It is certainly a beautiful image, but I do not believe it is a circumhorizontal arc. At least it does not look like any such arc I have seen. (They are actually more common in mid-latitudes than most realize.) I am no photographer, so I rarely try to capture pictures of atmospheric optical phenomena. Other people have, though. (In fact there is a sub-culture of photographers who practically make it a competition.)
A Google image search of "circumhorizontal arcs" reveals lots of great shots of circumhorizontal arcs.
This link also has some nice pictures: https://www.atoptics.co.uk/halo/chaim0.htm
(BTW The site connected with this link has a wonderful collection of images, descriptions, and pictures of many, many atmospheric optical phenomena. It might be of use for an optics class, particularly in a "remote teaching" environment.)
The common thread among all of the images and my experience with the circumhorizontal arcs is that the arcs are embedded in cirrus clouds. I find it hard to imagine that ice crystals were present at such a low altitude in Washington.
In the absence of much context, I would suggest this actually is a rainbow. It is simply just very low to the horizon. If I squint enough, I can pretend to see a tiny bit of curvature in the bow.
Just some observations. I would love to hear what others think.
On personal note, I would like to use this opportunity to thank some former professors. About a billion years ago I was an undergraduate student at RPI, and I had the great fortune of taking a course under Drs. Charles Bean and Ivar Giaever. The one-credit course was titled, "Light and Color in the Open Air" and followed the classic text, "The Nature of Light and Colour in the Open Air" by Minnaert. (I highly recommend this book.) I have never looked at a cloud, blue sky, or shadow the same since that course. And I still recall one of the final exam questions: "Describe six inaccuracies on the cover of the book."
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I'll be "teaching" optics soon. Anyone seen this before?
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