The condition that when the atmosphere is in equilibrium, incoming radiation must balance outgoing radiation. (Radiation balance also occurs inside stars and planets, but of course, different processe [..]

The difference between the absorbed solar radiation and the net infrared radiation. Experimental data show that radiation from the earth's natural surfaces is rather close to the radiation from a black body at the corresponding temperature; the ratio of the observed values of radiation to black body radiation is generally 0.90 - 1.0.