The present study is focused on the influence of atmospheric factors (ozone, aerosols, precipitable water and, mainly, clouds) on the ratio (measured and modelled) between UV erythemal (UVER) and total shortwave (SW) solar radiation. The ratio between UV total (UVT) and SW is also analysed. Previous studies showed that the shortest wavelengths are less attenuated by the presence of clouds than the longest ones. This effect is confirmed in this study; however, another effect is shown for low solar zenith angles because the UVER/SW ratio values present a decreasing trend with cloud cover, which is enhanced by different average ozone column values in each situation. Simulations performed with the libRadtran 1.4 model show different behaviour of UVER/SW and UVT/SW ratios too. These trends are produced by the effects of solar zenith angle, cloud properties, ozone absorption, aerosol load and Rayleigh scattering. The ozone produces an important fall in the values of this ratio with a factor of −0.3% DU−1. An increase of the precipitable water column of the atmosphere leads to a growth of the UVER/SW ratio, while its dependence on the aerosol optical thickness shows the opposite behaviour. Particularly, two cases are studied with changes on the aerosol optical thickness (AOT) and the total ozone column (TOC). Falls about 7% in the values of the UVER/SW ratio are observed under a high AOT (~0.3 at 1020nm) or under a significant decrease in the TOC (~15DU). With respect to UVT/SW ratio, measurements and simulations point out no dependence on solar zenith angle in a cloudy scenario.