Permafrost is a layer of permanently frozen ground that is found in polar regions. It typically forms when the temperature of the ground cools sufficiently during the winter period to create frozen layers that last throughout the following summer. Permafrost can also form at high altitudes, even in non-polar areas.
There are two main types of permafrost: continuous permafrost and discontinuous permafrost. Continuous permafrost is present over a large area and has few or no unfrozen areas within it. Discontinuous permafrost is less extensive and has larger areas of unfrozen ground within it.
Permafrost can be either seasonal or active. Seasonal permafrost forms and thaws on a yearly basis. Active permafrost is permanently frozen and does not thaw even during summer