Frequency of types of sky - Info

Monthly frequency of sky types - Europe

Resource information

The monthly frequency of sky types is useful to characterize the climatic trend of a site. For daylighting purposes, sky type information is particularly important because the performance of a system is typically estimated under standard sky conditions such as cloudy and sunny. The frequency of each of these sky conditions at a given location indicate how each is representative of the climate at that location.

Three sky types are available: cloudy, intermediate and sunny, for sites in Western and Central Europe. The frequencies are based on the analysis of the images produced by the METEOSAT satellite every half hour, for five years: 1996 to 2000. The sky classification is based on the range of brightness values in the METEOSAT image: cloudy corresponds to the brightest values, sunny corresponds to the darkest values, intermediate is in between. The frequencies are available for two daily schedules: sunrise to sunset and 8:00 to 18:00 (local time) which corresponds to the operating hours of most office buildings.


The Satel-Light server delivers daylight and solar radiation information over Western and Central Europe with a high spatial and temporal resolution (every 5 km, every half-hour). It was funded by the European Union (Directorate General XII). Initially restricted to two years 1996 to 1998, the information has been extended to five years: 1996 to 2000, thanks to the SODA project.

(Link to this info)