Opposition Glow by Russell Sampson shortly before landing at Whitehorse, Yukon in June '08. Image ©Russell Sampson, shown with permission.


"It was about 30 minutes before sunset and there was a great opposition effect in the distant pine trees.  It was a rather extreme contrast situation with low light levels.   Nonetheless I was impressed with the brightness of the effect."

The opposition effect is a bright patch at the antisolar point (ASP) directly opposite the sun. In this case part of the effect is caused by shadow hiding. The shadows of the trees directly opposite the sun are hidden by the trees themselves. Away from the ASP the shadows become increasingly visible (unhidden) and, counterintuitively, they appear closer to the ASP than the shadow casters. The lack of shadows at the ASP makes it appear brighter than the surrounding area. It also appears more yellow because it lacks bluish sky lit shadows.

That's not the whole story, retro-reflection from minerals sometimes brightens the ASP and there is also an interference effect - coherent backscattering.