Literature references:

  Feigelson, E. M., B. A. Fomin, I. A. Gorchakova, E. V. Rozanov,
        Y. M. Timofeyev, A. N. Trotsenko, and M. D. Schwarzkopf, 1991,
        Calculation of longwave radiation fluxes in the atmosphere,
        J. Geophys. Res., Vol 96, 8985-9001.

  Ellingson, R. G., J. Ellis, and S. Fels, 1991, The intercomparison of
        radiation codes used in climate models: Long wave results,
        J. Geophys. Res., Vol 96, 8929-8953.