The purpose of the paper is to evaluate our capability to model upwelling radiances at high resolution over broad spectral ranges in clear and cloudy conditions. Spectroradiometric measurements are taken from aircraft flying at low altitude over a clear scene and over a low-level stratus cloud. It is the ideal case to capture the change in signal due to the presence of a cloud without the interfering effect of the atmosphere above, unavoidable in higher-altitude measurements. The knowledge of the variability of the radiometric signal over the measurement periods and the availability of in situ measurement from the same aircraft of the meteorological parameters (temperature and humidity profile) and cloud parameters (depth and microphysical parameters of the cloud layer) allow a nearly complete control of the experiment. The results for the clear case show deviations between simulation and measurement generally smaller than 0.5%, while in the cloudy case, differences over most of the spectral range are less than the variability seen during the recording of the measured spectra. The cloud result is not significantly changed when tested with realistic changes in the measured cloud properties. Over cloud an absorbing medium alone is insufficient, and full scattering is required to achieve good agreement. This shows that care is required when assimilating infrared radiances from above such clouds, which can often extend over large areas. The adopted simulation methodology generates spectral radiances in very good agreement with the measurements both in clear air and above the stratus cloud layers. ¿ 2001 American Geophysical Union