Sky luminance distribution in the North of Iberian Peninsula during winter

This work is based on the study of outdoor daylight conditions during the winter period in Pamplona (the Iberian Peninsula). The 15 standard sky types proposed by the Commission Internationale de l’Eclairage (CIE) are used. The selected sky type in each moment is the one exhibiting the lowest root mean standard deviation (RMSD) when the theoretical and experimental luminance distributions in the sky hemisphere are compared. The selection was derived from luminance distribution data from 145 patches of the sky hemisphere and registered every 10 min from October to March from 2007 and 2008.The most frequent sky type observed in winter in Pamplona is V.5 (cloudless polluted with a broad solar corona), with a frequency of occurrence of 21%. Notwithstanding, the group of overcast skies exhibits a slightly higher frequency (40%) than the clear skies (37%). Seven out of the 15 standard sky types, i.e.V.5, I.1, III.4, II.1, IV.4, II.2 and III.3, are practically 80% of those under study. In order to provide a possible application in daylight climates studies, the frequency of occurrence of the 15 sky types is included in eleven intervals of solar elevation on the threshold corresponding to the time period and the location considered.