The hypothesis of large scale control of midlatitude water vapor is evaluated through reconstructions of the water vapor field using a Lagrangian advection-condensation model without microphysics or diffusion. The reconstruction is validated against satellite observations in the 183.31 ¿ 1 GHz band from the AMSU-B radiometer following a model-to-satellite approach. Because microwave radiation can penetrate clouds, the validation can be performed for cloudy as well as clear sky scenes, with the exception of very cold or precipitating clouds, which are screened out. The results show very good agreement between the simulated top of the atmosphere radiation and the observations, in clear as well as cloudy regions, with a general bias of less than 2K. The results suggest that cloud microphysics and small scale mixing play at most a secondary role in determining midlatitude free tropospheric humidity except perhaps indirectly through their effect on large scale circulation.