Sunshine duration and daily courses of illuminance in Bratislava

The occurrences of typical sky conditions during overcast days are close to the CIE Overcast Sky Standard in the Bratislava area; their variation is specified by the diffuse coefficient k(dd) = D-v/E-v where D-v is the horizontal diffuse illuminance and E, is the extraterrestrial illuminance. Luminance distributions on cloudless days are close to the CIE Clear Sky Standard. Complex daylight conditions can be expected during cloudy days when moving clouds cause frequent sun disk shading with abrupt changes in illuminance levels. Since sunshine duration is one of the basic climatic parameters that describes sunny situations, the present study attempts to investigate its relation to daily illuminance levels. However, the average daily value of sunshine duration does not exactly correspond to the same illuminance course, as it depends on the temporal distribution of sunny periods within the day. Anyhow, the estimation of daytime illuminance levels is mainly influenced by the presence or absence of sunshine with frequent sky luminance patterns. Copyright (C) 2004 Royal Meteorological Society.