Overcast sky conditions and luminance distribution in Hong Kong

In studying solar energy and daylight availability, particularly the sky luminance distributions, the required data are always analysed under various sky types which are categorised by some climatic parameters. Overcast days are important because they are used in more general sky models and appear quite frequent in some places. This paper studies some common climatic parameters including cloud cover, sunshine hour, solar irradiance, daylight illuminance and sky luminance and their applications in the classification of sky conditions. Frequency of occurrence of each parameter under overcast skies in Hong Kong has been established. It has been found that no single parameter can effectively describe the fully overcast skies and hybrid indices should be adopted. The general luminance distribution formula for the overcast sky with various luminance distribution parameter values has been used to test the selected sky luminance databases. The analysis indicates that the CIE standard overcast sky shows a good agreement with the overcast sky luminance distributions obtained in Hong Kong.