Abstract Data derived from a unique observational arrangement at Round Hill Field Station allow the investigation of the instantaneous vertical variation of wind-direction fluctuations in the lowest 44 m of the atmosphere. Three observational runs under different thermal conditions have been analyzed. An estimate, from these data, of the vertical distribution of lateral diffusion is given in terms of the variation of Sutton's lateral virtual diffusion coefficient with height. The computed values of this coefficient are found to be somewhat larger than those given by Sutton.