What Is a Circumhorizontal Arc?

Have you ever seen this optical phenomenon? Though it looks rainbow-esque, it's not a rainbow. Nor is it a 22-degree halo or an instance of cloud iridescence, though it is occasionally confused with that phenomenon. No, it's not the tracks left by a unicorn galloping across the sky either. Rather, this beautiful phenomenon is called a circumhorizontal arc, and if you spy one, you can consider yourself blessed, as they only form in certain parts of the world.

Circumhorizontal arcs, or "fire rainbows" as they are sometimes called, are essentially ice-halos formed by the refraction of sunlight or, occasionally, moonlight, in plate-shaped ice crystals suspended in the atmosphere. They are most commonly spotted in cirrus or cirrostratus clouds, and they can be easily distinguished from 22-degree halos depending on the distance they appear below the sun or moon — twice the distance of the 22s. (As their name suggests, 22-degree halos form a circle with a radius of about 22 degrees around).

Since they require that their light source be very high in the sky — at an elevation of 58 degrees or greater — it means circumhorizontal arcs cannot form north of 55 degrees North or south of 55 degrees South. Fortunately for those who live in the continental United States, the 55th parallel rests above the border, so the phenomenon is not an uncommon sight in the summer there.

It's a different story, however, for those living in far northern latitudes, where the phenomenon is impossible. And the closer you are to the 55th parallel, the rarer these spectacles become. For instance, in London, the sun is only high enough to form a circumhorizontal arc for about 140 hours between mid-May and late July.

Of course, those living in far northern latitudes get the privilege of regularly witnessing the aurora borealis, so perhaps it's a trade-off.