Frost line

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In soil, the frost line or freezing depth is the level down to which the soil will normally freeze each winter in a given area. The depth this reaches depends on three factors, the length of the period below freezing (two months of −5 °C freezes deeper than a month of −5 °C), how much below freezing the temperature is (a month of −10 °C freezes deeper than a month of −5 °C), and the amount of snow cover (snow insulates the ground from the effect of the cold. Thus, drier areas with less snow are more prone to deeper soil freezing).

In temperate regions of the Northern Hemisphere, freezing depth varies from only a few centimetres in locations with mild winters, such as Dublin or San Francisco, to as much as three metres in areas of central Russia with severe winters. In Arctic and Antarctic locations, the freezing depth is so deep that it becomes year-round permafrost, and there is instead a thaw line during the summer.

Building codes must take this into account, as foundations must be dug down to or below this point. Failure to do so will cause frost heaving to dislodge the building at least slightly, causing damage which may become a serious threat to the building's structural integrity.