COLUMN WATER-VAPOR CONTENT IN CLEAR AND CLOUDY SKIES

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 significantl y lower than in cloudy skies. Column water vapor content in tropical r egions varies only slightly with cloud cover, but at midlatitude stati ons, particularly in winter, clear-sky values are much lower. The vari ation in column water content with cloud cover is not simply due to va riations in atmospheric temperature, since the increase in water vapor with cloud cover is generally associated with a decrease in daytime t emperature. Biases in radiosonde instruments associated with cloudines s do not explain the station-to-station variations in the magnitude of the increase of column water vapor with cloud cover. Statistics are p resented that can be used as guidance in estimating the bias in water vapor climatologies based on clear-sky or partly cloudy-sky measuremen ts. These may be helpful in distinguishing the clear- and cloudy-sky g reenhouse effects of water vapor.