I saw an upside-down rainbow. What is it?

Circumzenithal arcs have been described as upside-down rainbows or “a grin in the sky.” They’re wonderful! See photos here.

A rainbow-like arc in the sky, with red on the bottom of the arc, behind a tree.

David Lamberti caught this circumzenithal arc in 2019. Notice the kite in the tree! He wrote: “It was a beautiful January day in southeast Michigan. I looked up, and there it was, a beautiful circumzenithal arc. It was enormous, and the colors were very deep. It faded within 5 minutes.” Thank you, David!

People who look up might 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 is fairly low and especially if sundogs are visible. The center of the bow is 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!

Partly cloudy sky with an upside down rainbow at the top of the photo.

Circumzenithal arc captured by Tony Bowman in Surrey, England.

Circumzenithal arc like upside down rainbow high above treetops.

Patricia Chambers caught this circumzenithal arc from Potomac, Maryland, on August 5, 2016. She wrote: “It had rained not too long before I took the pic. I went out in my deck and saw an unbelievable sight – an upside-down rainbow! I was truly mesmerized … Enjoy!”

Large backlit sunflower with arc like upside down rainbow in the sky above it.

Circumzenithal arc captured September 16, 2015, in Lancashire, UK, by Amanda Cross.

Semi-cloudy sky with fainter circumzenithal arc high up.

Dan Szulewski captured a circumzenithal arc from Hermiston, Oregon, on June 22, 2014.

Parallel horsetail cirrus clouds with circumzenithal arc across them.

Dorothy caught this circumzenithal arc on January 9, 2014.

Faint upside down arc in thin clouds.

Julie Gurnhill caught this one on February 27, 2013.

Short arc in partly cloudy sky with tree on left.

John Gravell captured this circumzenithal arc from Boston on October 17, 2012.

Ripped looking cirrus clouds with short arc.

Duke Marsh caught this circumzenithal arc on October 3, 2012, from New Albany, Indiana. Thanks, Duke.

Very faint arc in thinly clouded sky.

Rene Pennings captured this circumzenithal arc on May 21, 2012.

Long parallel streamer-like clouds in a blue sky with very bright arc across them.

A lovely circumzenithal arc amidst high clouds by Dudley Williams on December 18, 2011.

Wide, faint circumzenithal arc against a grayish sky.

Andrew R. Brown caught a circumzenithal arc in Ashford Kent in the UK, on November 19, 2010.

Bottom line: When you see what looks like an upside-down rainbow, you’re likely seeing a circumzenithal arc. It’s related to the halos often seen around the sun or moon, caused by ice crystals in the upper atmosphere.

Deborah Byrd