Spatial and temporal distribution of water vapor is important because local severe rainfall is caused by the convergence of water vapor. On the basis of field measurements in the Matsuyama Plain, we found that urban absolute humidity was lower than rural absolute humidity. Following the manner of urban heat island phenomenon, we call this phenomenon “urban dry island” (UDI). The object of this study was to analyze the features and causes of the UDI. The UDI phenomenon was significantly found at daytime during fine days due to the difference in water vapor fluxes at urban and rural sites. The UDI was most prominent in the morning and in the late afternoon, in time zones where the sea breeze and land breeze altered each other and the wind speed temporally became low. Strong sea breeze transported humid air from the coastal regions to the center of the city and then caused the UDI to diminish. The UDI intensity was large in summer and small in winter. This was attributed to the small latent heat fluxes in winter and strong north-westerly monsoon, which enhanced the mixing of humidity. In addition, we investigated the impact of UDI on the formation of clouds using datasets of cloud base level, which was obtained by a ceilometer in summer. Cloud base level over urban areas tended to be higher than that over rural areas under the condition that the wind blew along the border between urban and rural areas. This was demonstrated by lifted condensation level, which was estimated from surface air temperature and water vapor content.