CIRCUMZENITHAL ARC:When the afternoon sun is getting low in the sky, look up. You might see what Les Cowley calls "the most beautiful of all the halos," i.e., the circumzenithal arc. Charles Tilley of Statesville, North Carolina, photographed this one on June 1st: (continued below)

"With thin clouds passing overhead, I felt certain something would happen as the Sun began to set. The circumzenithal arc formed at 7:00 p.m. and lasted until around 7:25 p.m.," says Tilley.

Circumzenithal arcs are caused by sunlight glinting through flat six-sided ice crystals in high clouds--the same ice crystals that cause sundogs. So if you spot a sundog, be alert for circumzenithal arcs, too. They are at their best when the sun is at an altitude near 22º.