permafrost table

Permafrost refers to ground that remains frozen for two or more consecutive years, with temperatures consistently below 0°C (32°F). This frozen layer can be found at variable depths below the surface in frigid regions. The upper surface of permafrost is known as the permafrost table.12

In Arctic regions, the ground above the permafrost, called the active layer, thaws during the brief summer and refreezes in winter. The thickness of this active layer varies depending on moisture content and soil type, ranging from less than a foot in wet organic sediments to several feet in well-drained gravels. Permafrost underlies approximately 24 percent of the land surface in the Northern Hemisphere.31