Classification of sky conditions by the ranges of isolation indices considering CIE standard for general sky

Abstract

This study introduces a method to classify sky conditions from overcast to clear by the values of insolation indices. Number of sky types and combination of insolation indices to classify sky types were investigated. Fifteen groups were clustered regarding nine indices by nearest centroid sorting method. Then groups were put together into sky conditions by considering the frequency distributions of CIE general sky types for each group and the frequency distribution of groups for each luminance distribution type. Seven sky conditions were found as a result. Indices to discriminate next sky conditions were found and then the most appropriate combination for total discrimination was decided: turbidity, clearness index, brightness and normalized global illuminance. Sky condition discrimination tree on the basis of the value of the indices was formed by a yearly data of International Daylight Measurement Program at Kyoto station and verified by the data of another year. By this method, illuminance and irradiance measurements can be separated into seven sky conditions considering CIE general sky types by the ranges of four indices without sky luminance distribution measurements.