"This was just amazing and beyond exciting," says photographer Brenda Pun. "I grabbed my camera (a Nikon D40x) and quickly took some shots before it could disappear."
Despite its rainbow-like appearance, this is not a rainbow. It's a circumzenithal arc. Atmospheric optics expert Les Cowley calls it "the most beautiful of all ice haloes." The circumzenithal arc, or "CZA" for short, is formed by sunlight shining through plate-shaped ice crystals in high clouds. "The CZA is often described as an 'upside down rainbow' by first timers. Someone also charmingly likened it to 'a grin in the sky.'"
The same ice crystals that make circumzenithal arcs also make sundogs. Indeed, Brenda Pun saw a pair of bright sundogs flanking the sun while she was photographing her CZA. 'Dogs and grins naturally go together.
Circumzenithal arcs typically appear in late autumn and early winter when the air is icy and the sun is low. "The CZA forms only when sun is less than 32.3° high," notes Cowley, "and it is at its best when the sun is about 22° high." As winter solstice approaches, "upside down rainbows" will become increasingly common. Look for them!
more images: from Paul Tahan of Wayne, New Jersey; from Peg Zenko of Green Bay, Wisconsin; from the frontyard of Glenn Machesney;
