Effects of clouds and haze on UV‐B radiation

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Abstract
An experiment was conducted over a 6‐month period in Research Triangle Park, North Carolina, to investigate the effects of clouds and haze on ultraviolet (UV) radiation. Data were collected using a Yankee Environmental Systems UVB‐1 pyranometer, an Eppley Laboratory Precision Spectral Pyranometer, and a SCI‐TEC Brewer spectrophotometer. Hourly reports of total cloud cover and surface observations of air temperature, dew point temperature, barometric pressure, and visibility from the National Weather Service located at the nearby Raleigh‐Durham International Airport were also used in this study. An empirical relationship has been formulated for UV‐B attenuation as a function of total solar transmissivity and cloud cover. Cumulus‐type clouds were found to attenuate up to 99% of the incoming UV‐B radiation during overcast conditions. However, these same clouds were found to produce localized increases of UV‐B radiation of up to 27% over timescales less than 1 hour under partly cloudy skies when the direct solar beam was unobstructed. Summer haze was found to attenuate UV‐B radiation in the range of 5% to 23% when compared to a clear day in the autumn. In general, total radiation was attenuated more than UV‐B radiation under cloudy conditions.