Ball Lightning: The Real Phenomenon Science Still Can't Fully Explain

On a July evening in 2012, a team from Northwest Normal University in Lanzhou had cameras and two spectrographs pointed at the sky on the Qinghai Plateau in northwest China. They were there to study ordinary lightning, which is frequent in that region. During a late thunderstorm a bolt struck the ground about 900 metres from their equipment, and a glowing ball rose from the spot where it hit. It drifted about 10 metres sideways and climbed about 3 metres, faded from white to reddish, and was gone in little more than a second.

The recording, published by Jianyong Cen, Ping Yuan and Simin Xue in Physical Review Letters in January 2014, was the first time anyone had captured the spectrum of natural ball lightning. Before that, almost everything known about the phenomenon came from eyewitnesses.

Illustration: ball lightning is usually described as a glowing sphere lasting a few seconds.

What people have described

The accounts go back centuries and are surprisingly consistent. A sphere of light appears during or just after a thunderstorm, usually somewhere between the size of a golf ball and a football, though a few reports describe balls several metres across. It is often red, orange or bluish-white. It floats, drifts or rolls, sometimes against the wind, and lasts from about a second to, in rare reports, tens of seconds. It then fades or vanishes with a bang. Some accounts have the ball entering houses down chimneys or through closed windows. Burns, scorch marks and occasional deaths have been attributed to it.

Scientists long had trouble with this, because a witness report, however sincere, cannot be measured. Ball lightning is rare, impossible to predict and brief, so for most of the twentieth century there was nothing to put under an instrument. That left a respectable minority of physicists collecting reports and a larger number assuming the witnesses had seen afterimages, St Elmo's fire or something burning.

Silicon from the soil

The Chinese spectrum showed emission lines from silicon, iron and calcium, three elements common in soil. That fits a theory put forward in Nature in 2000 by John Abrahamson and James Dinniss of the University of Canterbury in New Zealand. When lightning strikes the ground, they argued, the heat can turn silica in the soil into silicon vapour, with carbon in the soil stripping away the oxygen. As the vapour cools it condenses into a loose, floating network of silicon nanoparticles. These then burn slowly in the air, releasing heat and light until the fuel runs out.

The idea explains several things witnesses describe: the size of the ball, the fact that it lasts seconds rather than milliseconds, and its fading or popping at the end. The glow the Lanzhou team recorded appeared about 5 metres across, although the authors note that the real ball was smaller than its blurred image.

The cases silicon can't explain

Burning silicon has a harder time with some of the classic reports. It does not obviously explain a ball forming inside a closed metal aircraft, as several pilots have described, or one passing through a window without breaking it. For those, other researchers prefer electromagnetic explanations. The Soviet physicist Pyotr Kapitsa suggested in 1955 that ball lightning could be powered by microwaves from the storm, trapped in a standing wave. In 2016 Hui-Chun Wu proposed in Scientific Reports that a lightning strike could produce an intense burst of microwaves that would be trapped inside a bubble of plasma it created, which would also explain balls forming behind glass.

Laboratories have made things that look like ball lightning, though none that matches the best natural reports. In 2007 a group in Brazil led by Gerson Paiva passed strong currents through silicon wafers and produced luminous balls that bounced along the floor for up to several seconds, which supports the silicon idea. Others have made glowing plasma balls from electrical discharges over water, or with microwave ovens. Each method copies part of the phenomenon. None has reproduced a stable ball lasting many seconds in open air.

Many researchers now suspect that "ball lightning" is a label for more than one thing. A slowly burning silicon cloud after a ground strike is one real mechanism, now with a spectrum to back it up. Some of the indoor and aircraft reports may involve a different process altogether. Telling them apart needs more recordings, which in practice means more lucky accidents like the one on the Qinghai Plateau. If you ever see one, the best advice is the obvious one: keep your distance, and if you can, film it.

Sources

1. Jianyong Cen, Ping Yuan and Simin Xue, "Observation of the optical and spectral characteristics of ball lightning", Physical Review Letters 112 (2014): 035001. https://doi.org/10.1103/PhysRevLett.112.035001
2. Philip Ball, "First spectrum of ball lightning", Physics 7 (2014): 5, American Physical Society. https://physics.aps.org/articles/v7/5
3. John Abrahamson and James Dinniss, "Ball lightning caused by oxidation of nanoparticle networks from normal lightning strikes on soil", Nature 403 (2000): 519–521. https://doi.org/10.1038/35000525
4. Gerson S. Paiva, Antonio C. Pavão, Elder A. de Vasconcelos et al., "Production of ball-lightning-like luminous balls by electrical discharges in silicon", Physical Review Letters 98 (2007): 048501. https://doi.org/10.1103/physrevlett.98.048501
5. Hui-Chun Wu, "Relativistic-microwave theory of ball lightning", Scientific Reports 6 (2016): 28263. https://doi.org/10.1038/srep28263