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Satellite observations, above very high clouds in the tropics, are analyzed. These observations reveal the influence of warm water vapor in the stratosphere; above those clouds, the radiance (equivalent temperature) in the water vapor channel (6.7 μm) is warmer than the equivalent temperature in the window channel (11.1 μm). When the clouds are well below the tropopause, the temperature in the water vapor channel is colder than the window channel temperature. Theoretical radiative calculations support the interpretation of the satellite observations.
Fritz et al. (Mon,) studied this question.