Abstract
•The sensitivity of the UVB/G ratio to SZA for each cloud category was investigated.•For CA⩽2octa, the average value of the UVB/G ratio was equal to ≌0.25%.•The average UVB/G under overcast sky conditions was estimated as (UVB/G)c=0.31%.•On average, (UVB/G)c represents 1.08 times the UVB/G under cloudless conditions.
Earlier studies have demonstrated that clouds modulate the intensity of solar spectrum bands reaching the Earth’s surface. However, their effect may appear as wavelength dependence. In addition, cloud effects on solar radiation variability dominate any other variable, and these effects usually mask the influences of any other atmospheric variables. The variability of the ratio of ultraviolet-B, UVB, to broadband solar radiation, G, (UVB/G) under all cloud conditions at Qena, Egypt was discussed for the period 2000–2009. The dependence of this ratio on cloud amount (CA) for different solar zenith angle (SZA) classes has also been analyzed. Four classes were considered (SZA⩽30°, 30°<SZA⩽45°, 45°<SZA⩽60°, and SZA>60°). The results obtained reveal that this ratio increased with increasing CA for all SZA classes except for SZA⩽30°. However, for the SZA class⩽30°, the UVB/G ratio decreased with increasing CA. In addition, the effects of cloud type on the UVB/G ratio were investigated. Under cloudy conditions (7–8octas), on average, both low-level and mid-level cloud increased the UVB/G ratio more than for the other cloud classes (1–2, 3–4, and 5–6octas). In contrast, under high-level cloud, the average UVB/G ratio of the 1–2octas cloud class was greater (7.56%) than for the 7–8octas cloud class. The average UVB/G ratio under low-level cloud was 1.1 and 1.2 times that under high- and mid-level cloud respectively. Finally, an assessment of the UVB/G ratio under all cloud conditions was performed. This ratio was averaged according to a narrow range of sky clearness, ε (the range is equal to 0.1), under all cloud conditions. The results showed that the average UVB/G ratio reached a maximum under cloudy sky conditions (0.31±0.02%), which represented 1.08 times the average UVB/G ratio under cloudless sky conditions.