Strong contrasts in daytime mixing height (boundary layer height or zi) between urban and rural areas were observed during the 1999 Nashville Summer Intensive field campaign of the Southern Oxidants Study. On occasion, the urban mixing height was as much as 45% (700 m) higher than that over the rural areas. The difference was quite persistent, showing strongly in statistical comparisons, with a mean difference over all hours available for comparison of 160 m. Clouds had higher bases and were more common over the urban area as well. In this paper, measurements from wind profiling radars, lidars, and aircraft are used to characterize mixing height and clouds. The urban--rural contrasts have important implications for regional air quality. The mixing height is a first-order control on pollutant concentrations. The urban--rural contrast also results in the venting of urban pollutants, affecting the local concentrations and the regional background. Clouds affect air quality by changing the radiative input for photochemistry and through changes in mixing and venting.