Radiative flux, also known as radiative flux density or radiation flux (or sometimes power flux density), is the amount of
power radiated through a given area, in the form of
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they alwa ...
s or other elementary particles, typically measured in W/m
2. It is used in
astronomy
Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
to determine the
magnitude and
spectral class of a star and in
meteorology
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...
to determine the intensity of the convection in the
planetary boundary layer. Radiative flux also acts as a generalization of
heat flux
Heat flux or thermal flux, sometimes also referred to as ''heat flux density'', heat-flow density or ''heat flow rate intensity'' is a flow of energy per unit area per unit time. In SI its units are watts per square metre (W/m2). It has both a ...
, which is equal to the radiative flux when restricted to the
infrared spectrum
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
.
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. The ratio of irradiance reflected to the irradiance received by a surface is called
albedo
Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...
.
Shortwave radiation flux
Shortwave flux is a result of specular and diffuse reflection of incident shortwave radiation by the underlying surface. 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. As it is the main energy source of most weather phenomena, the solar shortwave radiation is used extensively in
numerical weather prediction.
Longwave radiation flux
Longwave flux is a product of both
downwelling 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.
SI radiometry units
See also
*
Spectral flux density
*
Flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ...
References
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Physical quantities
Vector calculus