(Answer) (Category) The Solar-AC FAQ : (Category) Theory & thermodynamics corner, equations welcome : (Answer) How can I compute radiative cooling as a function of temperature?
The primary law governing blackbody radiation is the Planck Radiation Law, which governs the intensity of radiation emitted as a function of wavelength for a fixed temperature. The Stefan-Boltzmann Law gives the total energy being emitted at all wavelengths by the blackbody (which is the area under the Planck Law curve). The energy emitted grows proportional to the 4th power of the temperature.
 .                                     4
 .  Stefan-Boltzmann Law:   E = sigma T  
 .
 .             where        sigma = 5.67e-8 W / (m^2.K^4)
 .
 .             and          E is energy emitted per square meter, in Watts
 .
For example: your skin temperature is about 300 kelvins, and your surface area is about 1 sq meter, so you emit about 450 watts.

This calculation is relevant to us in the solar-ac group because we can compute the radiative losses (cooling) of an object from its surface area and temperature. Of course, we must also account for any simultaneous radiative heating of the object.

This description is cribbed from http://csep10.phys.utk.edu/astr162/lect/light/radiation.html and the example from http://www.met.fsu.edu/Classes/Met1010-Ahlquist/0118_bw.pdf .

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