The solar radiation climate of Athens: Variations and tendencies in the period 1992-2017, the brightening era

Solar radiation at the surface of the Earth undergoes short- and long-term variations mostly influenced by changes in atmospheric composition and cloud cover. These two factors characterise the solar radiation climate at a place. This study deals with the solar radiation climate over Athens focusing on the variations and trends of the global and diffuse horizontal irradiances recorded at the Actinometric Station of the National Observatory of Athens. The analysis shows that the global radiation over Athens in the period 1992-2017 presents a maximum in July and in May, June around noon under all and clear skies. For the diffuse component, this occurs in spring and in May (noontime) under all- and clear-sky conditions. The global radiation trends are positive (+0.41%/decade and +2.36%/decade for all and clear skies, respectively), while its diffuse counterpart shows negative trends (−6.93%/decade and −9.27%/decade for similar sky conditions, respectively) in the period 1992-2017. These findings confirm the recovery in the solar radiation levels over Athens in the recent 25 years.