EFFECTS OF CLOUDS AND HAZE ON UV-B RADIATION

An experiment was conducted over a B-month period in Research Triangle Park, North Carolina, to investigate the effects of clouds and haze o n ultraviolet (UV) radiation. Data were collected using a Yankee Envir onmental Systems UVB-1 pyranometer, an Eppley Laboratory Precision Spe ctral Pyranometer, and a SCI-TEC Brewer spectrophotometer. Hourly repo rts of total cloud cover and surface observations of air temperature, dew point temperature, barometric pressure, and visibility from the Na tional Weather Service located at the nearby Raleigh-Durham Internatio nal Airport were also used in this study. An empirical relationship ha s been formulated for UV-B attenuation as a function of total solar tr ansmissivity and cloud cover. Cumulus-type clouds were found to attenu ate up to 99% of the incoming UV-B radiation during overcast condition s. However, these same clouds were found to produce localized increase s of UV-B radiation of up to 27% over timescales less than 1 hour unde r partly cloudy skies when the direct solar beam was unobstructed. Sum mer haze was found to attenuate UV-B radiation in the range of 5% to 2 3% when compared to a clear day in the autumn. In general, total radia tion was attenuated more than UV-B radiation under cloudy conditions.