In the last years, many studies have focused on the study of the ultraviolet radiation or the total solar radiation in Southwestern Europe. This area presents a particular interest due to the intense solar radiation which reaches the Earth's surface at these latitudes. Despite of this high interest, the influence of the atmospheric factors on the relationship between both variables is not well documented for that region. In this work, the ultraviolet erythemal irradiance (UVER) and the total solar irradiance (310–2800 nm) at three locations in Southwestern Spain were measured between January 2001 and December 2005. The main goal of this paper is to analyze the relationship between ultraviolet and total solar irradiance as described by their ratio. This ratio represents the percentage of UV erythemal irradiance over the total solar irradiance at Earth's surface. The results show that this ratio varies in time. It shows a day-to-day and seasonal variability mainly due to changes in sky conditions (regarding cloudiness and aerosols) and in the slant column of ozone, respectively. Differences in ozone may cause a relative increase in the ratio higher than 100% between winter and summer. This fact stresses the importance of reporting information about the UV radiation at ground level to the people mainly during summer. Clouds play also an important role, since highly different conditions between consecutive days are responsible for up to 60% of the ratio variability. The results show that cloudiness attenuates more the total solar radiation than the UV solar radiation. Additionally, high changes in aerosol load can produce a short-term decrease in the ratio of 13–15%, attenuating differently UVER than total solar irradiance.