Spectral dependency of cloud enhanced UV irradiance

This paper addresses two questions of primary importance to the solar UV community: 1) “Are cloud induced UV enhancements always wavelength dependent?” and 2) “Are the enhancements greatest in the UVA or UVB wavebands?” The answer to the first question is a definite no, with the conclusion to the second question that most of the enhancements found at this southern hemisphere measurement site are in the UVB waveband. This research is based on the results from a scanning UV spectroradiometer and a colour, all-sky camera over a 19-month period. In both the UVB and UVA wavebands there were cases that showed increasing, decreasing and no spectral dependence towards the shorter and longer wavelengths respectively. This research has found that cases of spectral dependence that decreased with wavelength tended to correspond to cloud fraction distributed in the outer field of view of the sky camera images for relatively low solar zenith angles. It is speculated that this is most likely due to an increase of scattered UV, compared to cases of increasing trends with wavelength, which would be accounted for by an increase in reflected UV from cloud surfaces in closer proximity to the sun. It also appears that wavelength dependency trends are related to the overall cloud fraction.