Thermodynamics of the Gray Atmosphere. I. Reversible Adiabatic Processes.

For every pencil of radiation inside a gray atmosphere in strict radiative equilibrium, energy spectra characteristic of different net fluxes (effective temperatures) are linked by a certain similarity transformation if the source function, which need not be Planckian, undergoes the same transformation. Related transformations hold for the concomitant variations of the spectra of radiant entropy. The common physical correlates of these similarity laws are quasi-statical processes taking the radiation field of the gray atmosphere, a non-equilibrium system, from one steady state to another. Because they satisfy all appropriate criteria, it is well within the bounds of accepted usage to call them reversible adiabatic processes, a term that classical thermodynamics had reserved for passages through a sequence of equilibrium states.