Abstract
Measurements obtained from several Robertson-Berger (RB) meters over the course of one year define the role of cloud cover in moderating biologically effective ultraviolet radiation at the Earth's surface. In an annual mean sense, clouds reduce the erythemal irradiance to levels from 62% to 78% of the values that would exist if skies over the measurement sites remained clear and free of pollutants. The RB meter results combined with a simple model of radiative transfer allow one to estimate the response of erythemal irradiance to variations in fractional cloud cover and cloud optical thickness. If local fractional cloud cover during June and July varied by ± 10% of its monthly mean value, erythemal irradiance at the different sites would undergo charm ranging from 1.2% to 6.4% with the opposite sign. Changes in cloud optical thickness of ±10% generally have a smaller impact on surface irradiance than do changes in fractional cloud cover. Variations in erythemal irradiance predicted in these scenarios are, in some cases, similar to those associated with derived trends in total column ozone at middle latitudes between 1969 and 1986.
