Observed influence of clouds on ultraviolet-B radiation



A Brewer Spectrophotometer was utilized to make ground-based measurements of solar ultraviolet (UVB) radiation received at Black Mountain, NC (35.66°N) in order to quantify the reduction of UV-B due to cloud cover. Observations were made over the interval February–July 1995 with continuous coverage except for a three-week period in May. A wide-angle lens video camera at the site was used for the assessment of cloud conditions at the time of each scan. Integrated UV-B (290–320 nm) values were evaluated and a regression through the values for clear conditions was produced for the range of zenith angles from 25°–70°. This regression effectively represents an average effect of the ambient aerosols and attenuating gases and was used to normalize the UV-B values measured under different cloud cover amounts, providing an estimate of the reduction in irradiance due to a given cloud amount as a function of solar zenith angle. For a solar zenith angle of 50°, average UV-B transmission was observed to be 30% for overcast skies, 61% for 8–9 tenths cloud cover, 74% for 6–7 tenths cover, and 79% for the scans during 4-5 tenths cover. A number of fractional cloud cover cases were observed where cloud reflections resulted in measured surface irradiances exceeding the expected clear sky values by as much as 11 percent.