Measurements of Solar Radiation on Vertical Surfaces in the Shade of Individual Trees

Abstract

Shortwave radiation, measured by pyranometers at six sample points on an imaginary one-story south-facing wall tangent to the north edge of deciduous tree crowns, was compared to radiation on a parallel vertical surface in the open. Sampling periods were up to 8 h long on 22 d, some of which were partly cloudy. In March and April, with no snow on the ground, reductions of estimated daily insolation on the entire south-facing wall by leafless trees (Acer platanoides, A. saccharum, and Platanus acerifolia) were 2.0 to 3.2 MJ m^-2 or 13 to 18 percent of insolation without trees. In March, with snow, the largest of the three, P. acerifolia, reduced daily insolation by 4.5 MJ m^-2 or 19 percent. In June and July, the trees in leaf reduced daily insolation on the wall by 2.7 to 3.6 MJ m^-2 or 29 to 38 percent. On clear days, irradiance reductions on the south-facing wall within the shadow of the trees were 30 to 34 percent when trees were leafless and 65 to 75 percent when trees were in full leaf in midsummer.