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Outgoing longwave radiation

Spectral intensity of sunlight (average at top of atmosphere) and thermal radiation emitted by Earth's surface.

In climate science, longwave radiation (LWR) is electromagnetic thermal radiation emitted by Earth's surface, atmosphere, and clouds. It may also be referred to as terrestrial radiation. This radiation is in the infrared portion of the spectrum, but is distinct from the shortwave (SW) near-infrared radiation found in sunlight.[1]: 2251 

Outgoing longwave radiation (OLR) is the longwave radiation emitted to space from the top of Earth's atmosphere.[1]: 2241  It may also be referred to as emitted terrestrial radiation. Outgoing longwave radiation plays an important role in planetary cooling.

Longwave radiation generally spans wavelengths ranging from 3–100 micrometres (μm). A cutoff of 4 μm is sometimes used to differentiate sunlight from longwave radiation. Less than 1% of sunlight has wavelengths greater than 4 μm. Over 99% of outgoing longwave radiation has wavelengths between 4 μm and 100 μm.[2]

The flux of energy transported by outgoing longwave radiation is typically measured in units of watts per metre squared (W⋅m−2). In the case of global energy flux, the W/m2 value is obtained by dividing the total energy flow over the surface of the globe (measured in watts) by the surface area of the Earth, 5.1×1014 m2 (5.1×108 km2; 2.0×108 sq mi).[3]

Emitting outgoing longwave radiation is the only way Earth loses energy to space, i.e., the only way the planet cools itself.[4] Radiative heating from absorbed sunlight, and radiative cooling to space via OLR power the heat engine that drives atmospheric dynamics.[5]

The balance between OLR (energy lost) and incoming solar shortwave radiation (energy gained) determines whether the Earth is experiencing global heating or cooling (see Earth's energy budget).[6]

  1. ^ a b Matthews, J.B.R.; Möller, V.; van Diemenn, R.; Fuglesvedt, J.R.; et al. (2021-08-09). "Annex VII: Glossary". In Masson-Delmotte, Valérie; Zhai, Panmao; Pirani, Anna; Connors, Sarah L.; Péan, Clotilde; et al. (eds.). Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (PDF). IPCC / Cambridge University Press. pp. 2215–2256. doi:10.1017/9781009157896.022. ISBN 9781009157896.
  2. ^ Petty, Grant W. (2006). A first course in atmospheric radiation (2. ed.). Madison, Wisc.: Sundog Publ. p. 68. ISBN 978-0-9729033-1-8.
  3. ^ "What is the Surface Area of the Earth?". Universe Today. 11 February 2017. Retrieved 1 June 2023.
  4. ^ "Earth's Heat Balance". Energy Education. University of Calgary. Retrieved 12 July 2023.
  5. ^ Singh, Martin S.; O'Neill, Morgan E. (2022). "Thermodynamics of the climate system". Physics Today. 75 (7): 30–37. Bibcode:2022PhT....75g..30S. doi:10.1063/PT.3.5038. Retrieved 12 July 2023.
  6. ^ Kiehl, J. T.; Trenberth, Kevin E. (February 1997). "Earth's Annual Global Mean Energy Budget". Bulletin of the American Meteorological Society. 78 (2): 197–208. Bibcode:1997BAMS...78..197K. doi:10.1175/1520-0477(1997)078<0197:EAGMEB>2.0.CO;2.

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