 Inversion criterion with intrinsic planetary flux

An atmospheric thermal inversion has outward-increasing temperature. In a semi-grey irradiated atmosphere, define downward thermal optical depth $\tau$, constant opacity ratio $\gamma ={\kappa }_{\mathrm{vis}}/{\kappa }_{\mathrm{IR}}>0$, irradiation temperature $T_{\mathrm{irr}}$ and internal effective temperature of a planet $T_{\mathrm{int}}$. In the isotropic-irradiation closure, write the incident mean irradiance as $f_r{\sigma }_{\mathrm{SB}}{T}_{\mathrm{irr}}^4$, with $f_r>0$ explicitly specifying the averaging convention. Common choices are $1/4$ for a whole-sphere average and $1/2$ for a dayside average.

The analytic radiative equilibrium profile in this closure isDifferentiation givesSince $\tau$ increases inward, an atmospheric thermal inversion occurs when this derivative is negative. Thus $\gamma >1$ is necessary in this closure; a nonzero intrinsic radiative flux also requires sufficiently strong irradiation. For $T_{\mathrm{int}}>0$, any inversion is restricted toprovided the logarithm is positive. The $T_{\mathrm{int}}=0$ limit gives the threshold $\gamma >1$. These coefficients depend on the angular closure; a collimated beam has an incidence-angle dependence.^[ref]