The Clearness Index Kt  is defined as the ratio of the horizontal global irradiance to the corresponding irradiance available outside of the atmosphere  (i.e. the  extraterrestrial irradiance multiplied by the sinus of the sun height).

The extraterrestrial irradiance is the Solar constant  (1361 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".