Circumhorizon arc or iridescent cloud?

Circumhorizon arc is a rainbow stripe in the clouds.
View at EarthSky Community Photos. | Denice Hazlett took this image on August 5, 2022. Denice wrote: “Captured this image in Fredericksburg, Ohio.” Thank you, Denice! Is this a circumhorizon arc or iridescent cloud? Read on to find out.

Rainbows are big and curved and opposite a low sun in the sky. Nearly everyone can identify a rainbow. But two rainbow-like sky phenomena can be hard to tell apart. One is called a circumhorizon arc. The other is an iridescent cloud. And many people mistake one for the other, in photos submitted to EarthSky. Once you learn a couple of key features, you’ll be able to spot the difference.

In the past week, readers have submitted three images of circumhorizon arcs to our Community Photos. That’s one key right there. You can’t see circumhorizon arcs all year or from all locations. But, for us in the United States, summer is a great time to look for circumhorizon arcs.

In fact, the first three images on this page are the ones we received this past week, and all are examples of circumhorizon arcs. Look carefully at the pictures and notice the order of the colors, from top to bottom. Like a rainbow, circumhorizon arcs are red at the top and indigo or violet at the bottom. This is the key feature of a circumhorizon arc.

In contrast, iridescent clouds have more randomly distributed colors.

Wispy clouds with a rainbow streak parallel to the horizon.
View at EarthSky Community Photos. | Ardy Lyons in Vienna, Virginia, took this image on July 30, 2022. Ardy wrote: “While kite flying (see string) at a local elementary school, I looked up and saw a rainbow in the high clouds. I thought it was unusual, so I snapped a couple of photos. Turns out, it is a circumhorizon arc.” Thank you, Ardy!

Circumhorizon arc: When and where?

The lower your latitude, the greater your chances of spotting a circumhorizon arc. The world’s sky optics guru is Les Cowley of Atmospheric Optics. And Les has said that most locations in the U.S. can see a circumhorizon arc about five times a year. But, from northern Europe, you might see them only once or twice a year. Likewise, they’re more commonly seen in Australia than in New Zealand.

And, according to Les, you’ll never see a circumhorizon arc from latitudes above 56 degrees. That’s about the latitude of Copenhagen, Denmark, or the Alaska peninsula. Why not? The reason is that – to see a circumhorizon arc – the sun has to be high above the arc … or high in your sky. From high latitudes, the sun can’t get high enough.

And, unless you’re near Earth’s equator, you won’t see a circumhorizon arc throughout the year. Instead, these arcs are most commonly seen in summer, when, again, the sun is high.

Rainbow colors on the edge of a cloud seen through a car window.
View at EarthSky Community Photos. | Conrado Jocson took this image near Lawrenceburg, Tennessee, on August 7, 2022. Conrado wrote: “Caught this awesome cloud-based rainbow on our way home from a canoeing outing. Some pop-up storms were forming and fading quickly to the south and east.” Thank you, Conrado! This is an example of a circumhorizon arc.

Circumhorizon arc versus iridescent cloud

How can you tell the difference between an iridescent cloud and a circumhorizon arc in the sky, or in a picture?

George Preoteasa, a frequent contributor to our Community Photos, used to mistake one for the other until he made a study of how to tell them apart. He’s also helped identify the difference between the phenomena in EarthSky’s photos. George explained:

The circumhorizon arc is a band parallel to the horizon. So, to the extent that the horizon is an arc, this is one, too. The colors in a circumhorizon arc are well organized, red at the top, indigo at the bottom. With cloud iridescence, the colors are more randomly distributed.

Circumhorizon arcs have a certain fuzziness. Ice crystals in cirrus clouds create them, similar to solar and lunar halos. On the other hand, water droplets create iridescence.

For a circumhorizon arc to occur, the sun must be high up, over 58 degrees above the horizon. Iridescence usually occurs close to the sun, which makes it difficult to photograph. You need to hide the sun so that sunlight does not overwhelm the colors in the cloud.

