Ground illumination from a turbid cloudless sky.
An attempt is made to develop a set of empirical equations for the relationship between skylight illuminance, turbidity, and solar altitude under the condition of a cloud free sky. The maximum possible illuminance from a cloud-free sky is estimated using these equations. The illuminances are stated in relation to the illuminance produced by a Rayleigh sky. An improved approximation for Bemporad's airmass function is used in the computations.
Nagel
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