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That wasn’t a rainbow over the pope in New York City, it was a circumzenithal arc

This colorful atmospheric phenomenon appeared in the sky after Pope Francis arrived to the Lady Queen of Angels school in New York City on Friday. But it wasn’t a rainbow. (John Moore/Getty Images)
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Social media exploded with photos of what looked like a rainbow that appeared right over the pope’s location in New York City on Friday. It was a beautiful sight, but it wasn’t a rainbow. It was something much more interesting.

We know this wasn’t rainbow because a rainbow requires the sun to be directly behind the viewer’s head, and tiny water droplets must be present to reflect and refract the light. Obviously the sun wasn’t on the ground behind the heads of people looking up in New York City on Friday afternoon, and it wasn’t raining.

The beautiful display of color in the sky over the pope was actually a circumzenithal arc, which is relatively common but few people see them because they rarely look up. The arc was possibly in combination with a full halo around the sun, though it’s hard to tell with all the big city buildings in the photos.

Ice crystals high in the sky bend the light to create different kinds of halos, including the circumzenithal arc. The ice crystals make up thin, cirrus clouds like those over New York City on Sept. 25. Sometimes the clouds can be so thin you can hardly see them.

The circumzenithal arc is just one of the phenomenon that ice crystals can create. “Sun dogs—their formal name is parhelia—on either side of the sun are the most common kind of halo,” I write in the AMS Weather Book. “Across most of North America you might see at least one sun dog every few days when high, thin clouds cover at least part of the sky.

“If you see a sun dog or sun dogs, look straight up. If the cloud causing the sun dog stretches to the zenith — the sky directly above you — you have a good chance of seeing a circumzenithal arc as well.”

Many colorful atmospheric phenomena are often mistaken for rainbows, including sundogs, iridescent clouds, coronas and glories.

If you watch the sky, you have a good chance of seeing a circumzenithal arc maybe 25 times a year. If happen to see a sundog, which are the bright spots that sometimes appear on each side of the sun, you’ll also see a thin ring of light passing through the sundogs as it loops around the sun. This is a 22-degree arc. To look for such an arc hold your hand at arm’s length with your thumb covering the sun and your fingers spread out as far as possible. A 22-degree arc will touch your little finger. The colorful circumzenithal arc, if it exists, will appear on top of this ring.