The change in the radiative effect of water vapor in response to an external perturbation of the climate. Water vapor, the most important greenhouse gas, absorbs only a small amount of sunlight but is a very efficient absorber of the earth's thermal infrared emission. Changes in either the amount or the vertical distribution of water vapor can therefore change the planet's ability to radiate heat to space. Climate models predict, without exception, that the water vapor feedback is positive. Changes in the distribution of water vapor in the middle and upper troposphere are inordinately important to this feedback process, because molecules that absorb upwelling infrared radiation at these altitudes emit it to space at a much colder temperature and therefore emit less than would be the case in their absence. The concentration of high-altitude water vapor is controlled by poorly understood dynamic and thermodynamic processes and is inadequately observed, thus contributing to uncertainty in the magnitude of the water vapor feedback.