## 2 Experimental Data and Quality Control  
The SWIFT Research Group ground meteorological facility, located on the flat roof of the Higher Polytechnic School of the University of Burgos (42\({}^{\circ}\)21'04\" N, 3\({}^{\circ}\)41'20\" W, 856 m a.s.l. ), and shown in Figure 1, has provided the experimental data used for this study, in a 10 min basis, with an average scanning granularity of 30 s, recorded from September 2020 to June 2022. Various climatic parameters were recorded, including air temperature (T), relative humidity (RH), wind speed (WS), and direction. GHI and Diffuse Horizontal Irradiance (DHI) were measured with Hulseflux pyranometers (model SR11) and Direct Normal Irradiance (DNI) by means of a Hulseflux pyrheliometer (model DR01). A GEONICA-SEMS-3000 sun tracker equipped with a shading disc was employed for the measurement of DHI. Additionally, the pyrheliometer was mounted on the sun tracker to measure DNI. GHUVE values were obtained with a Kipp and Zonnen SUV-E radiometer. Sky luminance and radiance distributions, used to classify the sky condition, were determined with a sky-scanner EKO MS-321 LR. The cloud cover (CC) was calculated with a commercial all-sky camera (SONAD201D) that records every 1 s an RGB color image with 1158 \(\times\) 1172 pixels of resolution. A complete description of the experimental facility and instruments can be found in previous works [42; 43]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Ref.** & **Location** & **Model** & **Meteorological Variables** & **Models** & **Data** \\ \hline \([37]\) & Spain & UVER & m, TOC, VIS, latitude, cloudiness & ANN & 3 h \\ \([31]\) & Spain & UVER & m, TOC, k, k\({}_{\text{b}}\), k\({}_{\text{t}}\) & ANN & 30 min \\ \([38]\) & Spain & UVER & TOC, cloudiness, AOD & MLR & 1 min \\ \([27]\) & Thailand & UVER/GHI & \(\theta_{\text{z}}\), AOD, TOC, k\({}_{\text{t}}\), precipitable water & LR & 10 min \\ \([39]\) & Korea & UVER/GHI & CC, AOD, SCD & MLR & 10 min \\ \([40]\) & Spain & UVER & m, TOC,k\({}_{\text{t}}\) & LR & 30 min \\ \([34]\) & Germany & UVER & GHI, DHI, SD, TOC, VIS, \(\theta_{\text{z}}\) & ANN & daily \\ \([35]\) & European Sites & UVER & SD, GHI, DHI, \(\theta_{\text{z}}\), CC, TOC,VIS, Month, & ANN & 30 min \\ \([36]\) & Serbia & UVER & GHI, cloudiness, m, k\({}_{\text{t}}\), TOC & ANN & 10 min \\ \([12]\) & Argentina & UVER/GHI & GHI, \(\theta_{\text{z}}\), k\({}_{\text{t}}\), altitude & LR & 1 min \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Studies to estimate UVER ratio around the world. The collected GHI, GHUVE, DHI, and DNI data underwent the quality control procedure recommended by the MESoR project [44]. For the GHUVE data, it was determined that GHUVE values should not exceed the corresponding extraterrestrial UVER on the horizontal plane (UVEH\({}_{0}\)). The calculation of UVEH\({}_{0}\) involved applying a correction factor (f\({}_{\rm c}\)), which considered the estimated orbital eccentricity to the UVER solar constant (UVE\({}_{\rm sc}\)) multiplied by the cosine of the solar zenith angle (\(\theta_{z}\)), as described in Equation (1). \[\rm{UVEH}_{0}=f_{\rm c}\cdot UVE_{\rm sc}\cdot\cos\theta_{z} \tag{1}\]  
In the absence of a standardized value, UVE\({}_{\rm sc}\) was determined by integrating the product of the extraterrestrial solar spectrum [45] and the erythema spectral weighting function [14] over the wavelength range of 280 to 400 nm. This calculation yielded a value of 14.5 W\(\cdot\)m\({}^{-2}\). Data points corresponding to solar elevation angles below 5\({}^{\circ}\) were excluded from the analysis to mitigate the cosine response issues inherent to the GHI, and GHUVE measurement instruments. A summary of the variables used in this study is shown in Table 2. Among these meteorological variables, the following were directly obtained from the experimental measurements: GHI, DHI, DNI, CC, RH, T, and WS. \(\theta_{z}\), and \(\psi\) were calculated.