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ABSTRACT:This study is an attempt to assess atmospheric modulations of the radiometric ratio of ultraviolet-B (UVB) to broadband solar radiation (G). It examines the role of the atmosphere as a filter, absorbing and scattering portions of the solar spectrum, shielding the biosphere from damaging doses of UVB. The diurnal variability of UVB/G at the Earth's surface has been evaluated by analysing 10-year hourly measurements of UVB and G on a horizontal surface at Qena, Egypt. The results show that, throughout the whole period, the values of hourly UVB/G varied from 0.03 to 0.73%, with an average value of 0.27%. This average represents only 17% of the corresponding value outside the atmosphere (1.56%). Moreover, the study quantifies the impact of ozone, water vapour, and aerosols on UVB/G under cloudless conditions. An analysis of the relationship between this ratio and the slant total ozone column (Z ) was also carried out. It showed that the ratio decreased with increasing Z . For Z = 537 DU, the average UVB/G was almost 53.47% lower than that at low Z (277 DU). The influence of precipitable water (W ) on UVB/G was determined under limited change in Z (<8.72%) and almost constant turbidity level (Ångström turbidity coefficient, β, of 0.19). The values of UVB/G increased with increasing W . For an average W = 1.13 cm, the average UVB/G was approximately 3.93% lower than at high W (2.82 cm). Moreover, the study illustrated the effect of aerosols on UVB/G when their influence on this ratio is greater than the impact of other atmospheric factors such as ozone and water vapour. It can be concluded that the ratio of the change in UVB/G to the change in β varies from 0.16 to 0.59%, with an average value of 0.33%.

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