Planetary effective emissivity (${ϵ}_{\mathrm{eff}}=F_{\mathrm{out}}/(\sigma T_s^4)$)

The ratio of top-of-atmosphere outgoing thermal flux to the blackbody flux at a chosen representative surface temperature. In global equilibrium with a uniform surface, ${ϵ}_{\mathrm{eff}}=(T_{\mathrm{eq}}/T_s{)}^4$. It parametrizes the relation between surface and radiating temperatures and need not equal the material emissivity of the surface or an atmospheric layer. An effective value below one is compatible with a black surface when colder atmospheric layers contribute to radiation escaping to space.