The clearness index is a measure of the clearness of the atmosphere. It is the fraction of the solar radiation that is transmitted through the atmosphere to strike the surface of the Earth. It is a dimensionless number between 0 and 1, defined as the surface radiation divided by the extraterrestrial radiation. The clearness index has a high value under clear, sunny conditions, and a low value under cloudy conditions.
The clearness index can be defined on an instantaneous, hourly, or monthly basis. The clearness index values in HOMER's Solar Resource Inputs window are monthly average values. The symbol for the monthly average clearness index is Kt.
Typical values of Kt range from 0.25 (a very cloudy month, such as an average December in London) to 0.75 (a very sunny month, such as an average June in Phoenix).
The table of solar data contains monthly average clearness indices for numerous locations around the world.