## 3 Results
### Validation of Diurnal Solar Radiation in Different Months  
When evaluating GHI estimates from Heliosat-2 on a monthly basis, using a dataset which includes both clear and cloudy days, the R\({}^{2}\) values in the EU locations were consistently lower than any other location in the study (<0.65). The months of January, February, and November, in these locations, demonstrated very low R\({}^{2}\) values, going below 0.3. In contrast, R\({}^{2}\) values in the ABS and TAT locations of Japan remained impressively robust, consistently topping 0.75 (Figure 6A). Notably, in March, May, and June, these values often exceeded 0.85. In African locations, R\({}^{2}\) values remain exceptionally high throughout the year, particularly from July through November, frequently surpassing 0.92. Similarly, in the KSA location, R\({}^{2}\) values are consistently high, generally exceeding 0.93, with peaks often reaching above 0.97 during January, August, and September (Figure 6A). In Indian sites, during the monsoon months, the R\({}^{2}\) values were lower (<0.7) compared to the non-monsoon months (>0.7).  
EU sites, such as CAB and CNR, consistently display positive mean biases, indicating an overestimation of solar irradiance throughout the year, with CAB values ranging from 15.91 to 37.96 W\(\cdot\)m\({}^{-2}\) and CNR from 8.91 to 38.58 W\(\cdot\)m\({}^{-2}\). On the monthly scale, MB values at Japanese sites fluctuate between \(-\)30.6 and +40.7 W\(\cdot\)m\({}^{-2}\), with notable instances of overestimation, and often exceeding +30.0 W\(\cdot\)m\({}^{-2}\), particularly between March and April and August-September (Figure 6B). In contrast, at the African sites, GOB consistently shows a trend of underestimation, with values ranging from \(-\)11.1 to \(-\)50.0 W\(\cdot\)m\({}^{-2}\), while VUW exhibits a general overestimation, with values ranging from 0 to +38.1 W\(\cdot\)m\({}^{-2}\). The KSA location in Saudi Arabia sees MB values ranging from \(-\)13.5 to +12.2 W\(\cdot\)m\({}^{-2}\), with very slight underestimations typically observed during January, September, and November; these range from \(-\)5.9 to +2.2 W\(\cdot\)m\({}^{-2}\) (Figure 6B). In India, the performance of solar irradiance estimation using the Heliosate-2 method demonstrates a mixed pattern across the year. MB values for Indian sites, such as ASO and HON, vary notably, with ASO experiencing a range from \(-\)16.26 to 8.0 W\(\cdot\)m\({}^{-2}\) and HON from \(-\)17.8 to 15.8 W\(\cdot\)m\({}^{-2}\), indicating both overestimation and underestimation of solar irradiance.  
In a monthly study, the RMSE values for EU sites are notably higher, with CAB and CNR recording values ranging from 72.0 to 166.0 W\(\cdot\)m\({}^{-2}\) and 78.1 to 179.6 W\(\cdot\)m\({}^{-2}\), respectively (Figure 6C). In the Japanese sites, notably lower RMSE values are observed between January and March and October-December, typically falling below 65.0 W\(\cdot\)m\({}^{-2}\). African sites show a pattern of lower RMSE during the cooler months of July, August, and September (below 60.0 W\(\cdot\)m\({}^{-2}\)), whereas the warmer months of January, February, March, and December exhibit higher RMSE values, ranging from 95.0 to 130.0 W\(\cdot\)m\({}^{-2}\), as illustrated in Figure 6C. At the KSA site in Saudi Arabia, RMSE values are exceptionally low, mostly below 50.2 W\(\cdot\)m\({}^{-2}\), with January and September marking months where RMSE values are consistently below 30.0 W\(\cdot\)m\({}^{-2}\). In India, the RMSE values remain relatively lower, with ASO and HON registering values ranging from 35.4 to 215.5 W\(\cdot\)m\({}^{-2}\) and 60.1 to 146.8 W\(\cdot\)m\({}^{-2}\), respectively.