I am trying to model the cooling of an object (for example, a sheet of glass) placed outside at night. At the moment I am only considering heat loss by radiation. I know that the net radiation from the object will be: Rnet = Robj - Rsky where: Rnet = the net radiation from the object Robj = the total thermal radiation from the object Rsky = the thermal downwelling radiation from the sky I have come across a formula which does the above, but also takes into consideration the absorptivity of the sheet of glass at the 8-14μm wavelength (I have only considered emissivity of the glass in this wavelength as I'm only concerned with thermal radiation heat loss to the sky). The formula is: Rnet = A((εobj2/αobj2)σTobj4 - εskyσTamb4) where: εobj = the emissivity of the object at 8-14μm αobj = the absorptivity of the object at 8-14μm εsky = the emissivity of the sky Tobj = the temperature of the object Tamb4 = the ambient air temperature A = the area of the object What I would like to know is where does the εobj2/αobj2 bit come from? I know if I wasn't considering absorptivity then it would just be εobj, but why are εobj and αobj now squared? I have since lost where I saw it, and I'm pretty sure there wasn't an explanation there anyway. I have searched all over for a derivation of it but have had no luck. Can anyone help?