Evaluation of overcast-sky luminance models against measured Hong Kong data

Sky luminance distribution is one of the most important quantities for predicting indoor daylight illuminance levels. Overcast-sky types are essential because they are used in more general sky models and appear quite frequent in some places. This paper presents the work on the evaluation of six worldwide overcast-sky models against two-year (1999–2000) measured Hong Kong sky luminance data. Overcast-sky conditions were identified using cloud cover (CLD) and a subsequent interpretation the overcast skies into thin and heavy overcast types was conducted in conjunction with the clearness index (Kt). A statistical analysis of the models has indicated that the International Commission on Illumination (CIE) standard overcast sky model performed the best, in particular for the heavy overcast-sky condition. The Building Research Establishment (BRE) quasi-overcast-sky model showed a good agreement with the thin overcast distributions which may include a circumstance component and the sky luminance patterns being orientation dependent.