Theoretical Investigation of the Visibility Under Special Atmospheric Conditions (The Horizontal Visual Range of not Self-Luminous Objects under Overcast Sky)

摘要

The present report summarizes a theoretical study on the horizontal visual range of not self-luminous objects under overcast sky. After a brief review on the definitions of brightness contrast, contrast threshold of the human eye, and visual range, the dependence of contrast threshold on the visual angle presented by a graph of H.R.BLACKWELL, is expressed analytically.nThe luminance distribution of overcast sky is investigated by means of values of former authored and of own measurements at Mains, Germany.nthe exact definitions of total and visual albedo are given, and calculated and measured values of them are presented.nThe theory is applied on visibility in polar regions. A brief description of the “polar whiteout” is given, and the optical phenomena of whiteout are explained on the basis of our visual range theory.nFinally some remarks are made with regard to future improvement and extension of the theory.