Radiative flux

Last updated June 30, 2024

Radiative flux, also known as radiative flux density or radiation flux (or sometimes power flux density^[1]), is the amount of power radiated through a given area, in the form of photons or other elementary particles, typically measured in W/m².^[2] It is used in astronomy to determine the magnitude and spectral class of a star and in meteorology to determine the intensity of the convection in the planetary boundary layer. Radiative flux also acts as a generalization of heat flux, which is equal to the radiative flux when restricted to the infrared spectrum.

Geophysics

Shortwave

In geophysics, shortwave flux is a result of specular and diffuse reflection of incident shortwave radiation by the underlying surface.^[3] 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.^[4] As it is the main energy source of most weather phenomena, the solar shortwave radiation is used extensively in numerical weather prediction.

Longwave

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.^[5]

SI radiometry units

SI radiometry units
v
t
e
Quantity		Unit		Dimension	Notes
Name	Symbol^{[nb 1]}	Name	Symbol	Dimension	Notes
Radiant energy	$Q e$ ^{[nb 2]}	joule	J	M⋅L²⋅T⁻²	Energy of electromagnetic radiation.
Radiant energy density	$w e$	joule per cubic metre	J/m³	M⋅L⁻¹⋅T⁻²	Radiant energy per unit volume.
Radiant flux	$Φ e$ ^{[nb 2]}	watt	W = J/s	M⋅L²⋅T⁻³	Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power", and called luminosity in Astronomy.
Spectral flux	$Φ e, ν$ ^{[nb 3]}	watt per hertz	W/Hz	M⋅L²⋅T⁻²	Radiant flux per unit frequency or wavelength. The latter is commonly measured in W⋅nm⁻¹.
Spectral flux	$Φ e, λ$ ^{[nb 4]}	watt per metre	W/m	M⋅L⋅T⁻³
Radiant intensity	$I e,Ω$ ^{[nb 5]}	watt per steradian	W/sr	M⋅L²⋅T⁻³	Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is a directional quantity.
Spectral intensity	$I e,Ω, ν$ ^{[nb 3]}	watt per steradian per hertz	W⋅sr⁻¹⋅Hz⁻¹	M⋅L²⋅T⁻²	Radiant intensity per unit frequency or wavelength. The latter is commonly measured in W⋅sr⁻¹⋅nm⁻¹. This is a directional quantity.
Spectral intensity	$I e,Ω, λ$ ^{[nb 4]}	watt per steradian per metre	W⋅sr⁻¹⋅m⁻¹	M⋅L⋅T⁻³
Radiance	$L e,Ω$ ^{[nb 5]}	watt per steradian per square metre	W⋅sr⁻¹⋅m⁻²	M⋅T⁻³	Radiant flux emitted, reflected, transmitted or received by a surface, per unit solid angle per unit projected area. This is a directional quantity. This is sometimes also confusingly called "intensity".
Spectral radiance Specific intensity	$L e,Ω, ν$ ^{[nb 3]}	watt per steradian per square metre per hertz	W⋅sr⁻¹⋅m⁻²⋅Hz⁻¹	M⋅T⁻²	Radiance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅sr⁻¹⋅m⁻²⋅nm⁻¹. This is a directional quantity. This is sometimes also confusingly called "spectral intensity".
Spectral radiance Specific intensity	$L e,Ω, λ$ ^{[nb 4]}	watt per steradian per square metre, per metre	W⋅sr⁻¹⋅m⁻³	M⋅L⁻¹⋅T⁻³
Irradiance Flux density	$E e$ ^{[nb 2]}	watt per square metre	W/m²	M⋅T⁻³	Radiant flux received by a surface per unit area. This is sometimes also confusingly called "intensity".
Spectral irradiance Spectral flux density	$E e, ν$ ^{[nb 3]}	watt per square metre per hertz	W⋅m⁻²⋅Hz⁻¹	M⋅T⁻²	Irradiance of a surface per unit frequency or wavelength. This is sometimes also confusingly called "spectral intensity". Non-SI units of spectral flux density include jansky (1 Jy = 10⁻²⁶ W⋅m⁻²⋅Hz⁻¹) and solar flux unit (1 sfu = 10⁻²² W⋅m⁻²⋅Hz⁻¹ = 10⁴ Jy).
Spectral irradiance Spectral flux density	$E e, λ$ ^{[nb 4]}	watt per square metre, per metre	W/m³	M⋅L⁻¹⋅T⁻³
Radiosity	$J e$ ^{[nb 2]}	watt per square metre	W/m²	M⋅T⁻³	Radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area. This is sometimes also confusingly called "intensity".
Spectral radiosity	$J e, ν$ ^{[nb 3]}	watt per square metre per hertz	W⋅m⁻²⋅Hz⁻¹	M⋅T⁻²	Radiosity of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m⁻²⋅nm⁻¹. This is sometimes also confusingly called "spectral intensity".
Spectral radiosity	$J e, λ$ ^{[nb 4]}	watt per square metre, per metre	W/m³	M⋅L⁻¹⋅T⁻³
Radiant exitance	$M e$ ^{[nb 2]}	watt per square metre	W/m²	M⋅T⁻³	Radiant flux emitted by a surface per unit area. This is the emitted component of radiosity. "Radiant emittance" is an old term for this quantity. This is sometimes also confusingly called "intensity".
Spectral exitance	$M e, ν$ ^{[nb 3]}	watt per square metre per hertz	W⋅m⁻²⋅Hz⁻¹	M⋅T⁻²	Radiant exitance of a surface per unit frequency or wavelength. The latter is commonly measured in W⋅m⁻²⋅nm⁻¹. "Spectral emittance" is an old term for this quantity. This is sometimes also confusingly called "spectral intensity".
Spectral exitance	$M e, λ$ ^{[nb 4]}	watt per square metre, per metre	W/m³	M⋅L⁻¹⋅T⁻³
Radiant exposure	$H e$	joule per square metre	J/m²	M⋅T⁻²	Radiant energy received by a surface per unit area, or equivalently irradiance of a surface integrated over time of irradiation. This is sometimes also called "radiant fluence".
Spectral exposure	$H e, ν$ ^{[nb 3]}	joule per square metre per hertz	J⋅m⁻²⋅Hz⁻¹	M⋅T⁻¹	Radiant exposure of a surface per unit frequency or wavelength. The latter is commonly measured in J⋅m⁻²⋅nm⁻¹. This is sometimes also called "spectral fluence".
Spectral exposure	$H e, λ$ ^{[nb 4]}	joule per square metre, per metre	J/m³	M⋅L⁻¹⋅T⁻²
See also: SI Radiometry Photometry

