<P> Greenhouse gases, such as methane (CH), nitrous oxide (N O), water vapor (H O) and carbon dioxide (CO), absorb certain wavelengths of OLR adding heat to the atmosphere . Some of this thermal 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 may contribute to global warming 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 . </P> <P> 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 . </P> <P> Longwave radiation typically refers to radiation in the spectral region from 3 to 100 microns . In the Earth's climate system, longwave radiation involves processes of absorption, scattering, and emissions from atmospheric gases, aerosols, clouds and the surface . Measuring outgoing longwave radiation at the top of atmosphere and downwelling longwave radiation at the surface are important for understanding how much radiative energy is kept in our climate system, how much reaches and warms the surface, and how the energy in the atmosphere is distributed to affect developments of clouds . </P> <P> Outgoing longwave radiation (OLR) has been monitored globally since 1975 by a number of successful and valuable satellite missions . These missions include broadband measurements from the Earth Radiation Balance (ERB) instrument on the Nimbus - 6 and Nimbus - 7 satellites; Earth Radiation Budget Experiment (ERBE) scanner and the ERBE non scanner on NOAA - 9, NOAA - 10 and NASA Earth Radiation Budget Satellite (ERBS); The Clouds and the Earth's Radiant Energy System (CERES) instrument aboard NASA's Aqua and Terra satellites; and Geostationary Earth Radiation Budget instrument (GERB) instrument on the Meteosat Second Generation (MSG) satellite . </P>

The earth's surface emits terrestrial longwave thermal infrared radiation