## 3 Results
### Credibility of Results  
To ensure that the DCS, DIS, and DOS results are acceptable, comparison of all-sky events with measured or modelled ones must take place. All 43 selected sites correspond to meteorological stations belonging to the HNMS network. However, the HNMS has not yet provided SSD-accredited data for the period 2007-2016 (from which period the TMYs have been derived in the PV-GIS platform). To overcome this difficulty, use of the free Global Climate Data of the German Weather Service (DWD) was made from the Climate Data Centre (CDC) of the German Weather Service (DWD)--Climate-ADAPT (europa.eu, accessed on 13 June 2022) platform; yearly sunshine durations were downloaded for the available Elliniko, Irakleio, Kerkyra, and Larissa sites in the period 2007-2016. In addition, measured SSD data from the Institute of Applied Physics, Academy of Sciences of Moldova (ASM) at Chisinau, were used for the period 2007-2015. SSD averages from the five sites were computed and are compared with the annual ones estimated in this work. Table 5 shows this comparison. It is seen that large differences exist between the SSD values from the three sources (fourth column in Table 5). This outcome may be attributed to the fact that some \(\rm k_{d}\) values though classified as DIS may have DCS characteristics. This issue is discussed in the following paragraph. In order to investigate the above findings further, one might consider the sub-division of the IS cases into IS-1, -2, and -3 sky conditions, as mentioned in Section 3.4. This is performed because of a possible \"intrusion\" of \(\rm k_{d}\)-DIS values into the \(\rm k_{d}\)-DCS range. To limit this intrusion and make things clearer, consideration of the DIS-1, DIS-2, and DIS-3 sub-ranges was taken into account. These sub-ranges correspond to \(\rm k_{d}\in(0.26,0.43)\) for DIS-1, to \(\rm k_{d}\in(0.43,0.60)\) for DIS-2, to \(\rm k_{d}\in(0.60,0.78)\) and were defined as to have equal \(\rm k_{d}\) lengths, i.e., 0.17, 0.17, and 0.18, respectively, for DIS-1, DIS-2, DIS-3. It is now assumed (without \begin{table}
\begin{tabular}{l l l} \hline \hline
**Sky Conditions** & \multicolumn{2}{l}{**Regression Equation and Statistics**} \\ \hline Clear & \(\rm ASICS=0.0007\times\varphi+0.1834\) \\  & \(\rm Ave=0.2118,R^{2}=0.0330,\)\(\rm SE=0.0074,\)\(\alpha=0.05\) \\ \hline Intermediate-1 & \(\rm ASIS_{-1}=2.5292\times 10^{-5}\times\varphi+0.3287\) \\  & \(\rm Ave=0.3297,R^{2}=0.0002,\)\(\rm SE=0.0035,\)\(\alpha=0.05\) \\ \hline Intermediate-2 & \(\rm ASIS_{-2}=0.0002\times\varphi+0.4999\) \\  & \(\rm Ave=0.5095,R^{2}=0.0206,\)\(\rm SE=0.0032,\)\(\alpha=0.05\) \\ \hline Intermediate-3 & \(\rm ASIS_{-3}=0.0014\times\varphi+0.6243\) \\  & \(\rm Ave=0.6801,R^{2}=0.2243,\)\(\rm SE=0.0050,\)\(\alpha=0.05\) \\ \hline Overcast & \(\rm ASIG_{OS}=-0.0024\times\varphi+1.0613\) \\  & \(\rm Ave=0.9685,R^{2}=0.1018,\)\(\rm SE=0.0133,\)\(\alpha=0.05\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Linear expressions for the annual mean ASI for clear; intermediate-1, -2, -3; and overcast skies over Greece as function of the geographical latitude (\(\varphi\) in degrees N), of the sites in the range (35\({}^{\circ}\), 42\({}^{\circ}\)). The ASI values are averages over the 43 sites from their TMYs. Ave = annual mean; \(\rm R^{2}\) = coefficient of determination; \(\rm SE=standard\) error of the estimate. The subscripts CS; IS-1, -2, -3; and OS to ASI denote clear; intermediate-1, -2, -3; and overcast skies, respectively. All regression fits are at the 95% confidence level (\(\alpha\)). \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Site** & **SSD, External Source (h)** & **DCS, This Work (h)** & **SSD—DCS (h)** \\ \hline Chisinau, ASM & 2346 & 1213 & 1132 \\ Elliniko, DWD & 2909 & 1523 & 1386 \\ Irakleio, DWD & 2905 & 1548 & 1357 \\ Kerkyra, DWD & 2593 & 1240 & 1353 \\ Larissa, DWD & 2435 & 1185 & 1250 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Annual mean values of SSD for 4 sites from DWD (2007–2016), 1 site from ASM (2007–2015), and DCS from the TMYs of the sites.