• attenuation;
  • clearness index;
  • cloudiness;
  • ultraviolet radiation;
  • UV/G ratio


Measurements of global solar radiation (G) and ultraviolet (UV) radiation on a horizontal plane at Wuhan, China during 2006–2012 were reported in this study to reveal their hourly, daily and monthly variations. The influences of clouds on UV irradiation and UV/G ratio at different solar elevations were investigated. It was discovered that hourly UV irradiation decreased with cloudiness and UV/G increased with cloud cover, especially for cloudiness greater than 4 octas, which indicated that cloud effects on UV irradiances were more evident in overcast skies. Clearness index (Kt) and UV clearness index (Kuv) was used for characterizing the cloudiness, hourly dependence of UV/G ratio on Kt and complete features of monthly average hourly UV/G were analysed for studying the relationships between G and UV irradiations under different sky conditions. It was revealed that UV/G changed from 4.3% (hourly basis) and 4.4% (daily basis) under overcast skies to 3.8% (hourly basis) and 3.9% (daily basis) in clear skies. It was also observed a largest predominance (90.8%) for the interval 2.8% ≤ UV/G ≤ 5.6% with an average value being about 4.1% during the whole study period. Specific days were selected to further study the cloudiness influence on UV/G, a general daily pattern with lower values during sunrise/sunset and higher values around noon was observed. In addition, the relationship between Kuv and Kt at different solar elevations has been evaluated in detail, Kuv was about 65.8% of Kt during the study period. Finally, using the monthly evolution of the 10-day Kt averages for the UV range, the attenuation of UV radiation in Central China caused by clouds ranged between 65 and 31% in January and July, respectively.