How cold is permafrost?

Permafrost, or permanently frozen ground, is soil, sediment, or rock that remains at or below 0°C for at least two years. It occurs both on land and beneath offshore Arctic continental shelves, and its thickness ranges from less than 1 meter to greater than 1,000 meters. Seasonally frozen ground is near-surface soil that freezes for more than 15 days per year. Intermittently frozen ground is near-surface soil that freezes from one to 15 days per year.

Permafrost underlies 12 to 18 percent of the exposed land surface in the Northern Hemisphere. Seasonally frozen ground regions may cover as much as 55 percent.

Frozen ground data are critical to understanding environmental change, validating models, and building and maintaining structures in seasonal frost and permafrost regions.

Permafrost

Definition

Any rock or soil material that has remained below 32° F continuously for two or more years. The two-year minimum stipulation is meant to exclude from the definition the overlying ground surface layer which freezes every winter and thaws every summer (called the "active layer" or "seasonal frost").

By type

Cold permafrost — Remains below 30° F, and which may be as low as 10° F as on the North Slope; tolerates introduction of considerable heat without thawing.

Ice-rich — 20% to 50% visible ice.

Thaw-stable — Permafrost in bedrock, in well drained, coarse-grained sediments such as glacial outwash gravel, and in many sand and gravel mixtures. Subsidence or settlement when thawed is minor, foundation remains essentially sound.

Thaw-unstable — Poorly drained, fine grained soils, especially silts and clays. Such soils generally contain large amounts of ice. The result of thawing can be loss of strength, excessive settlement and soil containing so much moisture that it flows.

Warm permafrost — Remains just below 32° F. The addition of very little additional heat may induce thawing.

Varies between 31 Farenheit and minus 20 farenheit.