Re: Light effects - halo round a shadow

When an airplane flies above a cloud layer, the tiny water deoplets may
cause a set of colored rings called a glory around the shadow of the plane.
Light is reflected back toward you after traveling through the water 
droplets.
This effect can also occur when your back is to the sun (or other bright
light source) and a halo appears around the shadow of your head.  
In sunlight this phenomenon is a heiligenschein or brocken bow 
named after the Brocken Mtns. in Germany where this halo is quite common.
While much more common during the day (because of the sun)
a foggy night will do just as well. 
  http://pls.atu.edu/physci/physics/people/psjr/courses/PHSC3033/AtmOp
tics/
 http://members.tripod.com/~IrishPub/storms/w
eirdwx.html#halo