A new method of deriving high-resolution top-of-atmosphere spectral radiances over the entire outgoing longwave spectrum of the Earth is presented. Correlations between selected channels of the Infrared Atmospheric Sounding Interfermeter (IASI) on the MetOp-A satellite and simulated unobserved wavelengths in the far infrared are used to estimate radiances between 25.25–644.75 cm−1 at 0.5 cm−1 intervals. The same method is used in the 2760–3000 cm−1 region. Total integrated all-sky radiances are validated with broadband measurements from the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Terra and Aqua satellites at simultaneous nadir overpasses, revealing mean differences that are 0.3 W m−2 sr−1 (0.5% relative difference) lower for IASI relative to CERES with significantly lower biases in nighttime – only scenes. Averaged global data over a single month produces mean differences of about 1 W m−2 sr−1 in both the all and the clear-sky (1.2% relative difference). The new high – resolution spectrum is presented for global mean all and clear skies where the far infrared is shown to contribute 47 and 44% to the total OLR respectively, which is consistent with previous estimates. In terms of spectral cloud radiative forcing, the FIR contributes 19% and in some subtropical instances appears to be negative, results that would go un-observed with a traditional broadband analysis.