The longwave radiative cooling of the clear-sky atmosphere (QLWc) is a crucial component of the global hydrological cycle and is composed of the clear-sky outgoing longwave radiation to space (OLRc) and the net downward minus upward clear-sky longwave radiation to the surface (SNLc). Estimates of QLWc from reanalyses and observations are presented for the period 1979--2004. Compared to other reanalyses data sets, the European Centre for Medium-range Weather Forecasts 40-year reanalysis (ERA40) produces the largest QLWc over the tropical oceans (217 W m-2), explained by the least negative SNLc. On the basis of comparisons with data derived from satellite measurements, ERA40 provides the most realistic QLWc climatology over the tropical oceans but exhibits a spurious interannual variability for column integrated water vapor (CWV) and SNLc. Interannual monthly anomalies of QLWc are broadly consistent between data sets with large increases during the warm El Ni¿o events. Since relative humidity (RH) errors applying throughout the troposphere result in compensating effects on the cooling to space and to the surface, they exert only a marginal effect on QLWc. An observed increase in CWV with surface temperature of 3 kg m-2 K-1 over the tropical oceans is important in explaining a positive relationship between QLWc and surface temperature, in particular over ascending regimes; over tropical ocean descending regions this relationship ranges from 3.6 to 4.6 ¿ 0.4 W m-2 K-1 for the data sets considered, consistent with idealized sensitivity tests in which tropospheric warming is applied and RH is held constant and implying an increase in precipitation with warming.