Crown Flash Video from Greenwood, Indiana: Dancing Light Above a Storm

In June 2015 a cyclist in Greenwood, Indiana, noticed a strange light above an approaching thunderstorm, pulled over and started filming. The clip, uploaded to YouTube by QuadeM13 on 12 June 2015 under the simple title "Crown Flash", shows a bright plume on top of the storm cloud twisting, flickering and changing shape within seconds. Many people who saw it had no idea what they were looking at. The explanation is a rare and beautiful piece of storm physics.

What is a crown flash?

A crown flash is a brightening of the top, or crown, of a thunderhead, often followed by streamers or beams of light that look a little like an aurora. It is rarely seen and poorly documented, which is why videos like the Greenwood clip cause such excitement.

Despite its look, a crown flash does not produce any light of its own. Like a sundog or a halo, it is sunlight reflected and refracted by ice crystals, so it only appears from the right angle, with the sun lighting the top of the cloud. That is also why it is sometimes called a "leaping" or "jumping" sundog.

How electric fields make the light dance

A growing cumulonimbus cloud builds a powerful electric field, mainly as rising and falling ice particles collide and separate charge. Above the crown of the cloud float tiny, needle-like ice crystals. In a strong field these crystals line up together, and the aligned crystals reflect sunlight as a column, a curved streamer or a beam that can resemble a searchlight.

When lightning discharges inside the cloud, the electric field changes suddenly. Every crystal swings to a new orientation at the same moment, and the reflected light jumps to a new shape. Repeat that with each lightning stroke and the glow above the storm seems to dance, exactly as in the Greenwood footage.

Astronomer Phil Plait, who has written about crown flashes for years, described the process this way, and a laboratory experiment led by physicist Alberto Tufaile supported it: a ferrofluid stood in for the ice crystals, a laser stood in for the sun, and moving a magnet shifted the reflection much like a real crown flash.

A phenomenon described since 1885

Crown flashes are not a social-media discovery. Guinness World Records credits the first scientific description to the journal Monthly Weather Review in 1885, and the effect was discussed again in Nature in 1971. Since the first YouTube upload in 2009, dozens of videos have appeared, and the uploader of the Greenwood clip thanked commenters who recognised the effect and explained it.

How to watch and film storm light safely

Crown flashes are most often seen on the sunlit side of a tall, active thunderstorm, when the observer is far enough away to see the cloud top. When watching a clip, compare the bright area with the cloud edge and fixed objects such as the power lines in the Greenwood frame: they show that the light, not the camera, is moving.

If you see one yourself, film it from shelter. A storm close enough for thunder to be heard is close enough to strike, and the National Weather Service lightning advice is to get indoors or into a hard-topped vehicle until 30 minutes after the last thunder.

For more storm skies, explore Jeff Boyce’s Edge of Stability project and Mike Olbinski’s Monsoon II. For weather turned into an object, see Ken Kawamoto’s Tempescope.

Related reading

• Sam Smith’s Illinois Tornado Video: The April 2015 Storm in Context
• Twenty Thunderstorm Photographs: Light, Scale and Changing Skies
• Wave-Like Clouds over Nebraska
• Lightning Seen from Orbit: Reid Wiseman’s Space Station Footage
• Vincent Brady’s Toroidal Sky: A Four-Camera Night-Sky Experiment
• Thomas O’Brien’s Meteor-Shower Time-Lapse Around Aspen
• Four Storm Photographers: Different Ways of Working with Severe Weather

Sources