Effect of clouds on the diffuse component of the solar terrestrial erythemal UV

Abstract

[Abstract]: Broadband diffuse erythemal UV (SUV) and cloud cover were measured at five minute intervals for a 12 month period at a Southern Hemisphere site and for a solar zenith angle (SZA) range of 5o to 80o. Measured data for cloud conditions of 1 to 7 okta showed that variation in diffuse SUV due to cloud compared to modelled clear sky diffuse SUV, ranged from 0.8 to 1.2 for an SZA of 10o, 0.8 to 1.54 for 40o, and 0.9 to 1.8 for 70o. An empirical non-linear expression as a function of SZA has been developed for cloudy sky conditions of 1 to 7 okta to allow the evaluation of the diffuse SUV. The percentage of occurrences of the times required for an exposure of 1 standard erythemal dose (SED) due to the diffuse SUV for all sky conditions was a maximum in the range of 10 to 20 minutes with an occurrence of 32.1%, followed by 20.0% for 20 to 30 minutes, and 19.5% for 30 to 60 minutes. For the cases when the diffuse SUV was enhanced above that of modelled clear sky conditions for cloud cover of 1 to 8 okta, the maximum incidence for an SED was in the range of 10 to 20 minutes with an occurrence of 32.4%, followed by 19.9% for 20 to 30 minutes.