# Solar Ultraviolet Irradiance Characterization under All Sky Conditions in Burgos, Spain
## 3 Results and Discussion
### Sky Classification
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sky Type**} & \multirow{2}{*}{**Parameter**} & \multirow{2}{*}{**All Data**} & \multicolumn{5}{c}{**Season**} \\ \cline{3-7}  & & & **Winter** & **Spring** & **Summer** & **Autumn** \\ \hline \multirow{7}{*}{\begin{tabular}{c} Overcast skies \\ (\(0\leq k_{t}\leq 0.35\)) \\ \end{tabular} } & N & 11,971 & 3,281 & 3,584 & 1,314 & 3,792 \\  & \(GHI\) (W\(\cdot\)m\({}^{-2}\)) & 120.31 & 108.32 & 142.60 & 140.64 & 102.56 \\  & \(GHIU\) (W\(\cdot\)m\({}^{-2}\)) & 5.51 & 4.86 & 6.54 & 6.60 & 4.73 \\  & \(T\) (\({}^{\circ}\)C) & 9.87 & 6.10 & 11.24 & 17.33 & 9.24 \\  & \(RH\) (\%) & 82.41 & 85.64 & 79.15 & 71.97 & 86.30 \\  & \(WS\) (m\(\cdot\)s\({}^{-1}\)) & 2.26 & 2.53 & 2.15 & 1.74 & 2.31 \\ \hline \multirow{7}{*}{\
\begin{tabular}{c} Intermediate skies \\ (\(0.35<k_{t}\leq 0.65\)) \\ \end{tabular} } & N & 9,579 & 2,240 & 2,916 & 1,885 & 2,538 \\  & \(GHI\) (W\(\cdot\)m\({}^{-2}\)) & 312.54 & 278.81 & 361.07 & 335.61 & 269.44 \\  & \(GHIU\) (W\(\cdot\)m\({}^{-2}\)) & 12.08 & 10.43 & 14.22 & 13.20 & 10.25 \\  & \(T\) (\({}^{\circ}\)C) & 12.97 & 7.75 & 13.78 & 20.27 & 11.24 \\  & \(RH\) (\%) & 68.20 & 74.07 & 64.99 & 59.38 & 73.27 \\  & \(WS\) (m\(\cdot\)s\({}^{-1}\)) & 2.11 & 2.43 & 2.09 & 1.83 & 2.05 \\ \hline \multirow{7}{*}{
\begin{tabular}{c} Clear skies \\ (\(0.65<k_{t}\leq 1\)) \\ \end{tabular} } & N & 12,720 & 2,303 & 3,958 & 3,773 & 2,686 \\  & \(GHI\) (W\(\cdot\)m\({}^{-2}\)) & 603.17 & 451.58 & 694.03 & 706.59 & 454.00 \\ \cline{1-1}  & \(GHI\) (W\(\cdot\)m\({}^{-2}\)) & 21.63 & 14.75 & 25.43 & 26.40 & 15.22 \\ \cline{1-1}  & \(T\) (\({}^{\circ}\)C) & 16.51 & 9.05 & 15.83 & 23.59 & 13.97 \\ \cline{1-1}  & \(RH\) (\%) & 54.18 & 61.80 & 53.24 & 46.56 & 59.74 \\ \cline{1-1}  & \(WS\) (m\(\cdot\)s\({}^{-1}\)) & 2.08 & 2.08 & 2.24 & 2.11 & 1.82 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: _Cont._The number of experimental data classified in each sky type category is shown in Figure 3a. The CIE classifies sky conditions as being clear more frequently than \(k_{t}\), and as overcast or an intermediate sky type less frequently than \(k_{t}\). The number of classification matches obtained, according to the CIE and \(k_{t}\) classifications, can be seen in the confusion matrix in Figure 3b. \(k_{t}\) is closely linked to the irradiance value [1], whereas the CIE classification considers the angular distribution of sky radiance and luminance. In this sense, some of the CIE sky types included in the clear category are described as skies with a certain degree of turbidity or pollution, such as CIE sky type 13 (cloudless polluted with a broader solar corona), CIE sky type 14 (cloudless turbid with a broader solar corona), and CIE sky type 15 (white-blue turbid sky with a wide solar corona effect). This is linked to relatively low \(k_{t}\) values [66]. In any case, there is good agreement between the two classification criteria, as can be seen in Figure 3b. Figure 3: (**a**) Frequency of occurrence (FOC) of the CIE and \(k_{t}\) sky classifications in Burgos (Spain) from 14 September 2020 to 6 June 2022; and (**b**) confusion matrix showing the coincidences in the classifications obtained according to both criteria. Figure 2: Frequency of Occurrence (FOC) of each CIE standard sky type in Burgos (Spain) from 14 September 2020 to 6 June 2022.