The bible for climate modellers working on planetary radiation is the second edition of “Atmospheric Radiation: Theoretical Basis” by Goody and Yung (1989). In 1995 it was published as a paperback in the UK, and in 1996 in the USA. I have copies of both the first edition by Goody (1965) and the UK paperback edition. I bought them as the result of a challenge on a newsgroup to explain Section 2.2. Recently, I received a similar challenge, from which I can only deduce that I am not the only person to find section 2.2 difficult. Having studied that section in both editions over a period of about 10 years I think I now understand what what they are trying to say, and the reason that people find it difficult is that it is full of errors!
Section 2.2 Thermal Emission begins with a rather idiosyncratic description of Kirchhoff’s Radiation Law. There are, of course, two laws, the first of which was first discovered by the Scottish physicist Balfour Stewart, and can be stated as: for a body in thermodynamic equilibrium at each frequency absorption equals emission. This follows from the conservation of energy. Moreover, if absorption did not equal emission the temperature of the body would change and so it would not be in thermodynamic equilibrium, i.e.
α(λ) = ε(λ)
where α is the absorption coefficient, ε is the emission coefficient and λ is the wavelength.
to be continued.