Impact of cloud cover on erythemal UV-B exposure under vegetation canopies

The effect of cloud cover on the amount of solar UV radiation that reaches pedestrians under tree cover was evaluated using a three-dimensional canopy radiation transport model. The UVB irradiance across a horizontal domain at the base of a regular array of spherical tree crowns of varying radius was modeled under the full range of sky conditions: clear, few clouds, scattered clouds, broken clouds, and overcast. Differences in crown radius created differences in crown cover (m) with resulting differences in portions of the domain in direct beam shade. The spatial mean relative irradiance and erythemal irradiance of the domain and the spatial mean relative irradiance (I_r) and erythemal irradiance in the shaded regions of the domain were determined for solar zenith angles of 15°, 30°, 45°, and 60°. The mean I_r and erythemal UV irradiance under skies with 4 octas or less was not remarkably different from that under clear skies. Broken cloud cover reduces the spatial mean irradiance by approximately 20% to 30% across the 15° to 60° solar zenith range. In the shade, the irradiance was greater under partly cloudy than under clear skies. Partial cloud cover did not greatly influence the irradiance in the shade of the canopies. 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°. Consequently 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. These equations were then used to predict the variation in erythemal irradiance received across a region of suburban Baltimore, Maryland.