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The freezing level, or 0 °C (zero-degree) isotherm, represents the altitude in which the temperature is at 0 °C (the freezing point of water) in a free atmosphere (i.e. allowing reflection of the sun by snow, icing conditions, etc.). Any given measure is valid for only a short period of time, often less than a day as variations in wind, sunlight,
There are several different methods to examine the structure of the temperature of the
air masses
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...
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, 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
There are several different methods to examine the structure of the temperature of the atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
including its freezing level:
*A radiosonde attached to a weather balloon
A weather balloon, also known as sounding balloon, is a balloon (specifically a type of high-altitude balloon) that carries instruments aloft to send back information on atmospheric pressure, temperature, humidity and wind speed by means of a ...
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 permit reporting the isotherm, and its height from sea level, to aerial traffic.
* Weather satellites are equipped with sensors that scan the atmosphere and measure the infrared radiation it emits indicating its temperature.
* Weather radar 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, an upward pointing radar, can detect the speed of precipitation, which is different for rain, snow, slush and melting snow.
Depending on the frequency and resolution 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. This changes the isotherm gradually, over tens of kilometres for a cold front, and hundreds for a warm front, but the change spreads over a large area. #Local levels can be changed by wind, reflection of the sun, snow, and humidity 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
In the study of Earth's atmosphere, subsidence is the downward movement of an air parcel as it cools and becomes denser. By contrast, warm air becomes less dense and moves upwards (atmospheric convection).
Subsidence generally creates a high ...
and ascendence can contribute to variations in the isotherm.
These conditions imply that the 0 °C isotherm varies globally and more so locally.
See also
* Temperature inversion *Lapse rate
The lapse rate is the rate at which an atmospheric variable, normally temperature in Earth's atmosphere, falls with altitude. ''Lapse rate'' arises from the word ''lapse'', in the sense of a gradual fall. In dry air, the adiabatic lapse rate is ...
References
* ''The Mountain Manual'', Seuil, 2000 Mountains Montane ecology Meteorological phenomena