Net radiative fluxes in the short- and longwave (horizontal axis) as functions of optical depths (vertical axis) in case of isothermal atmosphere. Fluxes profiles are computed from the analytic formulae given by Eqs. (22) and (23) and normalised by the black body equilibrium flux (the equilibrium temperature T_eq being givenby Eq. (2)). Optical depths are normalised by their value at the planet’s surface, namely τ_s;S for the shortwave and τ_s;L for the longwave. Shortwave (blue lines) and longwave (red lines) net fluxes are plotted for various optical thicknesses, the optical depths at the planet’s surface taking the values τ_s;S = 0.1, 1, 10 (from light to dark blue lines) and τ_s;L = 0.1, 1, 10 (from light to dark red lines). Pure absorption is assumed (β_L0 = β_S0 = 1). The incident, atmospheric, and surface fluxes are arbitrarily set to F_i = 4F_eq, F_a = 0.7F_eq, and F_s = 2F_eq, respectively.