The lake-breeze circulation on the Great Lakes is often as vigorous as its oceanic counterpart. This paper shows that lake breezes frequently exert drastic control on mesoscale air pollution patterns in urbanized shore-line areas, in this case, Chicago, Ill. Observational data were gathered from a surface mesonetwork, surface and satellite cloud photography, a chain of pilot balloons normal to the shore, optically tracked constant-level balloons, and aircraft measurements of suspended particulate concentrations in several size ranges. On the 2 late summer days studied, the lake breezes were extremely well developed. Inflow depths ranged from 500 to 1000 m, with peak inflow velocities of 6–7 m/s. Beginning at the shore-line between 0800 and 0900 LST, the breezes penetrated inland over 40 km. Clearly defined return flow layers were present both days. Eulerian wind field measurements from serial pilot, balloon releases were used to make cross sections of the u wind component. Computed one-dimensiona... Abstract The lake-breeze circulation on the Great Lakes is often as vigorous as its oceanic counterpart. This paper shows that lake breezes frequently exert drastic control on mesoscale air pollution patterns in urbanized shore-line areas, in this case, Chicago, Ill. Observational data were gathered from a surface mesonetwork, surface and satellite cloud photography, a chain of pilot balloons normal to the shore, optically tracked constant-level balloons, and aircraft measurements of suspended particulate concentrations in several size ranges. On the 2 late summer days studied, the lake breezes were extremely well developed. Inflow depths ranged from 500 to 1000 m, with peak inflow velocities of 6–7 m/s. Beginning at the shore-line between 0800 and 0900 LST, the breezes penetrated inland over 40 km. Clearly defined return flow layers were present both days. Eulerian wind field measurements from serial pilot, balloon releases were used to make cross sections of the u wind component. Computed one-dimensiona...