Cloud cover and daylight illuminance

Abstract

Continous measurements of daylight illuminance were made at Nottingham, England, during 1985 and 1986. Cloud quantities were measured from simultaneous field photographs of the sky, and the association between global horizontal illuminance and cloudiness was calculated. It was found that cloud quantity, as a single variable, could account for between one-third and one-half of the variation in illuminance at a given solar altitude. The measured data are compared with values predicted using standard formulae and with calculations based on a model of light diffusion through a cloudy sky.