A "smile in the sky" over Baltimore
This morning's email included a wonderful photo, and this note from Kirstie Schraffenberger:
"I was with some friends down at the Inner Harbor last Wednesday afternoon when my toddler daughter in her stroller spotted what I don't think anyone else in the city noticed since we never bother to look straight up: these back-to-back double rainbows directly overhead.
"I've never heard of this phenomenon, and it was a sunny day with no recent rain. I thought you'd like to see this, and if you have any explanation that you have the time to share, I'd love to hear it! - Sincerely, Kirstie Schraffenberger"
You're right. Too many of us rarely if ever look up. And that means we miss many striking and memorable phenomena such as this one.
There appear to be two things going on in this photo. The first arc - the one that looks like (and is often called) a smile in the sky, is what meteorologists call a "circumzenithal arc." The "circum" part means "around," and the "zenithal" part refers to the "zenith." That's the point in the sky directly overhead.
Circumzenithal arcs are caused by the refraction of sunlight by ice crystals in the sky. It's a bit like the refraction by raindrops that create rainbows. But these are caused by horizontally aligned ice crystals. They are rarely more than a quarter of a full circle, and the arc curves away from the sun. And, if it were a complete circle, the zenith would be at the center. They're not all that rare, apparently, but they are rarely noticed, for the reason you cite - few of us ever look straight up.
For circumzenithal arcs to appear, the sun needs to be lower than 32 degrees above the horizon. The afternoon sun in late December would probably be well within that range.
The other arc is probably what's called a "supralateral arc." The cause is the same - refraction and reflection of sunlight within the ice crystals. But the geometry is different. If you could extend the supralateral arc, it would form a circular halo or bow around the sun. It could also be part of what's called a 46-degree halo. Hard to tell them apart sometimes, apparently.
Supralateral arcs come into contact with circumzenithal arcs when the sun is between 27 and 32 degrees above the horizon.
That's my take on it, anyway. I'd welcome any other thoughts readers might have. Your picture is one of the best I've seen of this phenomenon. Thanks so much for sharing it. Here's more on the topic.