Annual and Seasonal Trends of Solar Radiation in Athens, Greece

Abstract

This study deals with the solar radiation levels recorded at the Actinometric Station of the National Observatory of Athens, focusing on the trends of the global and diffuse horizontal irradiances in the period of 1992-2017 (26 years). The analysis shows that the annual global radiation time series over Athens in the above period presents a positive trend (+0.40%/decade and +2.38%/decade for all and clear skies, respectively), while its diffuse counterpart shows negative trend (-5.19%/decade and -6.77%/decade for the same sky conditions, respectively) during the same period. Also, the seasonal trends of both solar radiation components are examined. The global horizontal radiation shows positive trends in summer (+1.85%/decade and +2.10%/decade under all and clear skies, respectively), while a negative trend is calculated in the winter season (-2.46%/decade and -1.99%/decade for the same sky conditions, respectively). In the case of the diffuse solar radiation there are found negative trends in both seasons and both sky conditions. These findings confirm a recovery in the solar radiation levels over Athens in the recent 26 years.