Clouds are affected by aspects of climate change and are a major influencing factor on the pre-vitamin D3 effective solar UV (UVD3) exposures that humans receive. The aim of this research was to investigate the influence of clouds on the short wavelength cut-off of the spectral UVD3 irradiances, on the maximum of the UVD3 spectral irradiances and on the magnitude of the UVD3 irradiances.
The spectral pre-vitamin D3 effective solar UV irradiances on a horizontal plane were calculated by weighting the spectral UV measured every five minutes by a spectroradiometer with the action spectrum for the synthesis of pre-vitamin D3. The short wavelength cut-off was defined as the wavelength at which the UVD3 irradiances were 0.1% of the maximum biologically damaging irradiance for that scan.
Due to the higher relative effectiveness of the action spectrum at the shorter UV wavelengths, any variations will influence the UVD3 exposures and consequently has public health implications. The short wavelength cut-off, the magnitude of the irradiances and the maximum of the UVD3 spectral irradiances were investigated for a range of solar zenith angles for cloud free cases and cloudy cases. The results show that clouds do not influence the short wavelength cut-off or the wavelength at which the maximum spectral UVD3 irradiance occurs.
However, cloud cover higher than 5.5 octa reduces the amount of biologically effective UV for pre-vitamin D3 production. Any changes in cloud cover will not significantly influence the short wavelength cut-off or the wavelength at which the maximum spectral UVD3 irradiance occurs. However, any changes that cause the amount of cloud cover to vary between 5.5 and 8 octa or cause the cloud cover to increase above or decrease below 5.5 octa will have an effect on the UV for pre-vitamin D3 production.
Parisi, A.V., Turnbull, D.J. and Turner, J., 2007. Influence of clouds on pre-vitamin D3 effective solar UV exposures. Environmental Health, 7(3), p.75.