↑ Standards organizations recommend that radiometric quantities should be denoted with suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities.
1 2 3 4 5 Alternative symbols sometimes seen: $W$ or $E$ for radiant energy, $P$ or $F$ for radiant flux, $I$ for irradiance, $W$ for radiant exitance.
1 2 3 4 5 6 7 Spectral quantities given per unit frequency are denoted with suffix " ν " (Greek letter nu, not to be confused with a letter "v", indicating a photometric quantity.)
1 2 3 4 5 6 7 Spectral quantities given per unit wavelength are denoted with suffix " λ ".
1 2 Directional quantities are denoted with suffix "Ω".

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References

↑ "Communication Systems/Wireless Transmission". WikiBooks: Communication Systems/Wireless Transmission. Retrieved 2018-12-11.
↑ "Glossary of Meteorology: Radiative Flux" . Retrieved 2008-12-24.
↑ Kantha, L.H.; Clayson, Carol (2000). "Small scale processes in geophysical fluid flow". San Diego: Academic Press.{{cite journal}}: Cite journal requires |journal= (help)
↑ Yang, Rongqian; Friedl, Mark A.; Ni, Wenge (July 16, 2001). "Parameterization of shortwave radiation fluxes for nonuniform vegetation canopies in land surface models" (PDF). Journal of Geophysical Research. 106 (D13): 14275–14286. Bibcode:2001JGR...10614275Y. doi: 10.1029/2001JD900180 .
↑ Hoch, S. W.; Calanca, P.; Philipona, R.; Ohmura, A. (2007). "Year-Round Observation of Longwave Radiative Flux Divergence in Greenland". Journal of Applied Meteorology and Climatology. 46 (9): 1469–1479. Bibcode:2007JApMC..46.1469H. doi: 10.1175/JAM2542.1 .

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[note-suffix-e-6] Standards organizations recommend that radiometric quantities should be denoted with suffix "e" (for "energetic") to avoid confusion with photometric or photon quantities.

[note-alternative-symbol-radiometric-7] 1 2 3 4 5 Alternative symbols sometimes seen: $W$ or $E$ for radiant energy, $P$ or $F$ for radiant flux, $I$ for irradiance, $W$ for radiant exitance.

[note-suffix-nu-8] 1 2 3 4 5 6 7 Spectral quantities given per unit frequency are denoted with suffix " ν " (Greek letter nu, not to be confused with a letter "v", indicating a photometric quantity.)

[note-suffix-lambda-9] 1 2 3 4 5 6 7 Spectral quantities given per unit wavelength are denoted with suffix " λ ".

[note-suffix-omega-10] 1 2 Directional quantities are denoted with suffix "Ω".

[1] "Communication Systems/Wireless Transmission". WikiBooks: Communication Systems/Wireless Transmission. Retrieved 2018-12-11.

[2] "Glossary of Meteorology: Radiative Flux" . Retrieved 2008-12-24.

[3] Kantha, L.H.; Clayson, Carol (2000). "Small scale processes in geophysical fluid flow". San Diego: Academic Press.{{cite journal}}: Cite journal requires |journal= (help)

[4] Yang, Rongqian; Friedl, Mark A.; Ni, Wenge (July 16, 2001). "Parameterization of shortwave radiation fluxes for nonuniform vegetation canopies in land surface models" (PDF). Journal of Geophysical Research. 106 (D13): 14275–14286. Bibcode:2001JGR...10614275Y. doi: 10.1029/2001JD900180 .

[5] Hoch, S. W.; Calanca, P.; Philipona, R.; Ohmura, A. (2007). "Year-Round Observation of Longwave Radiative Flux Divergence in Greenland". Journal of Applied Meteorology and Climatology. 46 (9): 1469–1479. Bibcode:2007JApMC..46.1469H. doi: 10.1175/JAM2542.1 .

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