A one-dimensional numerical model of the formation and evolution of droplet spectra during the early stage of formation of advection fogs at sea is described. The model computes the evolution of a specified population of cloud condensation nuclei contained in an air parcel which is advected with the wind. The driving force for fog formation is a prescribed time rate of increase in the relative humidity of the parcel. This humidity increase is assumed to result from radiative cooling and from heat and moisture exchange with a cold sea surface. The model was used to predict the evolution of the droplet size distribution and visibility at an altitude of 20 m in two advection-type fogs which were encountered off the coast of Nova Scotia daring a cruise of the USNS Hayes in August 1975. Predictions were made for distances of up to 30 km inside the upwind boundary of these fogs. Observed and model-predicted visibilities agreed to within a factor of 2. In the size range above 5 μm diameter, the predicted droplet size distributions were in good agreement with impactor data. The model was able to predict the high number density of small (<2.0 μm diameter) droplets detected with an optical particle counter.