regions. The ground beneath glaciers and ice sheets is not usually defined as permafrost, so on land, permafrost is generally located beneath a so-called active layer of soil which freezes and thaws depending on the season.

Around 15% of the Northern Hemisphere or 11% of the global surface is underlain by permafrost, covering a total area of around 18 million km (6.9 million sq mi). This includes large areas of Alaska, Canada, Greenland, and Siberia. It is also located in high mountain regions, with the Tibetan Plateau being a prominent example. Only a minority of permafrost exists in the Southern Hemisphere, where it is consigned to mountain slopes like in the Andes of Patagonia, the Southern Alps of New Zealand, or the highest mountains of Antarctica.

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