Abstract
This study aims to explore the effect of anthropogenic factors on solar irradiation in Saudi Arabia. The concepts of the clearness index (CI), solar irradiation polygons (SIP) and solar Angstrom polygons (SAP) are for interpretation of solar irradiation. The solar irradiation records are grouped into two time periods. The SIPs show a decline in solar irradiation in the second time period for all regions except the eastern region. The maximum difference between the time periods, of approximately 10.9 (MJ/M-2 day), is found in October at Alhefa station in the southwestern region, while at Khurais station in the eastern region the values of solar irradiation are very close together, within 2 (MJ/M-2 day). Finally, CI concept is used in order to compare measured against estimated solar irradiation, where a maximum relative error of 11% was found. Finally, solar irradiation generally declined recently, which strongly indicates anthropogenic factors are at play.