You’re walking outside, maybe just grabbed a coffee, and you glance up. There it is. A giant, colorful grin beaming down from the blue. It looks like a rainbow, sure, but it’s upside down. People call it a smile in the sky, and honestly, it feels a little magical when you spot one. But here is the thing: if you call it a rainbow around a meteorologist, they might gently correct you.
It’s actually a circumzenithal arc.
That’s a mouthful. Most of us just stick with "upside-down rainbow" because it’s easier to say while you're fumbling for your phone to take a picture. These things are surprisingly common, yet most people go their whole lives without noticing them. Why? Because we don't look up enough. We’re usually staring at our feet or our screens, missing the physics-defying light show happening right over our heads.
The Science of the Upside-Down Grin
Standard rainbows happen when sunlight hits raindrops. The light enters the drop, reflects off the back, and bends as it leaves. That’s why you always see a rainbow opposite the sun, usually when it’s raining in front of you and clear behind you. A smile in the sky plays by a completely different set of rules.
You need ice. Specifically, hexagonal plate crystals.
These crystals live in cirrus clouds, those wispy, feather-like streaks way up in the atmosphere—usually around 20,000 to 30,000 feet. At that height, it’s freezing. Even in the middle of a sweltering July afternoon in Texas, it’s a winter wonderland up there. When sunlight hits the flat top face of these tiny ice plates and exits through a side face, it refracts. It’s basically a massive aerial prism.
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Les Cowley, a world-renowned atmospheric optics expert who runs the site Atmospheric Optics, points out that the purity of the colors in a circumzenithal arc is often way better than a regular rainbow. The colors don't overlap as much. You get these incredibly vivid violets and blues on the top (the "inside" of the curve) and a sharp red on the bottom. It’s crisp. It’s clean. It’s stunning.
Why You Keep Missing It
Timing is everything. You can’t just see a smile in the sky whenever you want. The sun has to be at a very specific height—specifically, lower than 32.2 degrees above the horizon. If the sun is higher than that, the light can’t exit the ice crystals correctly, and the "smile" vanishes.
This means your best window is usually early morning or late afternoon.
Most people miss it because the arc sits very high in the sky, often directly overhead (the "zenith"). We naturally scan the horizon. We look for rainbows near the ground. But the circumzenithal arc is shy. It hides at the top of the sky, waiting for someone to actually crane their neck back.
Interestingly, these arcs are often associated with "sun dogs"—those bright spots of light you see on either side of the sun. If you see sun dogs, there is a very high chance a smile in the sky is lurking directly above you. It’s a package deal provided by the same ice crystals.
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Debunking the Myths
People get weird when they see things they don't understand in the sky. I’ve seen Reddit threads claiming these arcs are "chemical rainbows" caused by pollution or "chemtrails."
That’s just not true.
Historical records show people have been documenting the smile in the sky for centuries. It’s a natural phenomenon. It’s not a sign of the apocalypse, and it’s not caused by "heavy metals" in the air. It’s just light meeting ice. Even the famous 17th-century polymath Christiaan Huygens spent time trying to map out the mathematics of how these arcs form. If it were a modern pollution issue, Huygens wouldn't have been sketching it in the 1600s.
Another common misconception is that it’s a "Fire Rainbow." Nope. Fire rainbows (circumhorizontal arcs) sit much lower in the sky and look like long, flat ribbons of color. They are beautiful, but they aren't the "smile." The circumzenithal arc is the only one that truly looks like a grin.
How to Spot One This Week
Want to find one? It’s easier than you think if you know the "formula."
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First, look for cirrus clouds. If the sky is a deep, solid blue with no clouds, you’re out of luck. You need those thin, wispy streaks. Second, check the sun's position. If it’s high noon in the summer, forget it. Wait until about two hours before sunset.
Third, and this is the "pro tip," use your hand to block the sun. It’s hard to see faint optical effects when you’re squinting against a direct glare. Block the sun with your palm and look straight up toward the top of the sky.
You might see a faint glimmer of color. If the conditions are perfect—if those ice crystals are perfectly horizontal and flat—the colors will be so bright they look like they were painted on the sky.
The Actionable Checklist for Skywatchers
If you’re serious about catching a smile in the sky, don't just leave it to chance.
- Monitor the clouds: Use an app like Windy or just look for high-altitude "mare's tails" (cirrus clouds).
- Check the Solar Elevation: Use a simple compass or stargazing app to see if the sun is below 32 degrees.
- Look for the "Friends": If you see a halo around the sun (a 22-degree halo) or sun dogs, stop what you’re doing and look straight up. The smile is likely there.
- Polarized Sunglasses: Wear them. They can actually help the colors of atmospheric arcs pop by filtering out some of the background scattered light.
- Photographing the arc: Use a wide-angle lens. Since these arcs are often directly overhead, you need a wide field of view to capture the context of the sky, or it’ll just look like a blurry smudge of color.
The next time you’re out and the sun is hanging low, take a second. Look up. You might just find the atmosphere smiling back at you, a reminder that even the air we breathe is full of hidden, icy geometry.