Impact of sky conditions on erythemal UVB exposure under tree canopies

Although international agreements to limit emissions of ozone-depleting substances will apparently prevent the huge global increases in ultraviolet-B (UVB, 280-320 nm) radiation that were feared a decade or two ago, over-exposure of people to UVB was a health hazard even before serious ozone depletion began, and UVB exposure remains a health problem today.  Seeking shade during the potentially high radiation portions of the day is one of the recommended behaviors for people to avoid excess exposure to ultraviolet radiation.  Because the very short wavelengths of solar radiation are widely scattered across the sky, UVB exposure near trees cannot be judged simply from the visible tree shade pattern.  The amount of solar UV radiation that reaches pedestrians under differing amounts of tree cover was evaluated using a three-dimensional canopy radiation transfer model.  The tree cover was represented in the modeling as a regular array of spherical tree crowns with differences in the cover established by varying the radius of the tree crowns.  UVB irradiance was then modeled across a horizontal domain at the base of the crowns under the full range of sky conditions: clear, few clouds, scattered clouds, broken clouds, and overcast.  The differences in crown radius resulted in differences in the portion of the domain in direct beam shade.  The spatial mean relative irradiance (I_r) and erythemal irradiance (I_e) on the entire domain and on the shaded regions of the domain were determined for solar zenith angles of 15^o, 30^o, 45^o, and 60^o.  The mean I_r and I_e under skies with 4 octas or less of cloud cover was not remarkably different from that under clear skies.  Broken cloud cover (5 to 7 oktas) reduced the spatial mean irradiance by approximately 20% to 30% across the 15 to 60^o solar zenith range.  In the shade, the irradiance was greater under partly cloudy than under clear skies.  Significant changes in erythemal irradiance in the shade did not occur except with cloud cover of 8 octas (overcast) with solar zenith angles less than 45^o.  The mean ultraviolet protection factor for vegetation canopies under partly cloudy skies (50% or less cloud cover) is nearly equivalent to that for clear sky days.  Regression equations were developed to estimate the areally averaged relative irradiances across the entire domain and only the shaded regions of the domain for each cloud cover fraction as functions of the solar zenith angle and canopy cover.