In this study, a dense fog episode that occurred near Windsor, Ontario, Canada, on 3 September 1999 is investigated. The fog patch, with a spatial scale of several kilometers, reduced visibility on a major highway to a few meters and led to a series of collisions and loss of life. Satellite imager y and surface obser vations are used to analyze the physics of the event, and several hypotheses on the origin of the fog are presented. A series of simulations of the event with the ?fth-generation Pennsylvania State University–National Center for Atmo- spheric Research Mesoscale Model (MM5) suggest that the fog formed because of convergence of land breezes developing along the shores of a lake and subsequent advection of moisture over the site of the accident. Tests indicate that the small scale of the modeled event contributes to sensitivity of the results to a broad range of factors. Sensitivity to the initial and boundar y conditions, including initial soil moisture content and parame- terization of turbulence, is discussed.