With radiosonde data from 15 Northern Hemisphere stations, surface-to-400-mb column water vapor is computed from daytime soundings for 1988–1990. On the basis of simultaneous surface visual cloud observations, the data are categorized according to sky-cover amount. Climatological column water vapor content in clear skies is shown to be significantly lower than in cloudy skies. Column water vapor content in tropical regions varies only slightly with cloud cover, but at midlatitude stations, particularly in winter, clear-sky values are much lower. The variation in column water content with cloud cover is not simply due to variations in atmospheric temperature, since the increase in water vapor with cloud cover is generally associated with a decrease in daytime temperature. Biases in radiosonde instruments associated with cloudiness do not explain the station-to-station variations in the magnitude of the increase of column water vapor with cloud cover. Statistics are presented that can be used as guidance in estimating the bias in water vapor climatologies based on clear-sky or partly cloudy-sky measurements. These may be helpful in distinguishing the clear- and cloudy-sky greenhouse effects of water vapor.

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