Statistical analysis of weather conditions based on the Clearness Index and correlation with meteorological variables

The Clearness Index and the ratio between diffuse and global irradiance for different time scales are used to define a taxonomy for sky conditions and to study patterns to provide information for production-oriented probabilistic risk analysis studies for utility-level PV arrays. The study is focused on the definition of probabilities and distribution functions for the defined taxonomy of sky conditions. Correlations of the Clearness Index are investigated with different meteorological variables, to finally highlight that the only important correlation besides the irradiance is given by the relative humidity, though with a negative pattern. In conclusion, the analysis has shown that the three constraints of the standard test conditions (1000 W/m2, 25 °C, air mass of 1.5 global) normally applied for laboratory characterisation of PV modules are actually never simultaneously met outdoor.