AbstractA one‐dimensional numerical model designed to study the formation and growth of radiation fog is described. the realism of the model in simulating the formation of fog is assessed by critical comparisons with detailed observations made on two nights. the comparisons show that the model is unable to reproduce the very light winds which occur near the surface in stable conditions. As a consequence the formation of radiation fog is inhibited in the model unless the afternoon relative humidity is very high. It is suggested that the discrepancy is caused by the use of a turbulence formulation tuned to match observations over an ideally flat site. the influence of turbulence, radiative cooling, advection and frost deposition on the formation of fog during the two nights is also discusse