Outgoing longwave radiation
Outgoing Longwave Radiation (OLR) is the energy radiating from the Earth as infrared radiation at low energy to Space.
OLR is electromagnetic radiation emitted from Earth and its atmosphere out to space in the form of thermal radiation. The flux of energy transported by outgoing longwave radiation is measured in W/m².
Over 99% of outgoing longwave radiation has wavelengths between 4 µm and 100 µm, in the thermal infrared part of the electromagnetic spectrum. 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 longwave radiation overlaps that of solar radiation, and for various applications different cut-off wavelengths between the two may be chosen.
Radiative cooling by outgoing longwave 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.
OLR is a critical component of the Earth's energy budget, and represents the total radiation going to space emitted by the atmosphere. 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 is affected by clouds and dust in the atmosphere, which tend to reduce it to below clear sky values.
Greenhouse gases, such as methane (CH4), nitrous oxide (N2O), water vapor (H2O) and carbon dioxide (CO2), absorb certain wavelengths of OLR adding heat to the atmosphere, which in turn causes the respective absorbing layer of the atmosphere to emit more radiation. Some of this radiation is directed back towards the Earth, increasing the average temperature of the Earth's surface. Therefore, an increase in the concentration of a greenhouse gas would contribute to global warming by increasing the amount of radiation that is absorbed and emitted by these atmospheric constituents.
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