Starting from the experimental values obtained on horizontal surfaces, Liu and Jordan have introduced a widely used method to calculate the monthly average solar radiation on inclined surfaces. This method is based on the division of radia­tion between its direct and diffuse components. Liu and Jordan discovered that the ratio between the monthly average diffuse radiation D and the global radiation H received on a horizontal surface can be correlated to a parameter called monthly clearness index K. This index is obtained by dividing the monthly average terres­trial radiation for every day by the monthly average extraterrestrial radiation for every day both received on a horizontal plane.

K=H/Hm (39)

To compute Hex the solar constant value used is 1394 W/m2 (instead of the more recent value of 1367 W/m2). For this reason, K values should be based on that value. As for the calculation of Hex, it has been suggested that eqn (38) be applied to a specific day of each month. That day must be chosen to get an extraterrestrial radiation Hex on a horizontal surface, which is equal to the monthly average extra­terrestrial radiation Hex.

If B (= H — D) is the monthly average direct component received on a horizontal surface for each day and E is the monthly average global radiation on an arbitrary oriented surface, we have:

E=RbB+RiD+Rr(B+D) (41)

where ha’ is the hour angle calculated using eqn (22); ha and ha are expressed in radians and Rb is the monthly average factor of inclination by direct radiation [1].