Iridescent cloud? Or circumhorizon arc?
Here at EarthSky, we often receive photos of various kinds of rainbow-like arcs and bands in the sky. Many are not rainbows, but other sorts of optical phenomena. Two of these that are commonly confused are iridescent clouds and circumhorizon arcs (also called circumhorizontal arcs). It was George Preoteasa responding to the photo below – first published at our site in July 2017 – taken by Joan Helle-Fasolo at Ship Bottom, New Jersey. EarthSky originally misidentified it as a photo of iridescent clouds. In fact, it’s a circumhorizon arc.
Mistaking one for the other is easy, especially if all you have to go on is a photo.
How can you tell the difference in the sky, or in a picture?
George Preoteasa had an answer for us. He said he’d also mistakenly identified one sky phenomenon for the other, and so made a study of how to tell them apart. George 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.
Read more about circumhorizon arcs on Les Cowley’s website, Atmospheric Optics
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 iridescent clouds with circumhorizon arcs. Here are some tips that can help you tell these two elusive, colorful, beautiful sky phenomena apart.