## 3 Results
### Intra-Annual Sky Status  
The present section provides an analysis of the sky conditions over Greece on a monthly basis. Figure 6 shows the monthly mean values of the duration of sky conditions (DCS, DIS, DOS) with the \(\pm 1\sigma\) band and the best-fitted curves to the mean ones at the confidence level of 95%. More particularly, Figure 6a depicts a smooth intra-annual variation in the DCS (or else SSD) with its maximum in July and its minimum values during winter, as expected. The best-fitted regression curve to the mean DCS values lies within the \(\pm 1\sigma\), a fact that implies 95% of the DCS events are fully described by the fitted curve. Figure 6b shows the intra-annual variation of the DIS conditions over Greece; two local maxima are found, one in May and another in August. The first maximum appears in spring time (transition season), while the second is due to the frequent thermal storms of short duration in early afternoon at near-shore areas because of temperature differences between land and water [36]. Figure 6c presents the intra-annual variation in the DOS cases over Greece; lowest values in the summer and highest in winter occur, as expected. Figure 4: _Cont_. Figure 4: Seasonal mean frequency of occurrence (FOO, in %) for the (**a**) DCS, (**b**) DIS, and (**c**) DCS cases from the TMYs of the 43 sites. Figure 5: _Cont._  
Figure 5: Seasonal mean duration of sky conditions (in h) for the (**a**) DCS, (**b**) DIS, and (**c**) DOS cases from the TMYs of the 43 sites. The black lines represent the means, the red and blue ones represent \(+1\sigma\) and \(-1\sigma\), respectively. The dashed green lines are the best-fitted curves to the mean ones. \begin{table}
\begin{tabular}{l l} \hline
**Sky Conditions** & **Regression Equation and Statistics** \\ \hline \multirow{2}{*}{Clear} & SSD = DCS = 983.0168 \(\times\) exp\(\left[-0.5\times\left[\ln(t/1.9320)/0.3804\right]^{2}/t\right]\) \\  & R\({}^{2}\) = 1, SEE = 0 h, \(\alpha\) = 0.05 \\ \hline \multirow{2}{*}{Intermediate} & DIS = 968.7578 \(\times\) exp\(\left[-0.5\times\left[\ln(t/1.9336)/0.3855\right]^{2}/t\right]\) \\  & R\({}^{2}\) = 1, SEE = 0 h, \(\alpha\) = 0.05 \\ \hline \multirow{2}{*}{Overcast} & DOS = 289.0910 \(\times\) (\(t^{-90.3392\times t^{-8.103}}\)) \\  & R\({}^{2}\) = 0.9993, SEE = 3.6101 h, \(\alpha\) = 0.05 \\ \hline \end{tabular}
\end{table} 
Table 2: Expression of the seasonal mean duration (in h) for the clear, intermediate, and overcast skies from the TMYs of the 43 sites as function of the season, t, in the range [1 (spring), 4 (winter)]. R\({}^{2}\) = coefficient of determination; SEE = standard error of the estimate. SSD/DCS = sunshine duration/duration of clear skies; DIS = duration of intermediate skies; DOS = duration of overcast skies (all durations in h). All regression fits are at the 95% confidence level (\(\alpha\)). \begin{table}
\begin{tabular}{l l} \hline
**Sky Conditions** & **Regression Equation and Statistics** \\ \hline \multirow{2}{*}{Clear} & SSD = DCS = \(-0.0110\)\(\times\)\(t^{6}+0.4560\times\)\(t^{5}-7.1042\times\)\(t^{4}+51.0454\times\)\(t^{5}\) \\  & \(-170.5219\times\)\(t^{2}+260.5100\times\)\(t-81.5485\) \\  & R\({}^{2}\) = 0.9838, SEE = 11.4553 h, \(\alpha\) = 0.05 \\ \hline \multirow{2}{*}{Intermediate} & DIS = \(0.0063\)\(\times\)\(t^{6}-0.2435\times\)\(t^{5}+3.7120\times\)\(t^{4}-28.1920\times\)\(t^{3}+106.7279\) \\  & \(\times\)\(t^{2}-160.6950\times\)\(t+175.3897\) \\  & R\({}^{2}\) = 0.9648, SEE = 0.9097 h, \(\alpha\) = 0.05 \\ \hline \multirow{2}{*}{Overcast} & DOS = \(0.0119\)\(\times\)\(t^{6}-0.5002\times\)\(t^{5}+8.0436\times\)\(t^{4}-61.1764\times\)\(t^{3}+223.5341\times\)\(t^{2}-366.5001\times\)\(t+323.0779\) \\  & R\({}^{2}\) = 0.9820, SEE = 7.3930 h, \(\alpha\) = 0.05 \\ \hline \end{tabular}
\end{table} 
Table 3: Non-linear expressions of the monthly mean duration (in h) for the clear, intermediate, and overcast skies from the TMYs of the 43 sites as function of the month (t), with t \(\in\)[1, 12]. R\({}^{2}\) = coefficient of determination; SEE = standard error of the estimate.