The frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the soil and adjacent materials, and on nearby heat sources. For example, snow cover and asphalt insulate the ground and homes can heat the ground (see also heat island). The line varies by latitude, it is deeper closer to the poles. Per Federal Highway Administration Publication Number FHWA-HRT-08-057, the maximum frost depth observed in the contiguous United States ranges from 0 to 8 feet (2.4 m). Below that depth, the temperature varies, but is always above 32 °F (0 °C).
Alternatively, in Arctic and Antarctic locations the freezing depth is so deep that it becomes year-round permafrost, and the term "thaw depth" is used instead. Finally, in tropical regions, frost line may refer to the vertical geographic elevation below which frost does not occur.
Frost front refers to the varying position of the frost line during seasonal periods of freezing and thawing.
Building codes sometimes take frost depth into account because of frost heaving which can damage buildings by moving their foundations. Foundations are normally built below the frost depth for this reason. Water and sewage pipes are normally buried below the frost line to prevent them from freezing. Alternatively, pipes may be insulated or actively heated using heat-tape or similar products to allow for shallower depths. Due to additional cost, this method is typically only used where deeper trenching is not an option due to utility conflicts, shallow bedrock, or other conditions that make deeper excavation infeasible.
There are many ways to predict frost depth including factors which relate air temperature, soil temperature and soil properties.