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Sunny sky
Clear Sky
Partly Cloudy sky
Cloudy sky
Overcast Sky
(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.
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