## 3 Results
### Relationships under Different Sky Conditions  
Figure 9 shows scatterplots for different pairs of sunshine recorders at the three synoptic stations for the different periods of measurement. The correlation between individual daily SD records is strong, as is indicated by the coefficient of determination (R\({}^{2}\)) which ranges between 0.84 and 0.96. The lowest correlation was recorded between the daily CS at Larnaca and CSD3 at Pentakomo. Scatterplots with the regression lines for four months of the year representing the four seasons of the year are shown in Figure 10. The parameters of the linear equations on a monthly and annual basis for all the pairs are shown in Table 6. The lower R\({}^{2}\) was obtained for the cases of comparisons of the sunshine recorders with the CSD3 sensors which are installed in the AWS close to the synoptic ones. A higher dispersion is observed in the middle of the graphs rather than towards the tops and bottoms of the regression lines. This can be attributed to the fact that on mostly cloudy (low daily SD) or mostly clear and sunny (high daily SD) days, the instruments tend to measure similar sunshine durations. In contrast, at intermediate sunshine conditions (middling daily sunshine durations), SD measured by each instrument tends to be more dissimilar, due to the different sensitivity of each instrument. For example, the CS device will require a certain time of sustained sunshine for a trace to burn through the sun card. In contrast, the automatic instruments are more sensitive to detecting cloud changes. As is expected, the length of the prediction intervals for the cases of comparisons between CS sunshine recorders and CSD3 sensors is higher, due to the fact that the cloud appearance in the neighbor stations will be probably slightly different than those at the synoptic stations (Table 6). Figure 9: _Cont._As was indicated earlier, all cases of sunshine duration measurements over the analyzed period were classified into four categories, according to the value of clearness index (K\({}_{\rm T}\)), namely, cloudy, partly cloudy with predominance of diffuse radiation (D), partly cloudy with predominance of direct radiation (B) and clear skies. The second class can be described as'mostly cloudy' and the third class as'mostly clear'. Figure 11 shows scatterplots and regression lines for the relationships between the manual (CS) and automatic (Gl) SD measurements recorded at Larnaca during the period 2009-2012 for the four sky conditions. The regression parameters of the relations between different pairs for the three synoptic stations are presented in Table 7. The closest relations are obtained in 'clear sky' and 'all sky' conditions, as is indicated by the high coefficient of determinations. R\({}^{2}\) reached the values of 0.91 and 0.96 under 'all sky' conditions, and between 0.80 and 0.91  
Figure 10: Scatterplots of daily SD for the pair of CS and Gl for Larnaca for the period 2009–2012 for four months representing the four seasons of the year. Figure 9: Scatterplots of different pairs of sunshine recorders at the three synoptic stations for the whole period of measurements. under 'clear' sky conditions. An increase in the amount of clouds causes an increase in the SD differences recorded by the relevant devices. The coefficients of determination for the regression equations under'mostly cloudy' and'mostly clear' conditions range between 0.53 and 0.86. The comparisons of the actual SD measured using CS and automatic sensors under various sky conditions were assessed through the average values of each instrument and the root-mean-square deviation (RMSD) which is given by the following equation:  
\[RMSD=\sqrt{\frac{\sum\limits_{i=1}^{n}{\left({AS_{i}-CS_{i}}\right)^{2}}}{n}} \tag{7}\]  
where AS\({}_{\text{i}}\) is the ith sunshine duration recorded with Automatic sensor, CS\({}_{\text{i}}\) is the ith sunshine duration recorded by CS recorder and n is the number of cases.