(Terms of daylighting)
For daylighting purposes, the CIE declares a number of sky conditions as standard skies. Those are defined by functions, depending on the solar altitude, even when the sun is hidden.
Sunny sky is any sky condition where the sun happens to shine (through the clouds, if there are any). This can be combined with any of the following three conditions.
Clear sky has less than 30 % cloud cover, or none. This sky is most likely to be combined with sun.
Partly cloudy sky has between 30 % and 70 % cloud cover. This sky can be combined with sun in some cases.
Cloudy sky has more than 70 % cloud cover. This sky normally excludes the sun.
Overcast sky has a completely closed cloud cover (100 %). Obviously, this sky can't be combined with sun in a meaningful way. This is the sky condition applied in daylight factor calculations.
The "cloudy sky" as used in the Radiance software (generated by the gensky(1) program) corresponds to the CIE overcast sky definition.
|clear sky||Der klare Himmel|
|cloudy sky||Der bewölkte Himmel|
|overcast sky||Der bedeckte Himmel|
|partly cloudy sky||Der teilbewölkte Himmel|
|sunny sky||Der sonnige Himmel|