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In environments containing
In environments containing permafrost
Permafrost is ground that continuously remains below 0 °C (32 °F) for two or more years, located on land or under the ocean. Most common in the Northern Hemisphere, around 15% of the Northern Hemisphere or 11% of the global surface ...
, the active layer is the top layer of soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
that thaws during the summer and freezes again during the autumn. In all climate
Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologic ...
s, 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
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
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
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
, 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
Vegetation is an assemblage of plant species and the ground cover they provide. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic character ...
).
When there is not sufficient heat to thaw the frozen soil completely, permafrost forms. The active layer in this environment consists of the top layers of soil which thaws during the summer, while the inactive layer refers to the