Radiances observed by the GOES 6.7-μm channel are compared to computations from simultaneous aircraft soundings of the temperature and moisture profiles. The soundings were measured using high precision instruments aboard the National Center for Atmospheric Research Sabreliner. An advanced line-by-line radiative transfer model is used to simulate the satellite observations. This comparison shows that the computed brightness temperatures overestimate the observed brightness temperatures but reproduce the full range in observed brightness temperatures. The computed brightness temperatures are, on average, 2.6 K higher than the observed. Possible sources for this bias are investigated, such as transmission through a cloudy lower boundary, scattering by thin high-level cirrus, and water vapor continuum absorption. It is likely that a combination of uncertainties in the radiative properties of water and unaccountable ice or aerosol absorption contributes to the disparity. ¿ American Geophysical Union 1996