Peak UV: Spectral contributions from cloud enhancements

We use multi-year datasets of UV spectral irradiances, measured to the exacting standards required by the Network for the Detection of Atmospheric Composition Change (NDACC), to investigate the enhancement effects of clouds and their wavelength dependence at sites that span a wide range of altitudes. These enhancements are derived by comparing weighted UV irradiance measurements with corresponding model calculations at each site for clear skies. We find that the frequency, magnitude, and wavelength-dependence of cloud enhancements are insufficient to explain the repeatedly high values of UVB and UVI observed by Cabrol et al., 2014, and that ozone amounts lower than have ever been seen there would be required.