We investigate the possibility of measuring water vapor column from diffuse irradiance, and thus the extension of optical retrievals to cloudy days. Data from the rotating shadowband spectroradiometer (RSS) during its winter deployment at the North Slope of Alaska (NSA) site are used: 20 days in March 1999 that include clear, partly cloudy and overcast days. During these days water vapor column varied between one and five mm according to the NSA site's microwave radiometer (MWR). The diffuse transmittance in the 760-nm oxygen absorption band is used to obtain the effective air mass. The end result is a correlation of 0.97 and 0.95 between MWR and RSS retrievals using 820-nm and 940-nm diffuse transmittances, respectively. These results, which are based on empirical data analysis only, imply that the diffuse irradiance may contain sufficient information to retrieve water vapor column.