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 (GHUV) 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 GHUV and GHI can be used to
model GHUV.
