As I was about to get into the car on Saturday afternoon, I noticed a funny rainbow-like smear on the windshield. At first I thought it was something greasy on the windshield, but I looked up in case it was reflecting something in the sky. It was.
Above me, in the weirdest location and orientation, was a rainbow. Weird because there was no rain — it was a clear afternoon — and because it was upside down. Its feet were in the air! I’d never seen anything like it.
This is what I saw.
My first (incorrect) notion was that it was a glory. But you see glories when you look away from the sun. Furthermore, this was a whole constellation of delicate curves and colored arcs. I couldn’t stop looking at it.
I tweeted that anyone in Boston should go outside and look up. My friend MechanicalTim, an erstwhile physicist, sent a quick note back. It turns out he’s spent a fair amount of time thinking about the physics of rainbows, and even he was stumped. But he was able to track down a name for the phenomenon. That upside-down rainbow is a rare ice-crystal halo called a circumzenithal arc. You can tell it’s rare by its unlovely name. The high-altitude hexagonal ice crystals that put on the show were driven there ahead of Hurricane Sandy’s approaching bulk.
So Tim identified the circumzenithal arc, but what about the rest of it? In particular, what about the graceful gull-wing curve below the arc? A little more research put me onto the Ice Halo page on the Atmospheric Optics website. What a treasure of strange solar fauna! From my pictures, I worked out that I had seen the aforementioned circumzenithal arc, the supralateral arc, the upper suncave Parry arc, and the upper tangent arc. Not to mention the more common 22 degree halo and its hovering parhelic hounds, the stately Sun Dogs. In one fortunate sky, I’d bagged the whole menagerie. A veritable crepuscular jackpot! Rare and beautiful things with fancy names! It all made me very happy.
The so-called Boston ice crystal halo event was noticed by plenty of people with better cameras than me. In fact, the circumzenithal arc Wikipedia page now features a picture taken from Salem, Massachusetts on the same day.
And by the way, if you want to do ice halos the hard way, Tim recommends Greenler’s Rainbows, Halos and Glories.