Ice Crystal Rainbow Graces Boulder Sky

Bad Astronomy
The entire universe in blog form
June 28 2014 7:30 AM

Sterling Arc-er

Regular readers know that one thing I do whenever I step outside is look up. Always. It’s a habit, and a good one: I see a lot of cool stuff going on in the sky I’d otherwise miss.

I recently ran an errand in town, and when I stepped out of the store, I did that very thing. And when I did look up, I was rewarded with a glimpse of a beautiful circumhorizonal arc.

circumhorizonal arc
Polychromatic refraction via hexagonally crystallized dihydrogen monoxide.

Photo by Phil Plait, used by permission


It’s so pretty! They’re caused by ice crystals in cirrus clouds bending sunlight toward you, and can only occur when the Sun is higher than 58° above the horizon. I was out right at noon, so the Sun was about 70° up—the Sun only gets that high at my latitude (almost exactly 40° north) in the summer around noon.

This one was patchy due to the bigger clouds blocking the cirrus, but I could see the arc extending well off to the left and right. It looked pretty much like you see it here; I enhanced the contrast a bit in the picture to bring out the colors, but it was nice and vivid by eye (which is why they’re sometimes called fire clouds).

A little bit closer in shows the colors more clearly.

Photo by Phil Plait

The Atmospheric Optics site says circumhorizonal arcs aren’t very rare, but I’ve only seen them a few times.* Maybe where you live they’re more common.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

But you won’t know unless you look up! I’m not saying don’t watch where you’re stepping (everyone I know like me who stargazes has a story about stumbling over something because they were looking up, not down). But once you have your footing, it pays to check out the sky. I’ve seen meteors, satellites, weird clouds, rockets re-entering (seriously), and just about every kind of bizarre rainbow-colored optical effect there is. And all it takes to broaden your horizon is to glance skyward. I recommend it.

*The last time I saw one I was in Gunnison, Colorado, doing a site visit for my company Science Getaways. What’s funny is that we’re back to Gunnison this week for that very vacation trip! Cool coincidence.




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