The freezing level, or 0 °C (zero-degree) isotherm, represents the

altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...

in which the temperature is at 0 °C (the freezing point of

water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

) in a free atmosphere (i.e. allowing reflection of the sun by snow,

icing conditions In aviation, icing conditions are atmospheric conditions that can lead to the formation of water ice on an aircraft. Ice accretion and accumulation can affect the external surfaces of an aircraft – in which case it is referred to as ''airframe ...

, etc.). Any given measure is valid for only a short period of time, often less than a day as variations in wind, sunlight, air masses and other factors may change the level. Above the freezing altitude, the temperature of the air is below freezing. Below it, the temperature is above freezing. The profile of this frontier, and its variations, are studied in

meteorology Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...

, and are used for a variety of forecasts and predictions, especially in cold weather. Whilst not given on general

weather forecasts Weather forecasting is the application of science and technology to predict the conditions of the atmosphere for a given location and time. People have attempted to predict the weather informally for millennia and formally since the 19th centu ...

, it is used on bulletins giving forecasts for mountainous areas.

Measuring

Dual-polarization radar image of an easily identifiable freezing layer, denoted by the yellow ring around the radar

There are several different methods to examine the structure of the

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

of the atmosphere including its freezing level: *A

radiosonde A radiosonde is a battery-powered telemetry instrument carried into the atmosphere usually by a weather balloon that measures various atmospheric parameters and transmits them by radio to a ground receiver. Modern radiosondes measure or calcula ...

attached to a weather balloon is the oldest and most common method used. Each area normally releases two balloons a day in locations hundreds of kilometers apart. *Measuring devices attached to commercial

airliners An airliner is a type of aircraft for transporting passengers and air cargo. Such aircraft are most often operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an ai ...

permit reporting the isotherm, and its height from sea level, to aerial traffic. *

Weather satellites A weather satellite or meteorological satellite is a type of Earth observation satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be polar orbiting (covering the entire Earth asynchronously), or ge ...

are equipped with sensors that scan the atmosphere and measure the

infrared radiation Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

it emits indicating its temperature. *

Weather radar Weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern weather radars are mostly pulse- ...

detects bright bands, which are radar echoes produced just underneath and within the isotherm caused by the melting of snow or ice in the layer below that's above 0 °C. *A

wind profiler A wind profiler is a type of weather observing equipment that uses radar or sound waves ( SODAR) to detect the wind speed and direction at various elevations above the ground. Readings are made at each kilometer above sea level, up to the extent o ...

, an upward pointing radar, can detect the speed of precipitation, which is different for rain, snow,

slush Slush, also called slush ice, is a slurry mixture of small ice crystals (e.g., snow) and liquid water. In the natural environment, slush forms when ice or snow melts or during mixed precipitation. This often mixes with dirt and other polluta ...

and melting snow. Depending on the

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

and

resolution Resolution(s) may refer to: Common meanings * Resolution (debate), the statement which is debated in policy debate * Resolution (law), a written motion adopted by a deliberative body * New Year's resolution, a commitment that an individual mak ...

at which these readings are taken, these methods can report the isotherm with greater or lesser precision. Radiosondes, for example, only report a reading twice daily and provide very rough information. Weather radar can detect a variation every five to ten minutes if there is precipitation, and can scan a radius of up to two kilometers.

Variations in the isotherm

The isotherm can be very stable over a large area. It varies under two major conditions: #A change in the density of air due to

weather fronts A weather front is a boundary separating air masses for which several characteristics differ, such as air density, wind, temperature, and humidity. Disturbed and unstable weather due to these differences often arises along the boundary. For ins ...

. This changes the isotherm gradually, over tens of kilometres for a

cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern ...

, and hundreds for a

warm front A warm front is a density discontinuity located at the leading edge of a homogeneous warm air mass, and is typically located on the equator-facing edge of an isotherm gradient. Warm fronts lie within broader troughs of low pressure than cold f ...

, but the change spreads over a large area. #Local levels can be changed by

wind Wind is the natural movement of air or other gases relative to a planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few ho ...

, reflection of the

sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout ...

, and

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...

level. These factors can cause the isotherm to change rapidly and sometimes constantly over several kilometres, in both winter and summer all year round. Also, atmospheric subsidence and ascendence can contribute to variations in the isotherm. These conditions imply that the 0 °C isotherm varies globally and more so locally.

References

* ''The Mountain Manual'', Seuil, 2000 Mountains Montane ecology Meteorological phenomena

Measuring

Variations in the isotherm

See also

References