The Airborne Research Interferometer Evaluation System (ARIES), flown on the Met Office C-130 aircraft, has been used to study clear-sky thermal infrared radiation in the range 550 - 3000 cm^-1 (18 - 3.3 µm). This spectral range coincides with that of the IASI satellite instrument, due for launch in 2005, and the airborne measurements provide a useful assessment of the spectroscopy and radiative transfer needed to utilise the IASI radiances effectively. Particular emphasis has been given to studies of water vapour, as IASI offers the potential to sound water vapour at high vertical resolution in cloud-free conditions for use by operational weather centres. Two campaigns, in the tropics and at high-latitude, were undertaken in order to obtain comprehensive measurements of radiation and atmospheric profiles. We have modelled ARIES radiances with the GENLN2 line-by-line radiative transfer code, and find good agreement in the atmospheric window (800 - 1200 cm^-1) using the latest spectral line parameters and CKD2.4 water vapour continuum. Some residual errors remain to be addressed here however, as well as outside the window region where larger errors persist. An important parameter for modelling upwelling atmospheric radiances in optically transparent regions is the surface emissivity, which we have probed with ARIES when flying over the sea. Our results show that the emissivity varies according to the characteristics of the sea surface, and that a single emissivity model is insufficient to describe the surface radiative boundary condition in all cases. In addition to measurements of upwelling radiation, ARIES spectra of downwelling sky radiances have also been compared with model simulations. Early results appear to show that the modelled water vapour continuum in these cases is in slight disagreement with the measurements.