A fogbow created by the tiny droplets of water in a fog with strong sunlight behind the observer. Credit: Phil Plait

A perfect morning, wrapped up in a fogbow

Contributed by
Oct 24, 2017

I used to not be a morning person. I guess I’m still not, in that I don’t leap out of bed with a smile and a carefree laugh to face the day. It’s more of a grimacing roll out of bed and trying not to stub my toe as I stumble across the bedroom to put on warm clothes before heading outside to feed the goats.*

Yeah, even in the summer it gets chilly here at night in Colorado (1700 meters elevation + dry air = lots of radiative cooling at night), and the goats don’t care if I’m warm and toasty and awake or if I’m cold and miserable and cranky. They want their hay, so up I get to get it to them.

Sometimes, though, getting up around the same time the Sun does pays off. Like it did in late September when I got to see a rare and wonderful thing: a fogbow!

These are like rainbows, but instead of large (like 1 millimeter or so) water droplets in the air bending light back to you, fog droplets do the trick. These droplets are far smaller, 1/10th of a mm or less. In a rainbow the water droplets spread out the colors, cleanly separating them into thin bows of different sizes, but in fogbows the color bands are much wider. The colors overlap, and add together to form a wide, diffuse, white light bow.

They’re also damnably hard to catch on camera. I’ve only seen one once before, in April 2017, and the pictures I got were, well, subtle. When I posted them I had to enhance them a bit to show what I was talking about.

But this autumn’s fogbow? Yeah, this came out way better:

A fogbow created by the tiny droplets of water in a fog with strong sunlight behind the observer. Credit: Phil Plait

A fogbow created by the tiny droplets of water in a fog with strong sunlight behind the observer. Credit: Phil Plait

Whoa. I used the panorama mode of my camera to get that (the vertical lines you can see in the sky are an artifact of the camera). I was surprised it was this vivid! Fogbows are hard to photograph for a couple of reasons. First, you have to be in the right place; if the fog is too thick and you’re in the middle of it the sunlight won’t be bright enough to make one. So you need to be on the edge of the fog, and it has to be thick enough to refract (bend) enough light back to you to be visible. And you have to be on the correct edge, so the Sun is behind you as you face the fog; the droplets bend the light back toward you, so geometry is critical here.

As if that’s not hard enough, fogbows don’t last long because generally speaking if the Sun is strong enough to make a fogbow then it’s also strong enough to evaporate the fog droplets. So they might last a few minutes and that’s that.

Knowing this, I grabbed my phone from my pocket and snapped away; I knew I wouldn’t have time to run inside and get my nicer camera to take a really good shot. Still, I think it came out pretty dang OK.

As a bonus, the dewy grass also created heiligenschein: the halo around the head of my shadow (or the shadow of my head, if you prefer). This is a fun little optical trick where light coming from the Sun hits something like droplets or dust and tends to be reflected directly back toward the source. In this case, the sunlight was coming from directly behind me, hitting the dew, then reflecting right back at me, so the grass around my head looks brighter.

I love the German word for this, but if you want to sound all sciencey, it’s also called the zero phase effect or opposition surge.

If you want to sound anti-sciencey then you can claim the Moon landings were faked because it looks like the astronauts were standing in a spotlight, but in fact heiligenschein shines a light (ha ha! ha!) on this common but fallacious claim.

The fogbow base shows a hint of color. Credit: Phil Plait

The fogbow base shows a hint of color. Credit: Phil Plait

My fogbow lasted a few minutes, and as I admired it I noticed something pretty cool: The southern base of the arc was showing a little color! That’s because all the colors emerging from the fog droplets overlap, but they’re bounded on the long wavelength side by red light, and on the short side by blue. So there’s no visible color outside red to mix with it and wash it out, and you get a red tinge to the outside of the bow (and the same goes for blue on the inside edge). It’s not strong, but it’s noticeable.

As you can see from the photos, I was right at the edge of the fog, and within a few more seconds the fogbow was gone. If I had gotten up earlier the fog would’ve been too thick to see it, and a few minutes later I would’ve missed it completely.

When I’m outside I spend a lot of time just looking around, because I love all the wonderful things in nature going on around us, especially the surprising ones. And it makes me wonder: How many things do we miss simply because they’re so ephemeral?

And if you don’t look around, you’ll miss all of it. Pay attention! There’s a whole Universe doing its thing out there whether you notice it or not. You should notice.

* Look, you gotta follow me on Instagram and Twitter so there’s no excuse for not knowing about my adorable ungulate pals.