Fig. 1 Photosynthetic photon flux (PPF) under variable skies in Columbus, OH. Measurements in full sun and deciduous, coniferous, and building shade are displayed hourly. Results are averaged across 12 spectral scans made biweekly from vernal equinox to autumnal equinox in 1997. Horizontal lines at 150 and 867 mol m-2 s-1 correspond with the approximate light compensation and saturation points of perennial ryegrass at 320 l l-1 COæ2 and 22°C. Notice the diminished amount of photon flux available in the afternoon in full sun illustrated by a vertical line drawn at solar noon (1330 h)
