Spectral shade ratios on horizontal and sun normal surfaces for single trees and relatively cloud free sky

Abstract

The spectral shade ratios defined as the ratios of the spectral irradiances on horizontal and sun normal planes in the tree shade to those on a horizontal plane in sunlight were calculated. These planes were in the shade of an isolated medium canopy density tree and a sparse canopy density tree at the tree shade sites of the centre, edge and trunk. The sun normal plane was employed as there are some activities that have exposures to parts of the body that are orientated in a sun normal plane. The horizontal plane shade ratios for the medium density canopy dropped by 47% - 56% from the ratios in the range 301 to 310 nm to the ratios in the range 391 to 400 nm. In absolute terms, the largest change in the shade ratio of 0.28 was for the centre and edge sites compared to 0.07 for the trunk. Similarly, for the sun normal plane, the ratio dropped by 40% - 49% with an absolute reduction of 0.19 for the edge and 0.04 for the trunk. For the sparse density canopy, the decrease in the shade ratios over the same wavelength range were a drop by 37% - 42% on a horizontal plane, or in absolute terms, a reduction by 0.22 for the edge and 0.13 for the trunk. Similarly, the decrease was 34 - 39% on the sun normal plane, or in absolute terms, a reduction by 0.19 for the edge and 0.12 for the trunk.