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Observed influence of clouds on ultraviolet‐B radiation
Author(s) -
Schafer J. S.,
Saxena V. K.,
Wenny B. N.,
Barnard W.,
De Luisi J. J.
Publication year - 1996
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/96gl01984
Subject(s) - overcast , zenith , solar zenith angle , cloud cover , irradiance , environmental science , cloud albedo , ultraviolet , atmospheric sciences , sky , remote sensing , radiation , meteorology , optics , physics , cloud computing , geography , computer science , operating system
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.