Net radiometer
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A net radiometer is a type of actinometer used to measure net radiation (NR) at the earth'surface in meteorological applications. The name net radiometer reflects the fact that it is supposed to measure incoming minus upwelling radiation.
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[edit] Terminology
Although there are many types of net-radiometers, the 4-component design at present is most popular for scientific applications.
A 4-component net-radiometer serves to measure the 4 separate components of the surface radiation balance. SWin or global solar radiation, SWout or reflected solar radiation, LWin or infra red emitted by the sky and LWout or infra red emitted by the ground surface. From these also parameter like SW “albedo”, “sky temperature”, “(ground) surface temperature” and off course “net-radiation” (net value of all SW and LW fluxes) can be calculated. A typical net radiometer does not require any power to operate.
[edit] Explanation
The SW solar radiation spectrum extends approximately from 300 to 2800 nm. Pyranometers usually cover that spectrum with a spectral sensitivity that is as “flat” as possible. The LW longwave or Far Infra RED (FIR) extends from 4500 to 50000 nm. This part of the spectrum is covered by a pyrgeometer.
[edit] Calculations
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In its most common application 4 component net-radiometers are used for measurement of net-radiation. The 4 components of the net-radiation are measured and the net-radiation is calculated:
NOTE: the following formula’s have T in kelvins. In order to have T in degrees Celsius, please add 273. With U voltage output of a sensor, up upfacing instrument, down downfacing instrument, SW shortwave or solar radiation, LW longwave or far infrared (FIR) radiation, in incoming, out outgoing, T temperature, NR net radiation.
SWin = 'Upyrano, up / Epyrano, up
SWout = Upyrano, down / Epyrano, down
LWin = (Upyrgeo, up / Epyrgeo, up) + 5.67.10-8 (Tpyrgeo)4
LWout = (Upyrgeo, down / Epyrgeo, down) + 5.67.10-8 (Tpyrgeo)4
NOTE: in the LWnet the instrument temperature is cancelled:
LWnet = Upyrgeo, up / Epyrgeo, up - Upyrgeo, down / Epyrgeo, down
SWnet = Upyrano, up / Epyrano, up - Upyrano, down / Epyrano, down
NR = SWnet + LWnet
Special parameters that can be deducted:
SW albedo = SWin / SWout NOTE: the following formula’s have T in Kelvin. In order to have T in degrees C, please add 273.
Tsurface = (LWout /5.67.10-8)1/4
Tsky = (LWin /5.67.10-8)1/4
The SW albedo and the Tsurface must be estimated from other sources, and the NR can be calculated using these plus the SWin and LWin measurements. SW albedo typically is assumed to be a constant, typically taken from local satellite observations; Tsurface can often be calculated from air temperature of ground temperature measurements.
[edit] Usage
Net radiometers are frequently used in meteorology, climatology, solar energy studies and building physics. They can be seen in many meteorological stations - typically installed horizontally.
[edit] Standardisation
Net-radiometers are not standardised.
[edit] References
[edit] See also
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