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.: 2251
Outgoing longwave radiation (OLR) is the longwave radiation emitted to space from the top of Earth's atmosphere.: 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 microns (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.
The flux of energy transported by outgoing longwave radiation is typically measured in units of watts per meter 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).
Emitting outgoing longwave radiation is the only way Earth loses energy to space, i.e., the only way the planet cools itself. Radiative heating from absorbed sunlight, and radiative cooling to space via OLR power the heat engine that drives atmospheric dynamics.
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).
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