Seasonally frozen ground (SFG) is a term denoting a type of ground that freezes and thaws annually. Although the extent of an SFG area is difficult to identify precisely, it may be assumed to cover more than 48 × 106 km2 in the northern hemisphere, which is twice as much as the permafrost area. Seasonally freezing depth (SFD), defined with the use of various empirical formulas, depends on the duration of frost period and severity, on the composition and properties of soil, on the moisture content, and on the heat flow from the earth. It is also influenced by the vegetation and snow cover. Some of the frost-induced soil structures and landforms which occur in the SFG zone, such as frost-boils, patterned ground, earth hummocks and ploughing boulders are characteristic of the periglacial zone. These are mostly ephemeral or extrazonal forms.
