Impact of sky conditions on erythemal UV-B exposure under tree canopies

A model of radiation transmission through tree canopies was developed to estimate erythemal ultraviolet (UV)-B irradiance below vegetation canopies in the Baltimore, Maryland area under variable cloud conditions and tree cover. The model developed assesses the UV-B irradiance at a point below a canopy given sky conditions and a canopy consisting of a finite number of ellipsoidal crowns that act as discrete scattering volumes. The UV-B protection factors for the tree canopies in Baltimore were on average 2.0 for the entire area under tree canopies and 4.0 for shaded areas. The UV-B protection afforded by tree canopies under partly cloudy skies was found to be similar to that for clear sky days except for zenith angle greater than 45°.