# Erythermal Solar Irradiance, UVER, and UV Index from Ground-Based Data in Central Spain
## 3 Results
### Solar IUI Study  
Solar UVI is a variable that provides information regarding UV radiation levels at the Earth's surface and which indicates the possibility of skin damage [31; 32]. UVI is a variable that alerts to the need to use special protective measures when there is human exposure to UV radiation. UVI was prepared by different international institutions, and since its publication in 1995 has improved its use as a health tool to encourage sun protection. At present, over 50 research laboratories monitor UVI in Europe, as can be seen in [16]. Using experimental UVER, the UV index has been evaluated from solar irradiance measurements using two different norms: (a) the value at midday, used up to 1998. (b) the value approved by all the various worldwide organizations, namely, the maximum daily value. Figure 4 offers a statistical analysis of the differences between the UVI values obtained with each norm at the study station. It shows an agreement percentage of 62.1%, but in 20.3% of cases, there was a difference of one UVI unit, while in 5.3% of cases there was a difference of several UVI units.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Mean** & **Median** & **SD** & **Kurtosis** & **Min** & **Max** & **Q1** & **Q3** & **P5** & **P95** \\ \hline
194 & 214 & 53.62 & 0.971 & 32.1 & 260 & 164 & 235 & 93.1 & 248 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Hourly UVER irradiance (mW m\({}^{-2}\)) statistical indices at solar noon in June, Valladolid, Spain.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Mean** & **Median** & **SD** & **Kurtosis** & **Min** & **Max** & **Q1** & **Q3** & **P5** & **P95** \\ \hline
29.4 & 30.9 & 12.31 & \(-\)0.651 & 3.68 & 61.03 & 18.7 & 38.63 & 9.41 & 48.39 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Hourly UVER irradiance (mW m\({}^{-2}\)) statistical indexes, in January, at solar noon.  
We, therefore, observe that the UV index at solar noon disagreed with the maximum daily value by one or less in 82.4% of the cases studied in Valladolid, Spain. These results are comparable to those obtained by [26] who analyzed 10 years of UV measurements at 16 of the Spanish UVB Radiometric  
Network stations. Results show that the percentage of coincidence is lower for stations at which cloudiness is more variable. Authors [26] have found that the percentage of cases for which the difference between the two criteria was zero or one UVI unit varied between 89%, in A Coruna, Spain, and 95%, in El Arenosillo, Huelva, Spain where the average value is 92%. The result is higher in the south of Spain.  
Table 5 displays the percentage (%) of maximum daily UVI values at the measurement station, classified according to exposure levels, using the color code approved by the WHO [10]. UVI reaches high values (6-7) on over 17% of days, very high values (8-10) on over 28%, and extreme values (\(\geq\)11) in 0.3% of cases in Valladolid, Spain. UVI values of above 10 were normally recorded in warm seasons; namely, spring and summer.  
It could be assumed that human skin requires maximum protection when there are high UVI values. This entails the need to wear appropriate clothing and to follow the established rules, e.g., by wearing long-sleeved shirts, sunglasses, hats, total sunscreen of solar protection factor (SPF) 30+, and by avoiding sunbathing between 12.00 and 15.00, the central hours of the day [10].