Frozen ground

Seasonally frozen ground, like snow, covers a large expanse of the globe. Its depth and distribution varies as a function of air temperature, snow depth and vegetation, and can exhibit high temporal and spatial variability. Most of the Earth's snow-covered area is located in the Northern Hemisphere, Although not completely known, the global extent of permafrost underlies about 24.5% of exposed Northern Hemisphere land areas. Thickness exceeds 600 m along the Arctic coast of northeastern Siberia and Alaska, but permafrost thins and becomes horizontally discontinuous towards the margins. Only about 2 million sq km consists of actual ground ice ("ice-rich"). The remainder (dry permafrost) is simply soil or rock at subfreezing temperatures.