Once or twice a year, after I’ve seen a CZA, I stop strangers on the street and point it out. It blows everyone’s mind, including mine, even after all these years. The colors are more vivid than a rainbow’s. And it’s upside-down – meaning, the arc is oriented like a smile. As we all know, rainbows are shaped like frowns, with the ends aiming downward.
But there’s more: The Circumzenithal Arc (you get used to saying it with a little practice) only materializes high overhead. Straight up. Actual rainbows, by contrast, are never overhead, ever. The very highest the tippy-top of a rainbow’s midpoint can ever reach is halfway up the sky, and all the rest is lower.
But the CZA, as the name reveals, is an arc that surrounds the zenith, the place directly straight up. And there are still more differences: A rainbow requires a sunshower. It must be raining. Thus, it’s usually seen against dark clouds. But the CZA can appear against a blue sky.
Okay, you’re hooked; you want to see one. Pull up a chair; I’ll tell you how.
You need a day when high thin cirrus clouds are in the sky. Look at the clouds in the photo on this page. That’s typical. But CZAs also form within a barely-there thin layer of ice haze, so that the blue sky is only slightly lightened, or, also, when there’s a thicker ice-cloud layer, called cirrostratus. In all cases the clouds are very high and nearly transparent. They don’t fully block out sunlight.
On such days, whenever the Sun is lowish – between 15 and 25 degrees high – look straight up. During the next few weeks, the correct Sun height will happen each day between 8 and 9 a.m. and again from 5 to 6 p.m. And there overhead, surrounding the zenith, is the brilliant spectral arc. It’s never a full circle; it’s one-third of a circle. The arc you see is always directly far above that lowish Sun.
Caused by refraction induced by hexagonal ice crystals, the length of the CZA as well as the arc’s thickness vary with the Sun’s elevation. When you show it to people, most will exclaim, “It’s a rainbow!” At which point, and at risk of making them feel deservedly ignorant, you might say, “Yes, this does display all of the Sun’s spectral colors, just as a rainbow does. But this is the circumzenithal arc. The arc is inverted, compared to that of a rainbow. It’s not caused by a rain shower. It’s high overhead, where a rainbow can never appear. And unlike a rainbow, it’s not opposite the Sun, but directly above the Sun. So in many ways, it’s a rainbow’s exact opposite!”
Then, according to the tradition of my grandparents, make them repeat its name three times and give you five bucks. Okay, I made that up.
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