Journal of Atmospheric and Solar-Terrestrial Physics 65 (2003) 113–119 www.elsevier.com/locate/jastp Sky luminance distribution in Central Europe and the Mediterranean area during the winter period A. Bartzokas a ; ∗ , S. Darula b , H.D. Kambezidis c , R. Kittler b a Laboratory of Meteorology, Department of Physics, University of Ioannina, 451 10 Ioannina, Greece b Institute of Construction and Architecture, Slovak Academy of Sciences, 842 20 Bratislava, Slovakia c Atmospheric Research Team, Institute of Environmental Research & Sustainable Development, National Observatory of Athens, P.O. Box 20048, 118 10 Athens, Greece Received 20 November 2001; received in revised form 28 May 2002; accepted 19 September 2002 Abstract Five-minute data of global and diuse irradiance and illuminance along with zenith luminance from Bratislava and Athens for a period of 5 years are used in order to dene the prevailing sky luminance distributions at the two sites in the winter period. The classication into 15 sky types is mainly based on the value of the ratio of zenith luminance to diuse illuminance with solar altitude as parameter (SSLD method). This is applied for the rst time in a research work. It is found that the most frequent sky type in Bratislava is the overcast (with steep luminance gradation and slight brightening towards the sun); while in Athens it is the cloudless polluted sky (with a broad solar corona). That is, the winter daylight climate is quite dierent in Central Europe in comparison to the Mediterranean. An analysis for various solar altitudes reveals the frequency distribution of the 15 sky types suitable for the application in daylight climate studies. c  2002 Published by Elsevier Science Ltd. Keywords: Sky luminance distribution; Zenith luminance; Diuse illuminance; Daylight climate; Central Europe; Mediterranean 1. Introduction Research work on daylight presents an increasing trend during the last 3 decades. However, daylight measurements cover limited areas globally, mainly due to the severe lack of appropriate stations. Moreover, in the past most daylight research was focused on clear or overcast skies only, be- cause sky luminance distribution on a partly cloudy sky is a very sensitive function mainly dependent on latitude, cloudi- ness, turbidity and air pollution. The rst research work on other than overcast or clear skies was attempted by Littlefair (1981) for ‘average’ skies, and Nakamura et al. (1985) for ‘intermediate’ skies. Later, Perez et al. (1993) developed the ‘all-weather’ model and dened sky luminance distribution ∗ Corresponding author. Tel.: +30-6510-98477; fax: +30-6510- 98699. E-mail address: abartzok@cc.uoi.gr (A. Bartzokas). using parameters of sky clearness and sky brightness based on the global and direct irradiances. This model was applied also for predicting luminance patterns derived from satellite data for European locations (Fontoynont et al., 1999). Re- cently, Kittler et al. (1997) and Igawa et al. (1997) presented new methods based on luminance gradation and indicatrix functions. Comparisons of daylight and sky luminance regimes in areas with dierent geographical and climatological char- acteristics have revealed important dierences (Tregenza, 1999). The purpose of this work is to dene typical day- light conditions for Central Europe and the Mediterranean with special attention to partly cloudy sky conditions. To do this, data from Bratislava, Slovakia and Athens, Greece are analysed following the standardskyluminancedistribution (SSLD) method proposed by Kittler et al. (1997). In daylighting practice, the momentary instantaneous, sometimes complex and uneven, sky patterns, due to the 1364-6826/03/$ - see front matter c  2002 Published by Elsevier Science Ltd. PII:S1364-6826(02)00283-3