Large portions of the land are also frozen during the year. Permafrost is substrate that has been frozen for a minimum of 2 years. There are two types of permafrost: discontinuous and continuous. Discontinuous permafrost is found in areas where the mean annual air temperature is only slightly below freezing (); this forms in sheltered locations. In areas where the mean annual soil surface temperature is below , continuous permafrost forms. This is not limited to sheltered areas and ranges from a few inches below the surface to over deep. The top layer is called the active layer. It thaws in the summer and is critical to plant life.