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Explaining Tuesday’s colorful clouds over Virginia and Maryland

The phenomena was a picturesque display of cloud iridescence

Cloud iridescence Tuesday evening. (Ashley Reid) (Ashley Reid)
4 min

Anyone who lives in the Mid-Atlantic knows that pop-up thunderstorms are a staple of the summertime. Tuesday was no exception, featuring a spattering of renegade downpours that blossomed during the heat of the day. But one of the storms west of D.C. wasn’t your typical late-day shower: The cloud appeared to be shimmering in rainbow hues.

It wasn’t a rainbow though, nor was it a sun halo. Instead, the cloud itself resembled the polished inside of an abalone shell. It was a rare instance of cloud iridescence, which is usually at its most visually appealing when occurring in tandem with strong thunderstorms.

The storm in question Tuesday night formed over the Alleghenies thanks to unequal heating over the varied terrain, then pushed east during the early evening.

Pictures began pouring in to Capital Weather Gang’s social media feeds from Loudoun County. At the time — around 7 p.m. — the storms were just pushing east of Interstate 81.

Subsequently, we received photos from Fairfax and Montgomery counties as the storms progressed eastward.

The storms weren’t overly intense and didn’t reach severe levels, but the exceptionally humid air mass in place translated to heavy downpours. Far and wide, though, residents broke out their smartphones to capture the stunning scene and post on social media.

Iridescence forms as a result of diffraction, which isn’t to be confused with refraction, which is the cause of rainbows and halos. Both pertain to the bending of sunlight by raindrops and ice crystals. There’s one key difference, however.

Refraction is an internal bending. Light enters an ice crystal or raindrop and slows down, but each color slows down, and is resultantly refracted, or bent, at a slightly different speed and angle from the others. That splits white light up into its component colors, which are then bounced back to the observer as a full color spectrum. The result? A ring of light — usually in the form of a semicircular rainbow or, in the case of ice, a halo around the sun.

Diffraction features bending, but it occurs outside the raindrop or ice crystal. It requires very small, round water droplets or ice crystals. Rays of light are bent around the periphery of the droplet, almost as if they’re skimming along its edge. The light from all the different droplets once again becomes split into component colors, but the randomness causes an interference pattern that we visually see as a marble wash of colors. It’s akin to the hues we see in a soap bubble, oil slick or on a DVD.

For a cloud to be iridescent, all the droplets must be close to uniform in size and the cloud needs to be close to the sun from the viewer’s vantage point. That doesn’t usually happen at the tops of thunderstorms, since water droplets there are entering a more frigid region of the lower atmosphere and transitioning into ice. But Tuesday’s storms had something a little special: a pileus.

That’s a “cap” of cloud cover that forms atop a quickly-rising towering updraft. It results when an updraft punches through a layer of warm air and bends that otherwise warm, stable layer upward — like someone’s pressing upward on a tablecloth or trampoline from below. Ordinarily we wouldn’t see this warm layer, but in the case of a pileus cloud, the moisture contained within that warm layer is forced higher into the atmosphere where it’s cooler, and resultantly condenses to form a visible cloud.

That explains why there was a cloud with water droplets perhaps higher than the freezing line (since that shallow warm layer got bent upward), and why the actual cloud was thin enough that light was able to penetrate through.

A similarly impressive display of iridescence occurred on July 20, 2019, and was visible from the D.C. and surrounding areas looking westward.

Here are a few of the many great shots of the iridescent thunderstorm anvil from Tuesday: