= Planetary effective emissivity
{title2=$\epsilon_{\rm eff}=F_{\rm out}/(\sigma T_s^4)$}
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, $\epsilon_{\rm eff}=(T_{\rm eq}/T_s)^4$. 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.
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