# Erythermal Solar Irradiance, UVER, and UV Index from Ground-Based Data in Central Spain
## 1 Introduction  
Ultraviolet solar irradiance (UV) causes chemical and biological processes in the lower atmosphere and at the Earth's surface and has been widely studied in recent decades [1]. UV ranges between 100-400 nm and scattering as well as absorption processes in the atmosphere attenuate the spectral interval. UV radiation (200-400 nm) at the top of the atmosphere represents less than 7% of total irradiance. Stratospheric ozone absorbs the shortest wavelengths (UV-C, 100-280 nm); ozone weakly absorbs wavelengths in the UVB range (280-315 nm) and as a consequence, a small part of UV-B and most UV-A (315-400 nm) radiation reach the Earth's surface [2]. Sun height and station altitude modify the solar UV irradiance that reaches the ground because they change the sunlight's path length into the atmosphere. UV variations at the Earth's surface [3, 4] can be caused by atmospheric components such as clouds, total ozone column, aerosols, and desert dust. As a result, 94% of UV energy at ground level corresponds to the UV-A component and 6% to UV-B [5]. UV effects are related to a major increase in illnesses, and UV-B solar irradiance exposure can cause skin cancer, sunburn, ocular damage, cataracts, allergies, and other effects in human beings. Biological response in the UV range is greater at lower wavelengths, such that small increases in UV-B irradiance can have substantial biological effects [6]. Biological sunburn or the erythema effect is oneof the most common reactions to UV-B radiation overexposure in humans. It is analyzed through ultraviolet erytnal radiation (UVER), which is defined as the spectrally integrated solar irradiance at ground level weighted with the standard erythema action spectrum curve adopted by the CIE (Commission Internationale de l'Eclairage) in 1987 [7; 8]. The effects of UVER on the skin that has not previously been exposed to solar irradiance are usually studied by the UVER minimum dose, which can cause intense reddening of human skin, [9]. Different institutions, such as the World Meteorological Organization (WMO), the (ICNIPR) International Commission on Non-Ionizing Radiation Protection, the World Health Organization (WHO), and (UNEP) the United Nations Environmental Programme, recommend use of the UV Index as an indicator of potential skin damage from solar UV irradiance [10]. In 1995, the global solar UV index, UVI, was recommended for forecasting and informing the public about UV irradiance values that reached the ground [10]. The UVI value is evaluated by multiplying the UVER irradiance value (expressed in W/m\({}^{2}\)) by 40 and is reported as a value rounded to the nearest integer [11]. As regards the harmful effects, stratospheric ozone depletion, and the increase of erytnal UV doses, authors propose the development of studies, programs, and strategies designed to raise awareness amongst the population of the need for protection from the sun [12]. In 2002, the UV Index (UVI) was standardized by the World Meteorological Organization (WMO) [10], and the World Health Organization (WHO) [10], as a simple indicator of potential skin damage from solar UV irradiance. Previously, within the framework of the European COST-713 Action, the forecast and dissemination of UVI were analyzed recommending the use of radiative transfer models to operationally determine UVI values [13]  
Additionally, the WMO (2007, 2008) [14; 15] and other national organizations have encouraged the establishment of networks to measure UV irradiance and, more specifically, erytnal irradiance. Many stations have recently started to take continuous measurements and, particularly in the Northern Hemisphere, long-term case studies and trend analyses are being carried out. Different authors (e.g., [PERSON] et al., 2007; [PERSON], 2008; [PERSON] et al., 2008; [PERSON] et al., 2011; [16; 17; 18; 19]) have therefore established modeling approaches to reconstruct past UV radiation time series.