People who look up a lot may occasionally see the rainbow-like arcs depicted in the photos on this page. They’re called circumzenithal arcs, and they’re not really rainbows. Instead, they’re caused by ice crystals in the upper atmosphere. These arcs are related to the frequently seen halos around the sun or moon. Les Cowley of the great website Atmospheric Optics says of these graceful and colorful arcs:
The circumzenithal arc, CZA, is the most beautiful of all the halos. The first sighting is always a surprise, an ethereal rainbow fled from its watery origins and wrapped improbably about the zenith. It is often described as an “upside down rainbow” by first timers. Someone also charmingly likened it to “a grin in the sky”.
Look straight up near to the zenith when the sun if fairly low and especially if sundogs are visible. The centre of the bow always sunwards and red is on the outside.
Les says that the most ideal time to see a circumzenithal arc is when the sun is at a height of 22 degrees in the sky. Look here to see Les Cowley’s illustration of the various kinds of halo phenomena, related to circumzenithal arcs. And enjoy the photos below, contributed by members of the EarthSky community. Thanks to all who contributed!
Bottom line: When you see an upside-down rainbow in the sky, you are likely seeing a circumzenithal arc. It’s related the halos often seen around the sun or moon, caused by ice crystal in the upper atmosphere.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.