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SYFY WIRE Bad Astronomy

Iridescent Sunset Mammatus

By Phil Plait

If ever there is a Colorado summer sunset that doesn’t leave me standing slack-jawed and awestruck, just turn me into Soylent Green. My wonder and joy will be dead.

This was a sunset in late July 2016. Lower-altitude cumulus clouds are silhouetted, but the trailing edge of a large anvil-headed cumulonimbus was lit at low angle by the Sun (which had already set behind the Rocky Mountains from my location). Mammatus clouds were forming; those are weird bulb-shaped clouds that are common in patches, but also sometimes (and more rarely) seen in huge fields across the sky. The low Sun lit only the bottom edges of the mammatus, giving them an eerie glow.

But there’s more! To the right, the edges of the mammatus clouds have a hint of color, pinks and greens. See it? That’s caused by iridescence, a complicated optical phenomenon that can create vivid and bizarre colors on the edges of clouds. As I've written about them before:

Iridescence is a weird phenomenon. You need lots of tiny raindrops ( or ice crystals) all the same size over a large portion of the cloud. In a rainbow, the lights goes into the droplets and gets bent (twice) to create colors. In iridescence, though, the light actually bends (diffracts) around the droplets. Different colors bend by different amounts, splitting the colors apart. The size of the raindrop needs to be roughly the same size as the wavelength of light, so when I say “tiny” I mean it: The drops must be less around a micron in size! A human hair, by the way, is about 100 microns in width, so these really are teensy drops.
But it’s more complicated than just that. The cloud also has to be what’s called optically thin; that is, mostly transparent so that on average a beam of light only hits one droplet and only gets bent once. If it hits multiple drops the colors get washed out. That’s why this happens more often near the edges of clouds, where they’re thinner. On top of that, the light waves interfere with each other, similar to how waves in a bathtub add together or subtract from each others’ wave heights as you wiggle around (and please, don’t deny you’ve ever done this playing in the tub; it’s fun, and educational!). These processes combine in complicated ways to produce these different colors.

So, what a performance by nature! An unusual cloud lit in an unusual way produced yet another unusual optical effect.

This is what you get for being aware of what’s going on around you, folks. Science is a party that never ends, never gets old, and is never ever uninteresting. And as a bonus, it’s beautiful, too.

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