Hourly observations of the 6.7 &mgr;m water vapor radiances from geostationary satellites are used to document the diurnal cycle in upper tropospheric water vapor and its relationship to cloud cover and convection. A coherent diurnal cycle in tropical water vapor is observed which lags the variations in cloud cover by approximately 2 hours. The variations in upper tropospheric cloud and water vapor occur (roughly) in phase with changes in deep convection over land, but nearly 12 hours out of phase with those over ocean. This feature is shown to be associated with differences in the vertical structure of land and ocean convection and offers a useful test of convective parametrizations in atmospheric models.