Abstract

The illuminance of a horizontal surface at the ground below an overcast sky is computed. The transmittance, T, of visible light is given as a function of the cloud optical thickness, h/L, of the sun’s zenith distance, $\mathcal{Z}$, and of the reflectance, a, of the underlying surface. For some cloud thicknesses, T varies markedly with $\mathcal{Z}$. Also, for thick clouds, T is several times larger over a fresh snow cover than it is over snow-free ground.

A formula for luminance distribution, B(θ), of a heavily overcast sky is presented. This formula agrees fairly well with existing measurements made over snow-free terrain; however, it further suggests that B(θ) will become much more uniform when a fresh snowfall uniformly covers the ground.

© 1955 Optical Society of America

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