Reversible and irreversible sources of radiation entropy

We discuss two practical issues concerning the relationship between the entropies of the atmosphere and of the radiation field. The first is whether the radiative flux of entropy can be determined from a satellite with sufficient accuracy for climate studies. We conclude that an accuracy much better than 1% is required, but that this can be achieved in the thermal spectrum with spectrometers having spectral resolutions ∼ 1 cm −1 ; it is also possible that the required accuracy can be achieved with a non‐spectral approach. The second is whether the rate of change of the atmospheric entropy inventory can be inferred from the outgoing radiative‐entropy flux. We conclude that, for clear skies in the thermal region of the spectrum, the two are directly proportional, and that the atmospheric term can be inferred with useful accuracy from an empirical relation.