{{Short description, Altitude in which the temperature is at 0 °C The freezing level, or 0 °C (zero-degree) isotherm, represents the

altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...

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

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

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

icing conditions In aeronautics, icing is the atmospheric icing, formation of water ice on an aircraft. Icing has resulted in numerous aviation accidents and incidents, fatal accidents in aviation history. Ice accretion and accumulation can affect the external s ...

, 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 In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to ...

and other factors may change the level. The 700

hPa The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI). It is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is an S ...

pressure level (or about 3000 m above

sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...

) is generally assumed as a rough estimate of the freezing 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 the scientific study of the Earth's atmosphere and short-term atmospheric phenomena (i.e. weather), with a focus on weather forecasting. It has applications in the military, aviation, energy production, transport, agricultur ...

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

weather forecasts Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmo ...

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

Measuring

There are several different methods to examine the structure of the

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

of the

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

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 calculat ...

attached to a

weather balloon A weather balloon, also known as a sounding balloon, is a balloon (specifically a type of high-altitude balloon) that carries instruments to the stratosphere to send back information on atmospheric pressure, temperature, humidity and wind spe ...

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 airplane for transporting passengers and air cargo. Such aircraft are most often operated by airlines. The modern and most common variant of the airliner is a long, tube shaped, and jet powered aircraft. The largest o ...

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 are mainly of two types: polar orbiting (covering the entire Earth asyn ...

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 that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

it emits indicating its temperature. *

Weather radar A weather radar, also called weather surveillance radar (WSR) and Doppler weather radar, is a type of radar used to locate precipitation (meteorology), precipitation, calculate its motion, and estimate its type (rain, snow, hail etc.). Modern w ...

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 Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

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 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 in ...

. 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 (meteorology), trough of Low-pressure area, low pressure. It often forms behind an extratropica ...

, and hundreds for a

warm front Warm, WARM, or Warmth may refer to: * A somewhat high temperature; heat * Kindness Music Albums * ''Warm'' (Herb Alpert album), 1969 * ''Warm'' (Jeff Tweedy album), 2018 * ''Warm'' (Johnny Mathis album), 1958, and the title song * ''Warm'' ( ...

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

wind Wind is the natural movement of atmosphere of Earth, air or other gases relative to a planetary surface, planet's surface. Winds occur on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heatin ...

, reflection of the

sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

snow Snow consists of 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 througho ...

, 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 (meteorology), precipitation, dew, or fog t ...

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