Seasonal caracterization of CIE standard sky types above Burgos, northwestern Spain

Abstract

Outdoor daylight conditions in Burgos, Spain, are studied throughout a full year. The CIE standard sky type is selected in accordance with the lowest RMSD (Root Mean Square Deviation) following the comparison of both the theoretical and the experimental luminance distributions in the sky hemisphere. The original criterion to determine the type of sky, the SSLD (the Standard Sky Luminance Distribution), is difficult to apply in certain places and at times when the solar elevation is higher than 35°. In consequence, two alternative procedures are used and compared in this study: the Tregenza method and the Normalization Rate (NR) introduced by Littlefair. The selection was taken from luminance distribution data of 145 patches of the sky hemisphere recorded between June 2016 and May 2017. The most frequent sky type observed in Burgos was V.5. (cloudless polluted with a broad solar corona), with a frequency of occurrence close to 20%. Notwithstanding that observation, the group of clear skies exhibited a higher frequency (in almost 50% of the cases under study, using both methods). The skies above Burgos were of an overcast sky type in less than 25% of cases, a situation with a higher likelihood in winter and in autumn, while in spring and summer the skies tended to be clear and cloud free. Both of the methodologies showed similar results in percentage terms and in confusion matrixes with almost insignificant differences when compared on a monthly, a seasonal, and an annual basis. Nevertheless, some mismatches were located in the highest solar elevation values.