# Trends from 30-Year Observations of Downward Solar Irradiance in Thessaloniki, Greece
## 4 Conclusions  
We have analyzed a 30-year dataset of SRD measurements in Thessaloniki, Greece (1993-2023), aiming to identify the long-term variability of solar irradiance under different sky conditions. Under all-sky conditions, there is a positive trend in SDR of 0.38%/y (brightening). A previous study [16] for the period 1993-2011 also reported a positive trend of 0.33%/y. The slight increase in this trend indicates that the brightening of SDR continues and is likely caused by continuing decreases in aerosol optical depth and the optical thickness of clouds over the area. A smaller trend has been found under clear-sky conditions (0.097%/y), which supports the notion that part of the brightening is caused by decreasing aerosols. [PERSON] et al. [32] showed that aerosol optical depth over Thessaloniki was decreasing constantly, at least up until 2018. The attenuation of SDR by aerosols over Europe has been proposed as major factor by [PERSON] et al. [3]. Unfortunately, for this study, aerosol data for the entire period were not available in order to quantify their effect on SDR. The brightening effect on SDR under cloudy-sky conditions (0.41%/y) suggests that cloud optical thickness is also decreasing during this period. As long-term data of cloud optical thickness are also not available for the region, we cannot draw quantitative conclusions. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sky Condition** & **Season** & **Trend** & **Trend S.E.** & **Pearson** & **Trend** \\  & & **[\%/year]** & **(2\(\sigma\))** & **Correl.** & **_p-Value_** \\ \hline \multirow{4}{*}{Cloudy skies} & Winter & 0.76 & 0.40 & 0.59 & 0.001 \\  & Spring & 0.06 & 0.23 & 0.10 & 0.593 \\ \cline{1-1}  & Summer & \(-\)0.08 & 0.27 & \(-\)0.11 & 0.560 \\ \cline{1-1}  & Autumn & 0.19 & 0.43 & 0.16 & 0.384 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._  
Figure 5: Time series of monthly mean anomalies of SDR by season (rows of plots) for the three sky conditions (columns of plots). The black lines represent the linear trends, (**a–c**) winter, (**d–f**) spring, (**g–i**) summer, (**j–l**) autumn, and also (**a,d,g,j**) all-skies, (**b,e,h,k**) clear-skies and (**c,f,i,l**) cloudy-skies. The observed brightening on SDR over Thessaloniki is dependent on SZA (larger SZAs lead to stronger brightening). The trend is also dependent on season, with winter showing the strongest statistically significant trend of 0.7 and 0.76%/y for all- and cloudy-skies, respectively, in contrast to spring and summer. The trends for autumn are also significant but smaller (0.42 and 0.19%/y for all- and cloudy-skies, respectively). The trend for clear skies is largest in winter (0.4%/y) and negligible in spring, summer, and autumn. Using the CUSUMs of the monthly departures for all and cloudy skies, we observed a 20-year period starting around 2000 where the CUSUMs remain relatively stable, with a steep decline before and a steep increase after. The rather smooth course of the CUSUMs suggests that no important change in the SDR pattern has occurred in the entire record. Continued observations with a collocated pyrheliometer, which started in 2016, will allow us to further investigate the variability of solar radiation at ground level in Thessaloniki. Also, additional data of cloudiness, aerosols, atmospheric water vapor, etc., will allow better attribution and quantification of the effects of these factors on SRD. Conceptualization, methodology, software, data analysis and original writing--draft preparation A.N. ; writing--review and editing, supervision, A.B. ; data curation, C.M. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. Not applicable. Not applicable. Data Availability Statement: Data as daily sums are available through the WRDC database, [[http://wrdc.mgo.rssi.ru](http://wrdc.mgo.rssi.ru)]([http://wrdc.mgo.rssi.ru](http://wrdc.mgo.rssi.ru)). One-minute data are available on request from the corresponding author.