Apparent Clearing of the Sky at Dusk

Abstract

CLOUDS may disappear by evaporation of the droplets, a purely physical effect, but they may also vanish to the eye, though in fact they remain. It is generally accepted that the eye can distinguish between brightnesses down to the limit of 2 per cent difference, which may be termed the discrimination factor. The best papers on this factor and its variation with intensity are those of Nutting¹ and Hecht². In average daylight for good eyes, the factor may be as low as 1 per cent, increasing for very bright light to about 6 per cent, and for much-reduced illumination rising steeply to more than 60 per cent; but the value 2 per cent holds approximately over a wide range of daylight.