This paper presents a report of some aspects of the effect of the dominant greenhouse gas in the earth's atmosphere, water vapour, on the radiative balance of the planet. Results from two satellite experiments which provide data on the concentration of water vapour in the upper and middle troposphere are presented. the first is the HALogen Occultation Experiment, HALOE, on the National Aeronautics and Space Administration Upper Atmosphere Research Satellite, for which retrievals of humidity have been made down to levels of about 150 hPa. These data reveal a number of interesting features in terms of the space and time variability of water vapour in the lower stratospherehpper troposphere. Comparison of HALOE measurements with the water vapour fields calculated in one general circulation climate model reveal substantial differences, which could significantly affect calculations of radiative balance. A second source of global data is the Television Infra-red Observation Satellite's Operational Vertical Sounder, TOVS, which uses infrared vertical soundings. Considerable variability of upper tropospheric humidity is revealed by TOVS measurements, and an interesting relationship between brightness temperature and relative humidity, identified by other workers, is discussed. Sensitivity studies are presented of the effect on the outgoing-radiation spectrum of varying the amount of water vapour throughout the troposphere. It is shown that uncertainties of only a few percent in knowledge of the humidity distribution in the atmosphere could produce changes to the outgoing spectrum of similar magnitude to that caused by doubling carbon dioxide in the atmosphere. Errors in the water vapour amounts generated in model simulations of the climate could, therefore, be significant in climate change calculations.