The relation between the solar radiation at the Earth’s surface and the extraterrestrial radiation gives a measure of the atmospheric transparency. In this way, a clearness index KTm is calculated for each month:
Note that the clearness index is physically related not only to the radiation path through the atmosphere, that is, with the AM value, but also with the composition and the cloud content of the atmosphere. Liu and Jordan  have demonstrated that, irrespective of latitude, the fractional time during which daily global radiation is equal to or less than a certain value is directly dependent on this parameter. Because of that, KTdm can properly characterise the solar climate of a particular location. This provides the basis for estimating solar radiation on inclined surfaces.