The observed mean values of the maximum hourly average surface sulphur dioxide concentration from generating station plumes (stack height 100 m and above) exhibit a marked dependence on wind speed. At night the lowest concentrations occur in light winds, and the concentration increases almost linearly with wind speed up to speeds of 14 m s-1 at stack top. In the daytime the lowest concentrations occur at about 4 m s-1 with a secondary maximum in light winds. The distributions of the individual hourly average values in both light and strong winds are discussed and some practical methods for predicting these distributions are suggested. The variations with distance from the source of the axial concentration, crosswind standard deviation and the concentration integrated across wind are discussed in terms of departures from the isotropic gaussian concentration distribution.