Active layer

This means that, over many years, the influence of cooling in winter and heating in summer (in temperate climates) will decrease as depth increases.

Liquid water cannot flow below the active layer, with the result that permafrost environments tend to be very poorly drained and boggy.

The thickness of the active layer is the average annual thaw depth in permafrost areas, due to solar heating of the surface.

Thus, in a continuous permafrost environment plants must have shallow roots, which restricts tree growth to specialised species such as Larix.

Cryoturbation is the dominant force operating in the active layer, and tends to make it generally uniform in composition throughout.

The slow rate of decomposition of organic material means gelisols (permafrost soils) are very important as a sink for carbon dioxide.

The red dotted-to-solid line depicts the average temperature profile with depth of soil in a permafrost region. The trumpet-shaped lines at the top show seasonal maximum and minimum temperatures in the "active layer", which commences at the depth where the maximum annual temperature intersects 0 °C. The active layer is seasonally frozen. The middle zone is permanently frozen as "permafrost". And the bottom layer is where the geothermal temperature is above freezing. Note the importance of the vertical 0 °C line: It denotes the bottom of the active layer in the seasonally variable temperature zone and the bottom limit of permafrost as the temperature increases with depth.