Sky luminance distribution in Central Europe and the Mediterranean area during the winter period

Abstract

Five-minute data of global and diffuse irradiance and illuminance along with zenith luminance from Bratislava and Athens for a period of 5 years are used in order to define the prevailing sky luminance distributions at the two sites in the winter period. The classification into 15 sky types is mainly based on the value of the ratio of zenith luminance to diffuse illuminance with solar altitude as parameter (SSLD method). This is applied for the first 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 different 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.