It wasn’t a rainbow although, nor was it a solar halo. Instead, the cloud itself resembled the polished inside of an abalone shell. It was a uncommon occasion of cloud iridescence, which is often at its most visually interesting when occurring in tandem with sturdy thunderstorms.
The storm in question Tuesday evening formed over the Alleghenies due to unequal heating over the numerous terrain, then pushed east through the early night.
Pictures started pouring in to Capital Weather Gang’s social media feeds from Loudoun County. At the time — round 7 p.m. — the storms had been simply pushing east of Interstate 81.
Subsequently, we additionally obtained pictures from Fairfax and Montgomery counties because the storms progressed eastward.
Storms weren’t overly intense, and didn’t attain extreme ranges, however the exceptionally humid air mass in place translated to heavy downpours. Far and huge, although, residents broke out their telephones and pointed them west, capturing the gorgeous scene and taking to social media to share what they’d seen.
Iridescence kinds as a outcome of diffraction, which isn’t to be confused with refraction, which is the trigger of rainbows and halos. Both pertain to the bending of daylight by raindrops and ice crystals. There’s one key distinction, nevertheless.
Refraction is an inside bending. Light enters an ice crystal or raindrop and slows down, however every colour slows down, and is resultantly refracted, or bent, at barely totally different speeds and angles. That splits white gentle up into its element colours, that are then bounced again to the observer as a full colour spectrum. The outcome? A hoop of gentle — often within the kind of a semicircular rainbow or, within the case of ice, a halo across the solar.
Diffraction options bending, nevertheless it happens exterior the raindrop or ice crystal. It requires very small, spherical water droplets or ice crystals. Rays of gentle are bent across the periphery of the droplet, virtually as in the event that they’re skimming alongside its edge. The gentle from all of the totally different droplets as soon as once more turns into cut up into element colours, however the randomness causes an interference sample that we visually see as a marble wash of colours. It’s akin to the hues we see in a cleaning soap bubble, oil slick or on the again of a DVD.
For a cloud to be iridescent, all of the droplets have to be near uniform in dimension and the cloud must be near the solar from the viewer’s vantage level. That doesn’t often occur on the tops of thunderstorms, since water droplets are within the course of of coming into a extra frigid area of the decrease ambiance and transitioning into ice. But Tuesday’s storms had one thing a little particular: a pileus.
That’s a “cap” of cloud cover that kinds atop a quickly-rising towering updraft. It outcomes when an updraft punches by way of a layer of heat air and bends that in any other case heat, secure layer upward — like somebody’s urgent upward on a tablecloth or trampoline from under. Ordinarily we wouldn’t see this heat layer, however within the case of a pileus cloud, the moisture contained inside that heat layer is pressured larger into the ambiance the place it’s cooler, and resultantly condenses to kind a seen cloud.
That explains why there was a cloud with water droplets maybe larger than the freezing line (since that shallow heat layer obtained bent upward), and why the precise cloud was skinny sufficient that gentle was capable of penetrate by way of.
A equally impressive display of iridescence occurred again on July 20, 2019, and was seen from the District of Columbia and surrounding areas trying westward.
Here are a few of the various nice photographs of the iridescent thunderstorm anvil from Tuesday: