Abstract
Knowledge of ultraviolet radiation (UVA) at the Earth’s surface is of great importance in different technological and scientific applications. In this study, UVA (315–400 nm) and global solar radiation (G, 400–1100 nm) data from Riyadh (Central Saudi Arabia) for the period between 2015 and April 2020 are used to characterize the UVA at different cloudiness levels. The sky was classified into several categories according to the clearness index. For all sky conditions, the mean values of UVA, G, and the ratio UVA/G were 11.58 ± 6.1 W/m
2
, 549.35 ± 264.55 W/m
2
, and 0.021 ± 0.003, respectively. While the mean UVA and G values under totally overcast skies were 5.51 ± 2.7 W/m
2
and 267.90120.44 W/m
2
, respectively, the UVA was 21.63 ± 2.14 W/m
2
and G was 933.19 ± 39.94 under clear skies. For partly cloudy skies, the amount of UVA and G increases as the sky gets clearer. The annual mean value of UVA decreased gradually from 2016 to 2018. In comparison with the mean value of UVA in 2016, the mean values of UVA were down about 7% and 18% in 2017 and 2018, respectively. This may be due to the heavy dust storms that occurred in those two years. The monthly and hourly variations in UVA and G are investigated and discussed. The distribution of the monthly values of UVA and G exhibits high symmetry. The UVA and G are at the maximum in summer and the minimum in winter. The amount of radiation appears similar in spring and summer and in autumn and winter. This may be explained by the seasonal symmetry of the summer and winter solstices. The hourly variations in the average UVA and G attain their minimum values in the early morning (06:00 local time) and reach their maximum values at around midday. Finally, several empirical models relating ultraviolet (UV), with solar global radiation (G), and the clearness index (Kt) under all sky conditions are established. The results reveal that the proposed empirical models accurately predict hourly values.