Gelisols are characterized by the presence of permafrost (soil temperature below 0 °C 【32 °F】) for at least two years in succession within two metres (about six feet) of the land surface. The permafrost layer may have a high organic carbon content, like Histosols (Histosol), or may exhibit substantial vertical mixing of the soil owing to cycles of freezing and thawing, resulting in structures similar to those found in Vertisols (Vertisol). In climates with significant precipitation and periods of annual warming, the soil distribution may be highly discontinuous. Gelisols with thick surface organic layers predominate in the Low Arctic, whereas those that exhibit pronounced vertical mixing are common in the High and Middle Arctic, where they are identified by hilly terrain and a landscape patterned with polygon-shaped cracking. Gelisols differ from Entisols (Entisol), Histosols, Inceptisols (Inceptisol), and Vertisols solely by the additional presence of permafrost.