Determination of fog horizontal visibility by means of NOAA-AVHRR

Two schemes for the determination of fog horizontal visibility using AVHRR data both, day and night, are presented. Ground truth has been performed by the comparison of calculated visibilities and simultaneous transmissometer measurements. The daytime approach is based on the calculation of fog albedo from AVHRR 1 and the relation of fog optical depth and geometrical thickness to spectral extinction and horizontal visibility. The mean deviation between observed and calculated visibilities is about 56 m. The nighttime approach uses the relation between fog and air temperature derived from AVHRR 4 and 5 and a simple model for the approximation of the fog droplet distribution. The resulting fog extinction is directly related to horizontal visibility. Because fog optical depth is not available at night, the calculation of horizontal visibility using the nighttime approach is only valid if fog extinction is nearly homogeneous over the whole fog layer. Mean deviations between observed and calculated visibilities therefore slightly increase to about 61 m.