Outgoing Long-wave Radiation (OLR) is

electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible ...

of wavelengths from 3–100 μm emitted from Earth and its atmosphere out to space in the form of

thermal radiation Thermal radiation is electromagnetic radiation generated by the thermal motion of particles in matter. Thermal radiation is generated when heat from the movement of charges in the material (electrons and protons in common forms of matter) is ...

. It is also referred to as up-welling long-wave radiation and terrestrial long-wave flux, among others. The

flux Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ...

of energy transported by outgoing long-wave radiation is measured in W/m². In the Earth's climate system, long-wave radiation involves processes of absorption, scattering, and emissions from atmospheric gases, aerosols, clouds and the surface. Over 99% of outgoing long-wave radiation has wavelengths between 4 μm and 100 μm, in the thermal

infrared 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 aroun ...

part of the

electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...

. Contributions with wavelengths larger than 40 μm are small, therefore often only wavelengths up to 50 μm are considered . In the wavelength range between 4 μm and 10 μm the spectrum of outgoing long-wave radiation overlaps that of

solar radiation Solar irradiance is the power per unit area ( surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre ...

, and for various applications different cut-off wavelengths between the two may be chosen. Radiative cooling by outgoing long-wave radiation is the primary way the Earth System loses energy. The balance between this loss and the energy gained by radiative heating from incoming solar shortwave radiation determines global heating or cooling of the Earth system ( Energy budget of Earth's climate). Local differences between radiative heating and cooling provide the energy that drives atmospheric dynamics.

Atmospheric energy balance

OLR is a critical component of the Earth's energy budget, and represents the total radiation going to space emitted by the atmosphere. OLR contributes to the net all-wave radiation for a surface which is equal to the sum of shortwave and long-wave down-welling radiation minus the sum of shortwave and long-wave up-welling radiation. The net all-wave radiation balance is dominated by long-wave radiation during the night and during most times of the year in the polar regions. Earth's radiation balance is quite closely achieved since the OLR very nearly equals the Shortwave Absorbed Radiation received at high energy from the sun. Thus, the Earth's average temperature is very nearly stable. The OLR balance is affected by clouds and dust in the atmosphere. Clouds tend to block penetration of long-wave radiation through the cloud and increases cloud albedo, causing a lower flux of long-wave radiation into the atmosphere. This is done by absorption and scattering of the wavelengths representing long-wave radiation since absorption will cause the radiation to stay in the cloud and scattering will reflect the radiation back to earth. The atmosphere generally absorbs long-wave radiation well due to absorption by water vapour, carbon dioxide, and ozone. Assuming no cloud cover, most long-wave up-welling radiation travels to space through the

atmospheric window An atmospheric window is a range of wavelengths of the electromagnetic spectrum that can pass through the atmosphere of Earth. The optical, infrared and radio windows comprise the three main atmospheric windows. The windows provide direct channels ...

occurring in the electromagnetic wavelength region between 8 and 11 μm where the atmosphere does not absorb long-wave radiation except for in the small region within this between 9.6 and 9.8 μm. The interaction between up-welling long wave radiation and the atmosphere is complicated due to absorption occurring at all levels of the atmosphere and this absorption depends on the absorptivities of the constituents of the atmosphere at a particular point in time.

Role in greenhouse effect

The reduction of the surface long-wave radiative flux drives the

greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...

Greenhouse gases A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), meth ...

, such as

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ear ...

(CH₄),

nitrous oxide Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has ...

(N₂O),

water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous p ...

(H₂O) and

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

(CO₂), absorb certain wavelengths of OLR, preventing the thermal radiation from reaching space, adding heat to the atmosphere. Some of this thermal radiation is directed back towards the Earth by scattering, increasing the average temperature of the Earth's surface. Therefore, an increase in the concentration of a greenhouse gas may contribute to

global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate variability and change, Climate change in a broader sense also includes ...

by increasing the amount of radiation that is absorbed and emitted by these atmospheric constituents. If the absorptivity of the gas is high and the gas is present in a high enough concentration, the absorption bandwidth becomes saturated. In this case, there is enough gas present to completely absorb the radiated energy in the absorption bandwidth before the upper atmosphere is reached, and adding a higher concentration of this gas will have no additional effect on the energy budget of the atmosphere. The OLR is dependent on the temperature of the radiating body. It is affected by the Earth's skin temperature, skin surface emissivity, atmospheric temperature, water vapor profile, and cloud cover.

OLR measurements

Measuring outgoing long-wave radiation at the top of atmosphere and down-welling long-wave radiation back towards the surface is important for understanding how much energy is kept in Earth's climate system; for example how thermal radiation cools and warms the surface, and how this energy is distributed to affect the development of clouds. Observing this radiative flux from a surface also provides a practical way of assessing surface temperatures at both local and global scales. Outgoing long-wave radiation (OLR) has been monitored and reported since 1970 by an ongoing progression of satellite missions and instruments. Earliest observations were with infrared interferometer spectrometer and radiometer (IRIS) instruments developed for the Nimbus program and deployed on Nimbus-3 and Nimbus-4. These

Michelson interferometer The Michelson interferometer is a common configuration for optical interferometry and was invented by the 19/20th-century American physicist Albert Abraham Michelson. Using a beam splitter, a light source is split into two arms. Each of those ...

s were designed to span wavelengths of 5-25 microns. Improved measurements were obtained starting with the Earth Radiation Balance (ERB) instruments on Nimbus-6 and Nimbus-7. These were followed by the Earth Radiation Budget Experiment) scanners and the non scanner on NOAA-9, NOAA-10 and Earth Radiation Budget Satellite; the Clouds and the Earth's Radiant Energy System instruments aboard Aqua, Terra, Suomi-NPP and NOAA-20; and the Geostationary Earth Radiation Budget instrument (GERB) instrument on the Meteosat Second Generation (MSG) satellite. Down-welling long-wave radiation at the surface is mainly measured by Pyrgeometer. A most notable ground-based network for monitoring surface long-wave radiation is Baseline Surface Radiation Network (BSRN), which provides crucial well-calibrated measurements for studying

global dimming Global dimming is the reduction in the amount of global direct irradiance at the Earth's surface that has been observed since systematic measurements began in the 1950s. The effect varies by location, but worldwide it has been estimated to be of ...

and brightening.

OLR calculation and simulation

Many applications call for calculation of long-wave radiation quantities: the balance of global incoming shortwave to outgoing long-wave radiative flux determines the Energy budget of Earth's climate; local radiative cooling by outgoing long-wave radiation (and heating by shortwave radiation) drive the temperature and dynamics of different parts of the atmosphere. By using the radiance measured from a particular direction by an instrument, atmospheric properties (like

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...

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 ...

) can be inversely inferred. Calculations of these quantities solve the radiative transfer equations that describe radiation in the atmosphere. Usually the solution is done numerically by atmospheric radiative transfer codes adapted to the specific problem. Another common approach is to estimate values using surface temperature and emissivity, then compare to satellite top-of-atmosphere radiance or brightness temperature.

References

External links