Ball lightning captured on film

BALL lightning—a rare and transient electrical phenomenon—is still very poorly understood. It typically appears during thunderstorms as a glowing sphere whose size may range from that of a golf ball to several metres in diameter, floating in the air for up to tens of seconds. Its colour can be white, orange, yellow, purple or green. Its rarity makes its systematic study virtually impossible.

But a fortuitous observation during field experiments on ordinary lightning allowed a Chinese research team comprising Jianyong Cen, Ping Yuan and Simin Xue of Northwest Normal University, China, to record a ball lightning event and analyse its size, colours and light. The results offer important clues about what the glowing balls are made of and have been published in the latest issue of Physical Review Letters.

In 2012, the researchers were studying thunderstorms in the Qinghai plateau with high-spectral-resolution high-speed cameras to record the spectral characteristics of ordinary lightning. But, by pure chance, they saw a glowing sphere about 5 metres in diameter and moving at about 8.6 m/s, which followed just after one of the ordinary lightning strikes they were recording. It lasted for just over 1.5 seconds. It started out as a bright purple-white ball, which gradually became orange, then white and finally red. They managed to record the entire event on a digital high-speed (3,000 frames per second) video camera and the latter half was also recorded on a high-speed camera that records 3,000 frames per second. Unfortunately, they missed the initial strike as the instrument was off.

The analysis of the pictures taken showed emission lines of silicon, iron and calcium, the main components of soil. The researchers say this supports the idea that the energy source for ball lightning is a burning core of soil, an idea that was advanced first by John Abrahamson and James Dinniss of the University of Canterbury in New Zealand in 2000. Interestingly, however, there are also emission lines of nitrogen and oxygen, which oscillate rapidly with time, whereas those of silicon, iron and calcium change only slowly. Experts believe this suggests that there may be two different sources of radiance from the same ball.