This study delineates some general but important features of the meteorological potential for air pollution in small and large cities throughout the contiguous United States. The potential is evaluated by means of a simple model of dispersion over urban areas in which theoretical concentrations are a function of mixing height, wind speed, and city size (along-wind distance across city). Mixing height and wind speed data are derived for mornings and afternoons from five years of surface and upper air measurements at 62 National Weather Service stations. Mean annual values of mixing height and wind speed as well as median annual and upper decile annual values of X/Q for 10- and 100-km cities are presented in the form of isoplethed maps. The X/Q maps show how theoretical concentrations vary between morning and afternoon, with city size, with frequency of occurrence, and from place to place. In addition, the potential for episodes with limited dispersion conditions lasting at least two days is evaluated objectively and depicted on a map by the total number of episode-days in five years.