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| Earth on Jul 03, 2014

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

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

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

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

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 EarthSky friends on Facebook. Thanks to all who contributed.

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

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

Julie Gurnhill caught this one on February 27, 2013.

Julie Gurnhill caught this one on February 27, 2013.

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

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

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

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

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

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

Here's that same circumzenithal arc from Andrew R. Brown again, minus the jet!

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

Dorothy caught this circumzenithal arc on January 9, 2014.

Dorothy caught this circumzenithal arc on January 9, 2014.

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. The photos here are by EarthSky friends on Facebook. Thanks to all who contributed!

Gallery: Rainbows around the world