Freezing, available onlin
library.wmo.int
/ref> or frost occurs when the

air temperature Atmospheric temperature is a measure of temperature at different levels of the Earth's atmosphere. It is governed by many factors, including incoming solar radiation, humidity and altitude. When discussing surface air temperature, the annual at ...

falls below the

freezing point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depe ...

of water (0

°C The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The d ...

, 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 – sourc ...

s, although this is highly undesirable and efforts are done 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 The World Meteorological Organization (WMO) is a specialized agency of the United Nations responsible for promoting international cooperation on atmospheric science, climatology, hydrology and geophysics. The WMO originated from the Internati ...

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 Frost is a thin layer of ice on a solid surface, which forms from water vapor in an above-freezing atmosphere coming in contact with a solid surface whose temperature is below freezing, and resulting in a phase transition, phase change from wa ...

)

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 In Big Bang cosmology the cosmic microwave background (CMB, CMBR) is electromagnetic radiation that is a remnant from an early stage of the universe, also known as "relic radiation". The CMB is faint cosmic background radiation filling all spac ...

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.

References

{{Authority control Snow or ice weather phenomena Atmospheric temperature

Terminology

Relation between freezing and hoar frost or ground frost

See also

References