Simulations of the HIRS 6.7 µm (water vapor) radiances, computed within the Hadley Centre atmospheric climate model (HadAM3) are compared with the observations over the period 1979-1998. The model is forced with observed sea surface temperature and sea-ice fields. HadAM3 overestimates the HIRS 6.7 µm brightness temperature (T_6.7) over regions of strong descending motion, particularly over the southern hemisphere. Some of the discrepancy between model and observed T_6.7 are related to the clear-sky sampling differences between the model diagnostic and the measured quantity. The interannual variation of T_6.7 over tropical oceans is of similar magnitude in the model and observations suggesting small changes in upper tropospheric humidity over the period. Peaks in T_6.7 during 1983 and 1998 coincide with strong El Niń o events. The effect on the simulations of prescribing volcanic, solar and greenhouse gas forcing and observed ozone variations is assessed.