Shadows.

We investigate the brightness distribution in and around outdoor shadows (for a variety of sky conditions) using modeling and field measurements. The dominant factor influencing the brightness of a shadow is the solid angle subtended by the object blocking the Sun. Occulters at the zenith that subtend a small solid angle cast shadows that are bright and possess a nearly uniform brightness across their extent. Shadows from large occulters are much darker and their brightness varies considerably, being darkest at their centers. For nonzenith occulters, the proximal (nearest the Sun) side of the shadow is darker than the distal side and the shadow will be darkest beneath the center of the occulter. Occulters (e.g., tree or cloud) influence the brightness of sunlit portions near the shadow because they block part of the sky and reflect light into the shadow. The aureole has a significant influence on the brightness of shadow edges. Semi-analytic formulations for the brightness in shadows are presented, and analytic expressions in wells and tunnels are derived.