# Solar Ultraviolet Irradiance Characterization under All Sky Conditions in Burgos, Spain
###### Abstract  
Solar Ultraviolet Radiation (UVR), which is identified as a major environmental health hazard, is responsible for a variety of photochemical reactions with direct effects on urban and aquatic ecosystems, human health, plant growth, and the deterioration of industrial systems. Ground measurements of total solar UVR are scarce, with low spatial and temporal coverage around the world, which is mainly due to measurement equipment maintenance costs and the complexities of equipment calibration routines; however, models designed to estimate ultraviolet rays from global radiation measurements are frequently used alternatives. In an experimental campaign in Burgos, Spain, between September 2020 and June 2022, average values of the ratio between horizontal global ultraviolet irradiance (_GHIUV_) and global horizontal irradiance (_GHI_) were determined, based on measurements at ten-minute intervals. Sky cloudiness was the most influential factor in the ratio, more so than any daily, monthly, or seasonal pattern. Both the CIE standard sky classification and the clearness index were used to characterize the cloudiness conditions of homogeneous skies. Overcast sky types presented the highest values of the ratio, whereas the clear sky categories presented the lowest and most dispersed values, regardless of the criteria used for sky classification. The main conclusion, for practical purposes, was that the ratio between _G  
UVR is divided into three wavebands: UV-C (100-280 nm), UV-B (280-315 nm), and UV-A (315-400 nm). UV-C is absorbed in the stratosphere, whereas both the UV-A and the UV-B bands reach ground level in amounts that depend on several factors. Column ozone content, atmospheric composition [12] (aerosols, water vapor, and other gases), cloud type and distribution [13; 14], and geometrical and geographic variables, such as solar zenith angle, and surface albedo [15; 16], all contribute to UVR variability; however, it is difficult to evaluate the individual role of each variable, due to their combined effects with other variables under different atmospheric conditions [17]. UVR ground measurements are scarce, and spatial and temporal coverage around the globe is limited, which is mainly due to equipment maintenance costs and the complexities of measurement equipment calibration routines [17; 18; 19]; however, the models used to estimate solar UVR from global radiation measurements, with relatively few data inputs, are frequently used alternatives [11; 12; 15; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36]. Several authors have studied the influence of clouds and sky conditions on surface UV measurements [12; 13; 17; 22; 33; 37; 38]. The presence of cloud cover and its type and distribution can both enhance total solar UVR by as much as 15% and reduce it by over 50%; this is due to the superposition of the albedo reflection, and distribution and extinction effects [39]. Conversely, atmospheric pollution and dust can drastically reduce total solar UV irradiance [33; 40; 41]. The ratio of global horizontal ultraviolet irradiance (_GHUV_) and global horizontal irradiance (_GHI_), measured using different (ten-minute data intervals, hourly and daily values, and monthly and annual averages) time scales around the world and under different sky conditions, has been widely studied, as shown in the literature review summarized in Table 1. As is evident, the _GHUV:GHI_ ratio ranges between 2.9 and 7%, with the highest values of the ratio corresponding to cloudy days, regardless of the index used for sky condition classification. The scattering effects on the UVR band caused by cloud cover increases the intensity of the _GHUV_, whereas the stronger absorption effect of water vapor on the infrared band contributes to lower levels of _GHI_[33]. The location of the measurement site is another important factor: _GHUV_ at ground level increases when the site is closer to the Equator [42] and at a higher height above sea level (h.a.s.l.