Fog Luminance Evaluation in Daytime

Abstract

In daytime, fog creates a luminous veil that impairs the visual indices required by motorists to coordinate their task of driving. The level of fog luminance depends on many parameters. A method is described to compute the luminance of a fog layer on the basis of its optical depth, the droplet size distribution, and the relative position of both the sun and the motorist. The results are then analyzed and indicate that fog luminance depends to a great extent on the direction of observation with respect to the sun. Except when the motorist is traveling toward the sun, the fog luminance follows the variations of horizontal illuminance, which increases with the altitude of the sun yet decreases with droplet size distribution. In most instances, the influence of the optical depth calculated over the height of the fog layer remains small. Only when facing the sun in a fog with droplet diameters less than or equal to 1 μm does the luminance increase significantly as optical depth decreases. The impact on the visibility parameters is presented.