A case study of a thunderstorm cloud on 9 September 1955 was made to investigate the effect of London's urban area on its growth and precipitation amounts. Radar evidence was used to follow the development of the cloud, and dense observation networks provided data on synoptic meteorological elements and rainfall amounts. The cloud originated to the west of London and moved eastward with the mid-tropospheric wind. As it crossed the city, rapid growth occurred and precipitation amounts were heavy. The cloud growth was due to the high values of potential and wet-bulb potential temperatures in the urban area. It is concluded, in this case, that the urban effect was real, but it is stressed that generalization from this conclusion may not be valid.