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To gauge the impact of clouds on erythemal (sunburn causing) UV irradiances under different surface albedo conditions, UV measurements from two Antarctic sites (McMurdo and South Pole Stations) and a midlatitude site (Lauder, New Zealand) are examined. The surface albedo at South Pole remains high throughout the year, at McMurdo it has a strong annual cycle, and at Lauder it is low throughout the year. The measurements at each site are divided into clear and cloudy subsets and are compared with modeled clear-sky irradiances to assess the attenuation of UV by clouds. A radiative transfer model is also used to interpret the observations. Results show increasing attenuation of UV with increasing cloud optical depth, but a high surface albedo can moderate this attenuation as a result of multiple scattering between the surface and cloud base. This effect is of particular importance at high latitudes where snow may be present during the summer months. There is also a tendency toward greater cloud attenuation with increasing solar zenith angle.

Recent extratropical ozone depletion and the concomitant increase in surface ultraviolet (UV) radiation may be expected to adversely influence the biosphere. Since few long-term, high quality datasets of surface UV are available for assessing these effects, there is a need to develop techniques for estimating past levels of biologically harmful UV at a particular location and thus derive long-term trends. This paper presents a semiempirical algorithm, making use of readily available meteorological variables and total column ozone, for inferring historical UV levels at a particular location, including cloud cover effects. Where input data are available for a network of locations, the technique can be used to generate geographical distributions of surface UV. Measurements made at Lauder (45.04°S, 169.68°E), from November 1993 to October 1994, were used to establish the relationship between cloud-induced reductions of erythemal UV and broadband irradiance, as a function of solar zenith angle—termed ...