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

Though numerous daylight studies have been conducted worldwide, works for the daylight climate and/or the daylight variations/trends are rare. This gap is treated here. Therefore, the innovative trend analysis and the month-hour diagrams are applied to the illuminance time series at Athens over the period of 1992-2017 under all-/clear-sky conditions. For all skies, the global illuminance shows a broad maximum at summer noon, while the diffuse counterpart is in spring. Under clear skies, the global illuminance maximum becomes broader, covering spring/summer. However, the diffuse illuminance maximum splits into minor ones in April and June. The innovative trend analysis method finds that the identified trends are + 2.83%/decade (-1.89%/decade) for the global (diffuse) illuminance, under all skies. In clear skies, the above values are modified to +2.66%/decade (-2.00%/decade). These results confirm a recovery in the Athens daylight levels as in the solar radiation ones found by many researchers. Another outcome of the study concerns the observed decline in diffuse illuminance levels in the examined period, which may be attributed to a decline in cloudiness and an increase of sunshine over the area. Also, the variation of the global and diffuse illuminance efficacies on a month-hour basis is examined using the month-hour diagram method.