Investigations of the nocturnal temperature and wind structure of the planetary boundary layer over a city were conducted in the Cincinnati, Ohio, metropolitan area. Temperatures near the surface were obtained by means of automobile traverses across the city, the vertical distributions of temperature were measured at several sites with a helicopter, and wind velocities were derived from pilot balloon observations. Results of the investigations demonstrated a pronounced modification of the vertical temperature structure as air with a rural history traversed the city. The vertical extent of the modification, referred to herein as the “urban boundary layer,” gradually increased with downwind distance over the urban area. Beyond the downwind side of the urban area, relatively unstable air was found aloft over a stable surface layer, suggesting a layer of outflowing urban air aloft that is called the “urban heat plume.” When a strong inversion existed in the planetary boundary layer upwind from the city, the urban boundary layer extended 150 to 300 ft above the surface. A superadiabatic lapse rate was observed within the urban boundary layer in the central business district and an isothermal lapse rate or weak inversion in the downwind suburban areas. A strong inversion, similar to that of the upwind rural environment, was maintained above the urban boundary layer.