Radiative flux, or radiative flux density, is the amount of power radiated through a given area, in the form of photons or other elementary particles, typically measured in W/m2.[1] It is used in astronomy to determine the magnitude and spectral class of a star. Radiative flux also acts as a generalization of heat flux, which is equal to the radiative flux when restricted to the infrared spectrum.
When radiative flux is incident on a surface, it is often called irradiance. Flux emitted from a surface may be called radiant exitance or radiant emittance.
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Shortwave flux is a result of specular and diffuse reflection of incident shortwave radiation by the underlying surface Kantha, L.H.; Clayson, Carol (2000). Small Scale Processes in geophysical fluid flow. San Diego: Academic Press. This shortwave radiation, as solar radiation, can have a profound impact on certain biophysical processes of vegetation, such as canopy photosynthesis and land surface energy budgets, by being absorbed into the soil and canopies.[2]
Longwave flux is a product of both down welling infrared energy as well as emission by the underlying surface. The cooling associated with the divergence of longwave radiation is necessary for creating and sustaining lasting inversion layers close to the surface during polar night. Longwave radiation flux divergence also plays a role in the formation of fog.[3]