A study of overcast, partly cloudy and clear skies by global illuminance and its variation features

The diffuse radiance and luminance distribution in the skydome can determine the solar energy on the building envelope in various orientations and under complex obstructions. The diffuse sky is anisotropic for clear and partly cloudy skies, while isotropic for overcast skies. Thus, identifying the sky conditions are essential to the sky radiance and luminance studies. Previously developed approaches using statistic data suffered from data shortage and inaccuracy. Sunlight under cloudy skies may frequently be blocked, causing the high-frequency illuminance fluctuations. The illuminance should variate smoothly under clear skies without clouds. In this work, we studied the horizontal irradiance time series, its variation frequency, and its relationship to the sky conditions determined by the luminance scan. The work contributes to identifying the sky conditions and skylight distributions using the horizontal global daylight illuminance that is measured by the fixed sensors.