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Freezing, available onlin
library.wmo.int
/ref> or frost occurs when the air temperature falls below the
freezing point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...
of water (0  °C, 32  °F, 273 K). This is usually measured at the height of 1.2 metres above the ground surface. There exist some scales defining several degrees of frost severity (from "slight" to "very severe") but they depend on location thus the usual temperatures occurring in winter. The primary symptom of frost weather is that water freezes. If the temperature is low for sufficiently long time, freezing will occur with some delay in lakes, rivers, and the sea. It can occur even in
water supply network A water supply network or water supply system is a system of engineered hydrologic and hydraulic components that provide water supply. A water supply system typically includes the following: # A drainage basin (see water purification – sour ...
s, although this is highly undesirable and efforts are made to prevent this from happening.


Terminology

The English word "frost" has 2 base meanings that are related to each other but nevertheless sufficiently different: * temperature of air below the freezing point of water (ca 273 K) * deposit of ice on cold surfaces The WMO avoids the word "frost" alone and uses * "freezing" for temperature of air below the freezing point of water * "hoar frost" and "ground frost" for deposit of ice on cold surfaces (see Hoar frost)


Relation between freezing and hoar frost or ground frost

A temperature at or below freezing is not absolutely necessary to get ground frost or hoar frost; they can form even if air temperature is marginally above freezing point if the sky is clear. This is because the ground loses heat due to radiation. It radiates its heat to the sky/space. The amount of heat that radiates is proportional to the difference of the fourth power of the temperatures between the two objects. At night, the atmosphere is not being warmed by the sun and the sky/space can approach 2.7 K (the blackbody temperature of the
cosmic microwave background The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...
radiation). On a clear night the ground can become colder than the air because it radiates its heat to the sky, and ground frost can form. On the other hand, a temperature below the freezing point of water does not mean that hoar frost must occur.


See also

*
Freezing Freezing is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point. For most substances, the melting and freezing points are the same temperature; however, certain substances possess dif ...
(the physical aspect) *
Permafrost Permafrost () is soil or underwater sediment which continuously remains below for two years or more; the oldest permafrost has been continuously frozen for around 700,000 years. Whilst the shallowest permafrost has a vertical extent of below ...
(permanently frozen ground) *
Frost Frost is a thin layer of ice on a solid surface, which forms from water vapor that deposits onto a freezing surface. Frost forms when the air contains more water vapor than it can normally hold at a specific temperature. The process is simila ...
and Rime (deposit of ice on cold surfaces)


References

{{Authority control Snow or ice weather phenomena Atmospheric temperature