The authors have empirically examined the dependence of the outgoing longwave radiation (OLR) on sea surface temperature (Ts), precipitable water (W), and height-mean relative humidity (RH¯). The OLR is obtained from 4 yr of data from the Earth Radiation Budget Experiment (ERBE), while Ts, W, and RH¯ are obtained from objective analyses of rawinsonde and ship data. It is found that in the midlatitudes, the surface temperature explains over 80% of the variability in the clear-sky OLR (Fcs) and almost half of the variability in the total OLR (Ftot). It fails badly in the tropics and subtropics, however, where (Ts explains only about 20% of the variability in (Fcs, and is largely decoupled from Ftot. The two-dimensional contour plot of the OLR binned with respect to Ts and RH¯ is marked by distinct changes in the gradient that are consistent with inferences from earlier investigations. For low values of Ts (OLR depends mainly on Ts. For values of Ts above 10°C, the OLR depends increasingly on RH¯. Specifically, in the tropics (Ts˜25°C), the total and clear-sky OLR depend significantly on both Ts and RH¯. The well-known drop in OLR in the tropics with increasing Ts correlates directly to an increase in RH¯, and not to changes in Ts. The authors suggest that the observed dependence of the OLR on Ts and RH¯ be a minimum performance standard for climate models. This approach is illustrated by comparing the observed dependence with the results of a radiative transfer model and an R 15 general circulation model, and by discussing the strengths and limitations of using RH¯ to parameterize the OLR.