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Definition:
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The thickness of the layer of the ground that is subject to annual thawing and freezing in areas underlain by Permafrost. The thickness of the Active Layer depends on factors such as the ambient air temperature, vegetation, drainage, soil or rock type (thermal properties of soil or rock), total water content, Snow Cover, and degree and orientation of slope. The thickness of the Active Layer can vary from year to year, primarily due to variations in the mean annual air temperature, distribution of soil moisture, and Snow Cover. The thickness of the Active Layer includes the uppermost part of the Permafrost wherever either the salinity or clay content of the Permafrost allows it to thaw and refreeze annually, even though the material remains cryotic (T < 0C).
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