Spatial variability in UV radiation during the growing season across the continental USA

¶A major limitation in predicting the ultraviolet-B irradiance on humans, plant leaves and flowers and aquatic organisms is the difficulty in estimating exposure. This study analyzes the spatial variability in the daily exposure of narrow band 300 nm and 368 nm and broadband 290–315 nm (UVB) solar radiation between twelve paired locations in the United States Department of Agriculture (USDA) UVB Climate Network over two summer growing seasons (May through August of 2000 and 2001). The spatial correlation of the UVB, 300 nm and 368 nm daily exposures between locations was approximately 0.7 to 0.8 for spacing distances of 100 km. The 300 nm daily exposure was typically more highly correlated between locations than the 368 nm daily exposure. Both the diffuse and direct beam components to the 300 nm daily exposure were similarly correlated with distance between locations. The 368 nm diffuse component of the daily exposures was less correlated with distance than the direct beam component, limiting the ability to interpolate daily exposures from measurement locations. In general the variability in daily exposures of UVB in the USDA UVB Climate Network is too large to interpolate daily exposures of solar radiation, with estimated 300 nm, 368 nm and broadband UVB errors at one-half the mean station spacing of the USDA Network of 22%, 21% and 16% respectively. More accurate interpolations of UVB exposure from this network will require either the incorporation of cloud cover variability from satellite imagery for daily exposure or the use of longer periods of accumulated exposure.