## 2 Materials and Methods  
### Study Region and Data Collection  
Poland is situated between 49.0\({}^{\circ}\)N and 54.5\({}^{\circ}\)N in the temperate transitional climate. According to long-term measured data (1971-2000), annual sunshine duration varies from 1377 h to 1700 h, and global solar radiation is in the range of 3600-3800 MJ\(\cdot\)m\({}^{-2}\). Cloud cover varies between 4.8 to 5.4 octas. The annual average air temperature lies between 6.5 \({}^{\circ}\)C in the northeast part of Poland to 8.5 \({}^{\circ}\)C in the southwest [30]. The 16-year-long data series used for this study come from 15 actinometric stations located across the country and concern the years 2000-2015. The stations were selected considering the availability of a representative sample and the geographical location. Their spatial distribution and characteristics are shown in Figure 1 and Table 1, respectively. The data were sourced from actinometric stations run by the Institute of Meteorology and Water Management--National Research Institute (IMGW-PIB), which is responsible for the operation of meteorological stations, maintenance of regular measurements and observations, and processing, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Station** & **\(\mathbf{\lambda}\) (E)** & **\(\mathbf{\Phi}\) (N)** & **h (m)** & **Gaps** \\ \hline Gdynia & 18\({}^{\circ}\)33\({}^{\circ}\)34\({}^{\circ}\) & 54\({}^{\circ}\)31\({}^{\circ}\)08\({}^{\prime\prime}\) & 2 & I,II,III 2001; XII 2003; I,IV 2004 \\ Gorzów & 15\({}^{\circ}\)16\({}^{\circ}\)38\({}^{\circ}\) & 52\({}^{\circ}\)44\({}^{\circ}\)28\({}^{\prime\prime}\) & 71 & 2000–2002 \\ Wielkopolski & 21\({}^{\circ}\)58\({}^{\circ}\)24\({}^{\prime\prime}\) & 51\({}^{\circ}\)48\({}^{\circ}\)53\({}^{\circ}\) & 180 & V,VI 2003; VII 2004; XI 2005; IX,X,XI 2015 \\ Kolobrzeg & 15\({}^{\circ}\)34\({}^{\circ}\)47\({}^{\circ}\) & 54\({}^{\circ}\)10\({}^{\circ}\)57\({}^{\circ}\) & 3 & \\ Legina & 16\({}^{\circ}\)12\({}^{\circ}\)28\({}^{\circ}\) & 51\({}^{\circ}\)113\({}^{\circ}\) & 122 & X, XI 2014 \\ Leba & 17\({}^{\circ}\)32\({}^{\circ}\)05\({}^{\circ}\) & 54\({}^{\circ}\)45\({}^{\circ}\)13\({}^{\circ}\) & 2 & VIII, IX 2012 \\ Lódz & 19\({}^{\circ}\)23\({}^{\circ}\)14\({}^{\circ}\) & 51\({}^{\circ}\)43\({}^{\circ}\)06\({}^{\circ}\) & 175 & IV 2015 \\ Lesko & 22\({}^{\circ}\)20\({}^{\circ}\)30\({}^{\circ}\) & 49\({}^{\circ}\)27\({}^{\circ}\)59\({}^{\circ}\) & 420 & III 2003 \\ Mikolajki & 21\({}^{\circ}\)35\({}^{\circ}\)23\({}^{\circ}\) & 53\({}^{\circ}\)47\({}^{\circ}\)21\({}^{\circ}\) & 127 & \\ Pifa & 16\({}^{\circ}\)44\({}^{\circ}\)50\({}^{\circ}\) & 53\({}^{\circ}\)07\({}^{\circ}\)50\({}^{\circ}\) & 72 & XI, XII 2014; I,II,III,IV 2015 \\ Pulawy & 21\({}^{\circ}\)57\({}^{\circ}\)58\({}^{\circ}\) & 51\({}^{\circ}\)24\({}^{\circ}\)46\({}^{\circ}\) & 142 & XI 2001; II,III,IV 2005;V 2004, 2015 \\ Torun & 18\({}^{\circ}\)35\({}^{\circ}\)44\({}^{\circ}\) & 53\({}^{\circ}\)02\({}^{\circ}\)31\({}^{\circ}\) & 69 & \\ Warszawa & 20\({}^{\circ}\)57\({}^{\circ}\)48\({}^{\circ}\) & 52\({}^{\circ}\)16\({}^{\circ}\)53\({}^{\circ}\) & 98 & \\ Wieluń & 18\({}^{\circ}\)33\({}^{\circ}\)24\({}^{\circ}\) & 51\({}^{\circ}\)12\({}^{\circ}\)37\({}^{\circ}\) & 199 & X,XI,XI 2014 \\ Wlodawa & 23\({}^{\circ}\)31\({}^{\circ}\)46\({}^{\circ}\) & 51\({}^{\circ}\)33\({}^{\circ}\)12\({}^{\circ}\) & 177 & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of stations and dataset used in the analysis (www.imgw.pl). storing, and making meteorological data available [31]. The data for Warszawa and Kolobrzeg stations were obtained from a database of the World Radiation Data Centre [32]. An initial data quality analysis was performed by the IMGW-PIB [33], and the data transferred to the WRDC are also subject to inspection and flagging (quality assurance). Daily sums of global solar radiation (H, [MJ-m\({}^{-2}\)]) provided the basis for this study.