Thank you, George!

Treetops below blue sky with wispy clouds, rainbow stripe across clouds.
This is the image that set George Preoteasa on a quest to distinguish iridescent clouds from circumhorizon arcs. He said: “Imagine a horizontal band at the level where you see the colors. If you had cirrus clouds with the same properties as the one with the colors, you would get a nice colored arc parallel to the horizon. For a circumhorizon arc to occur, the sun must be high up in the sky, above 58 degrees. The fact that the sun does not appear in this picture is another clue it’s not iridescence.” Thank you, George!

Another tip from Les

Les Cowley of Atmospheric Optics also said that:

… one key difference between a circumhorizon arc and iridescence is color structure. A circumhorizon arc has a spectral sequence of color with red at top and blue/violet lowest.

A circumhorizon arc is always about two outstretched hand-widths below the sun. Iridescent clouds are usually rather closer.

Thank you, Les.

Read more about circumhorizon arcs on Les Cowley’s website, Atmospheric Optics

Huge clump of randomly rainbow-colored clouds, with a big bird flying in front of them.
View at EarthSky Community Photos. | Karl Diefenderfer caught this image of a bird soaring in front of iridescent clouds on June 14, 2019, from Blue Bell, Pennsylvania. Notice that the colors in the cloud are randomly distributed. Thank you, Karl!
Beach with umbrellas. Above, cloudy sky with rainbow stripe across the clouds.
Here’s Joan Helle-Fasolo’s July 4, 2017, image of a circumhorizon arc.

Circumhorizon arcs versus iridescent clouds

Now it’s your turn. Which images below are iridescent clouds and which are circumhorizon arcs? The answers are in the captions.

A tree in the foreground, with the sun behind it, and, to its side, in the sky, a randomly colored cloud.
View at EarthSky Community Photos. | Henry Malinda took this photo in Spring Mountains National Recreation Area in Nevada on June 12, 2020. Notice how Henry placed the nearby sun behind a tree? That’s one clue that he saw a true iridescent cloud. Thank you, Henry!
Wispy clouds over the sea. A line of rainbow colors in them, with blue on the bottom and red at top.
View at EarthSky Community Photos. | Eric Broneer in Marseille, France, caught this circumhorizon arc on June 3, 2019. He wrote, “Beautiful weather, very dry, sun behind me.” Notice how organized the colors are red at the top, indigo at the bottom.
Sky above conifer trees, wide rainbow stripe on clouds.
Hints: the band is parallel to the horizon, red at the top, indigo at the bottom, with the sun out of the picture. For circumhorizon arcs such as this, the sun is always at least twice the span from thumb to little finger of your outstretched hand, held at arm’s length. Photo taken May 31, 2016, by Laura Berry.
Sun peeking out from behind structure, high clouds with patches of color.
This is an iridescent cloud: colors less organized and near the sun. The best way to see one is to place the sun behind a foreground object, such as a building or mountain. Duke Marsh captured this image in 2012 in New Albany, Indiana.
Sky with puffy clouds, short rainbow stripe about halfway up.
A circumhorizon arc can look slightly curved in photographs, but the curvature isn’t real; it’s due to the distortion that camera lenses can make. In the sky, circumhorizon arcs are completely straight. Photo taken May 27, 2013, by Mike O’Neal.
Streaks of wispy clouds, with short rainbow patch on one part.
If there were more cloud here, you could see more of this circumhorizon arc, which is parallel to the horizon with red at the top, indigo at the bottom. Photo taken May 24, 2017, by Zaneta Kosiba Vargas in Santa Barbara, California.

Bottom line: It can be easy to confuse circumhorizon arcs and iridescent clouds. Circumhorizon arcs follow the order of a rainbow, form a straight light above the horizon, are far from the sun, and only appear at certain times of year, such as in the summer.

August 14, 2022

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