Abstract
A method is described by which illumination intensities were measured by a photo-electric cell and galvanometer, and a continuous photographic record obtained of the variations of intensity over a period of time during which the ground was covered with snow.It was found that a remarkable increase in the illumination was caused by the presence of cloudiness with full sunshine. The percentage increase due to this condition in many cases was 20–30%, and on one occasion it was as high as 40%.When there was a cloud over the sun, with most of the sky clear, the decrease in illumination was found to vary up to 35%, depending on the density of the cloud, and on many occasions it was observed that the increase in indirect illumination due to overhead cloudiness more than balanced the decrease of direct illumination when the sun was partly hidden. From this it is concluded that the intensity may be greater with the sun partly hidden than at the same time of day with a clear sky.
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