The clearness index is a measure of atmosphere clearness. It is calculated as the fraction of the actual total
solar radiation
Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically p ...
on the surface of the Earth during a certain period over the theoretical maximum (i.e., clear sky) radiation during the same period. The clearness index is a
dimensionless quantity
Dimensionless quantities, or quantities of dimension one, are quantities implicitly defined in a manner that prevents their aggregation into unit of measurement, units of measurement. ISBN 978-92-822-2272-0. Typically expressed as ratios that a ...
and can vary from 0 (sky is completely covered) to 1 (perfectly sunny).
See also
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Cloud cover
Cloud cover (also known as cloudiness, cloudage, or cloud amount) refers to the fraction of the sky obscured by clouds on average when observed from a particular location. Okta is the usual unit for measurement of the cloud cover. The cloud c ...
References
category:Atmospheric radiation
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