## 2 Experimental Data and Quality Control
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Variables.** & **Meteorological Variables** & **Expression** \\ \hline GHI & Global Horizontal irradiance & measured \\ DHI & Diffuse Horizontal irradiance & measured \\ DNI & Direct Normal irradiance & measured \\ CC & Cloud cover & measured \\ RH & Relative Humidity & measured \\ T & Air temperature & measured \\ WS & Wind speed & measured \\ \(\psi\) & Solar azimuth angle & calculated \\ \(\theta_{\mathrm{z}}\) & Solar zenith angle & calculated \\ D & Diffuse fraction & \(\mathrm{D}=\frac{\mathrm{DHI}}{\mathrm{GHI}}\) \\ \(\mathrm{k_{t}}\) & Clearness index & \(\mathrm{k_{t}}=\frac{\mathrm{GHI}}{\mathrm{B_{\mathrm{sc}}}\mathrm{c},\cos{ \psi_{z}}}\) \\ \(\mathrm{k_{d}}\) & Diffuse to extraterrestrial irradiance & \(\mathrm{k_{d}}=\frac{\mathrm{DHI}}{\mathrm{B_{\mathrm{sc}}}}\) \\ \(\Delta\) & [PERSON]’s brightness factor & \(\Delta=\frac{\mathrm{m}-\mathrm{DHI}}{\mathrm{B_{\mathrm{sc}}}\mathrm{c}_{t}, \cos{\theta_{z}}}\) \\ \(\varepsilon\) & [PERSON]’s clearness index & \(\varepsilon=\frac{\mathrm{MHI}\mathrm{D}\mathrm{s}\mathrm{s}\mathrm{s}\mathrm{s} \mathrm{s}\mathrm{s}\mathrm{s}}+(\delta_{\mathrm{z}})^{3}\) \\ \hline \hline \end{tabular} \(B_{\mathrm{sc}}\) is the solar constant (1361.1 W\(\cdot\)m\({}^{-2}\)[49]). \(\mathrm{\ell_{z}}\) is the average value of the orbital eccentricity of the Earth, \(k=1.04\) (\(or\) 5.56\(\cdot 10^{-6}\) if \(\theta_{z}\) is expressed in degrees).  
\end{table} 
Table 2: Variables measured and calculated in Burgos. Figure 2: Frequency of occurrence (FOC) in Burgos (Spain) (**a**) of each ISO/CIE standard sky type, and (**b**) for each ISO/CIE sky type group. deviations around \(6\cdot 10^{-3}\%\). The maximum value was recorded in July (\(4.2\cdot 10^{-2}\cdot\%\)), while the minimum was reached in April (\(1\cdot 10^{-3}\%\)). Figure 4: Box and whisker plot of the GHUVE/GHI ratio based on 10 min data grouped by months. Figure 5: Box and whisker plot of the GHUVE/GHI ratio based on 10 min data grouped by seasons. Figure 3: Box and whisker plot of the GHUVE/GHI ratio based on 10 min data grouped by UTC hours. Gray crosses indicate the mean, and the white lines inside the box indicate the median. The limits of the boxes define the first, second, and third quartiles, whereas the extreme whiskers show the minimum and the maximum points. Black and gray circles represent outliers. Figure 5 shows the greatest dispersion of values for the summer months and the highest value of the GHUVE/GHI ratio. Conversely, the dispersion of values in winter was relatively smaller. The interquartile ranges were \(1.1\cdot 10^{-2}\%\) and \(5\cdot 10^{-3}\%\) with standard deviations of \(6\cdot 10^{-3}\%\) and \(4\cdot 10^{-3}\%\), respectively. Upon analyzing the GHUVE/GHI ratio based on the 15 ISO/CIE sky types (Figure 6), it is evident that sky type 15 exhibits the highest ratio value and the lowest data dispersion. Conversely, sky type 12 demonstrates the lowest ratio value, with an average level of data dispersion. Notably, both sky types belong to the category of clear skies. Figure 7 shows that the variation between the mean \(\left(1.1\cdot 10^{-2}-1.2\cdot 10^{-2}\%\right)\) and the dispersion (interquartile range: \(8\cdot 10^{-3}-9\cdot 10^{-3}\%\), standard deviation: \(5\cdot 10^{-3}-6\cdot 10^{-3}\%\)) of the three main ISO/CIE sky types is very similar.