Frost line

Frost line

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The frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...

 in soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...

 is expected to freeze. The frost depth depends on the climatic conditions
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

 of an area, the heat transfer
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...

 properties of the soil and adjacent materials, and on nearby heat sources. For example, snow cover and asphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...

Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...

 the ground and homes can heat the ground (see also heat island). The line varies by latitude, it is deeper closer to the poles. It ranges in the United States from about zero to six feet. Below that depth the temperature varies, but is always above 32 degrees Fahrenheit.

Alternatively, in Arctic
The Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...

 and Antarctic
The Antarctic is the region around the Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica and the ice shelves, waters and island territories in the Southern Ocean situated south of the Antarctic Convergence...

 locations the freezing depth is so deep that it becomes year-round permafrost
In geology, permafrost, cryotic soil or permafrost soil is soil at or below the freezing point of water for two or more years. Ice is not always present, as may be in the case of nonporous bedrock, but it frequently occurs and it may be in amounts exceeding the potential hydraulic saturation of...

, and the term "thaw depth
Thaw depth
In soil science, the thaw depth or thaw line is the level down to which the permafrost soil will normally thaw each summer in a given area....

" is used instead. Finally, in tropical regions, frost line may refer to the vertical geographic elevation
The elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ....

 below which frost
Frost is the solid deposition of water vapor from saturated air. It is formed when solid surfaces are cooled to below the dew point of the adjacent air as well as below the freezing point of water. Frost crystals' size differ depending on time and water vapour available. Frost is also usually...

 does not occur.

Frost front refers to the varying position of the frost line during seasonal periods of freezing and thawing.

Building codes

Building code
Building code
A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...

s sometimes take frost depth into account because of frost heaving
Frost heaving
Frost heaving results from ice forming beneath the surface of soil during freezing conditions in the atmosphere. The ice grows in the direction of heat loss , starting at the freezing front or boundary in the soil...

 which can damage buildings by moving their foundations. Foundations are normally built below the frost depth for this reason. Water pipes are normally buried below the frost line, or insulated to prevent them from freezing.

There are many ways to predict frost depth including factors which relate air temperature to soil temperature.

Sample frost lines for various locations

  • Minnesota
    Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...

    , USA (2007):
    • Northern counties: 5 feet (1.5 m)
    • Southern counties: 3.5 feet (1.1 m)