Directional variation of visual range due to anisotropic atmospheric brightness

摘要

The meteorological optical range (MOR) is the greatest distance at which a nonluminous object is barely discernible. Koschmieder related the optical atmospheric extinetion coefficient to the contrast between the object and its background. His law assumes a uniform atmosphere and yields an isotropic value of the MOR. The model presented here incorporates the effect of anisotropic atmospheric brightness on contrast (and thus on the MOR). The Koschmieder MOR is then decreased by a factor according to the brightness prevailing in the direction of assessment. Manual assessments of the MOR made in arbitrary directions are shown to agree satisfactorily with the derated MOR. Minor modifications to existing instruments at airports would enable the instruments to register true values of the MOR and the runway visual range in directions relevant to the pilot, i.e., along the runway.