Abstract
Daylight is composed of direct sunlight and skylight. Skylight is direct sunlight and light reflected from the earth's surface that have been scattered by the atmosphere and clouds. The correlated color temperature (CCT) of daylight varies hourly, and the CCT of skylight varies extensively due to changes in the spectral alvedo of the earth's surface, the solar altitude and the angular distance from the sun to a sky component, the atmospheric transmittance of visible radiation and the amount of cloud. This paper discusses causes of variations and local differences of skylight CCT and proposes a formula for skylight CCT as a function of cloud amount.
