Climatological diurnal variations of atmospheric physical properties are well known, particularly in the tropics. We have analyzed Meteosat data obtained every 3 hours during 3 summers (JJA 1983--85) in the ''watervapor'' (WV, 6.5 &mgr;m) band which depends on temperature and humidity in layers above 500 millibars. We find that spatially coherent diurnal variations of a few percent appear to extend well beyond the regions of convection into cloud-free areas. Furthermore, we observe propagation of a diurnal pulse in the WV signal, eastward and westward from the land areas of deep convection. We discuss the mechanisms that may be involved and conclude that we are observing the forced response of the middle and upper troposphere to the diurnally pulsating release of latent heat above central Africa and South America. ¿ American Geophysical Union 1988