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SYFY WIRE Bad Astronomy

Somewhere over the fogbow

By Phil Plait

Here’s a thing you may not know about a lot of astronomers: A lot of us are fans of the daytime sky, too*. We love interesting atmospheric phenomena like clouds, optical effects and the like.

I’m no exception, and probably on the more excitable end of that spectrum. I love just any ol’ kinds of clouds, but the rarer the better. That’s a reason I love Colorado: we get lenticular clouds, Kelvin-Helmholtz wave clouds, iridescence, mammatus fields, gigantic cumulonimbus that sometimes sport pileus caps … there’s a lot of cool stuff going on.

Having said that, there are still a ton of phenomena I want to see someday. They include a total solar eclipse (this August ought to fix that), a good aurora display, noctilucent clouds, and many more.

However, I’m pretty happy that I can now check one of these rare sights off my list: a fogbow. I finally saw one! And it caught me by surprise.

It’s hard to see in the image at the top of this article, so here is the same thing with arrows pointing it out:


I know, it doesn’t look like much. But that’s part of the fun of this; they’re not just rare, but usually subtle and difficult to spot. A fogbow is pretty much what the name says: It’s like a rainbow, but instead of raindrops in the air bending sunlight back toward you, the fog droplets do the deed.

This actually makes a big difference! Raindrops are big; millimeters in size. Fog droplets are far smaller, less than a tenth of a millimeter across. In a raindrop, the light from the Sun moves geometrically, along straight paths. But because the fog droplet is smaller, the light diffracts, or spreads out. This is what waves do when they pass a small obstacle; it’s a little bit like the patterns of waves you see when water flows past a rock.

In a raindrop, the colors of light separate to create a rainbow (each colors bends a little bit differently as they pass through the drop), and the colors are neatly laid out. But in a fogbow the colors get smeared out by diffraction and overlap, so it appears white. I was fortunate that the fog was to the west of me and the sky to the east was clear enough to let sunlight through. Even so, the fogbow was so faint I thought at first it was just a blob of denser fog reflecting the rising Sun’s light, but then I saw the base arc on the other side (to the right but out of the frame of the photos above) and suddenly realized what I was seeing. I grabbed my phone, but the fogbow’s appearance changed in mere seconds, and was hard to snap to boot. That photo is about the best I could do.

But the sky that morning wasn’t quite done with me yet …

The fog dissipated pretty quickly, and while it was still clear to the east where the Sun was rising, the sky itself to the west remained pretty gray. After a few minutes it started raining to the west, and when I happened to look up again, guess what I saw:


Oof. Yup, a double rainbow! A single rainbow happens when light reflects one time inside a raindrop, but sometimes multiple reflections can occur, creating secondary (or even higher order) bows. They’re fainter than the primary, so you’ll usually see them when the primary rainbow is really bright. This one was good enough to have a noticeable Alexander’s dark band —more light gets bent toward you from raindrops inside the primary rainbow, making it brighter there, while drops just outside the primary bend light away from you, making it look darker — and even some lovely supernumerary arcs which unfortunately aren’t easily visible in the photos I took. However, they’re very obvious in a video I took of a rainbow a while back … where I got another big surprise:

So, I have to say, for a meteorology-loving nerd astronomer, that was a pretty good day. The fogbow maybe wasn’t as dramatic and splashy as its polychromatic sibling a few minutes later, but because it was so rare and ephemeral I appreciated it all the more.

You really do need to look up every now and again, folks. There’s a whole Universe out there waiting for you to notice it.

* This is trivially true for a) solar astronomers, and 2) radio and/or space-based observational astronomers, who don’t generally care of the Sun is up or not.

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