UV exposure in the shade

Ultraviolet radiation (UVB) irradiance across a horizontal domain at the base of the crowns under the full range of cloud conditions from clear to overcast was modeled using a 3D-canopy-radiation-transfer model. Modeling showed decreased spatial mean erythemal irradiance at pedestrian height as tree cover increased, but the irradiance was similar for all cloud classes up to overcast. Using tree cover determined by a field survey in Baltimore, Maryland, it was estimated that for people walking in high-building-density residential landuse class, where tree cover averages 20%, UVB exposures would be 27%-29% higher than in the low- to medium-density residential land use class, where tree cover is 32%. In the open away from trees, erythemal irradiance exposure with 4 octas (SCT) or less of cloud cover is not remarkably different from that under clear skies, but exposure decreases significantly with broken cloud cover. In tree shade, the modeled irradiance under partly cloudy conditions was as great as or greater than under clear skies.