Clearness Index

The clearness index Kt is defined as the ratio of horizontal global irradiance to corresponding irradiance above the atmosphere (that is, extraterrestrial irradiance multiplied by the sine of sun height).

The extraterrestrial irradiance is the Solar constant (1367 W/m²) corrected by a yearly sinus function of amplitude 3.3% accounting for earth's orbit ellipticity.

Therefore the Clearness Index Kt may be considered as an attenuation factor of the atmosphere.

The monthly average of Kt should usually lie between about Kt = 0.12 and Kt = 0.82 at any place and for any month. If these limits are exceeded (in very particular cases), you may modify them in the "Advanced parameters", topic "Size and Meteo".