Iridescent cloud? Or circumhorizon arc?
Here at EarthSky, we often receive photos of rainbow-like arcs and bands in the sky. Most aren’t true rainbows, but instead are other examples of the many sorts of optical phenomena you can see in the sky. Two that are commonly confused are iridescent clouds and circumhorizon arcs. It was an alert EarthSky reader – George Preoteasa, who responded to a photo we had mistakenly identified at our website – who set us straight. We hope this post will let you learn to tell the difference between these sky phenomena, too.
Mistaking one for the other is very common! But it’s also easy to learn which is which.
How can you tell the difference between an iridescent cloud and a circumhorizon arc in the sky, or in a picture?
George Preoteasa said he used to mistake one for the other, too, and had made a study of how to tell them apart. He wrote:
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. They are caused by ice crystals in cirrus clouds, much as solar and lunar halos are. Iridescence, on the other hand, is caused by water droplets.
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.
It’s funny, but I made the same mistake. I was using the CloudSpotter app from the Cloud Appreciation Society. If you see clouds or cloud features or optical phenomena, you can take a picture and submit it for verification. I submitted the shot below as iridescence, and the moderator pointed out it’s not, but rather a fragment of a circumhorizon arc.
After that, I went to Les Cowley’s website – Atmospheric Optics – and immediately it became clear that the Cloud Appreciation Society moderator was right. So now I’m spreading the knowledge :-)
Thank you, George!
George also very kindly went into an EarthSky article about iridescent clouds and found three photos that are really circumhorizon arcs. We next sent those three photos to the world’s sky optics guru, Les Cowley of Atmospheric Optics, for confirmation. Les – who is a long-time friend of EarthSky and often helps us identify sky phenomena – confirmed that, yes, the photos below are all circumhorizon arcs. He also confirmed 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.
Below are the three photos EarthSky had misidentified:
The Cloud Appreciation Society had this to say about the likelihood of seeing a circumhorizon arc:
The rarity of the circumhorizon arc depends on where you’re based. The lower the latitude, the greater your chance of spotting a circumhorizon arc when Cirrus or Cirrostratus clouds are in the sky. Les Cowley … reports in his Atmospheric Optics site that from most locations in the U.S. they can be observed about five times a year, but from locations in northern Europe you might see them only once or twice. Likewise, they’re more commonly seen in Australia than in New Zealand. You’ll never see a circumhorizon arc, however, from latitudes above 56 degrees – in the Northern Hemisphere, that’s anywhere north of Copenhagen, Denmark – since the sun never climbs high enough in the sky.
Nor is it possible, unless you’re near the equator, to see a circumhorizon arc throughout the year. For most of us, the dependence of this vibrant optical effect on a such high sun means that its horizontal streak of pure, spectral color will only ever grace our skies during the summertime.
Bottom line: It’s easy to confuse a circumhorizon arc with an iridescent cloud, and vice versa. Here’s how to tell these two elusive, colorful, beautiful daytime sky phenomena apart. As for frequency … we see many, many more photos of circumhorizon arcs than of true iridescent clouds.