Cloudy infrared spectra are calculated from forecast model output and compared to AIRS (Atmospheric Infrared Radiance Sounder) observations from about 200 channels, with special focus in the Arctic region. Retrieved cloud parameters from AIRS are also compared to the equivalent forecast quantities. These parameters are the equivalent height and cloud fraction derived from the CO2-slicing technique. The potential ambiguity created by a different definition of forecast and observed cloud parameters is eliminated by applying the same retrieval technique using observed and calculated radiances from model output. Systematic differences in the cloudy spectra and cloud parameter distributions are analyzed as well as the sensitivity to the assumed cloud optical properties. The variation of the error in radiance space as a function of forecast time can also be evaluated.