What is ball lightning? asks Kelly Corcoran, a student in Babylon, NY.

Stories of ball lightning date back to at least the Middle Ages, and scientists estimate that at least five percent of our planet’s population have had the privilege (or, sometimes, the misfortune) to have seen the glowing, floating spheres.

According to eyewitnesses, ball lightning appears as a radiant sphere, ranging in size from a baseball to a basketball. It may be white, yellow, red, orange, or blue, and is usually no brighter than a 100-watt light bulb.

The glowing ball often floats 2 to 6 feet or more above the ground as it travels, sometimes spinning as it moves back and forth, here and there. After seconds—or minutes—the ball goes out with a hiss, pop, or a loud bang. Like a July 4th sparkler, ball lightning may emit an acrid smell like burning sulfur or ozone, and leave smoke behind. Most ball lightning is seen just before or during a thunderstorm, within seconds of a lightning strike.

Millions have seen ball lightning float through windows and screen doors, cut telephone wires, fall into barrels of water with a sizzle. Passengers and crew on an airliner even watched ball lightning float down the aisle after their plane was struck by lightning.

But ball lightning is still an elusive mystery of science – hard to catch on film, with no agreed-upon explanation for how it works. Most ball lightning theories are extremely complicated, involving, for example, standing radio waves in spheres of plasma. But a newer, simpler theory -- based in chemistry -- seems to explain more of the features of this otherworldly display than have prior explanations. The ingredients for ball lightning, the theory says, are everyday minerals and metals, whipped up by an energizing lightning strike.

The explanation has been developed by John Abrahamson, a chemical engineer who teaches at the University of Canterbury in Christchurch, New Zealand. Abrahamson says that when a bolt of lightning hits the ground, it triggers a chemical reaction between silicon compounds and carbon in the soil. The huge blast of energy with its searing heat acts like a factory smelter, creating pure metallic silicon.

The resulting hot vapor of silicon is composed of tiny particles called nanospheres, each as small as a billionth of a meter. Trailing out of the hole in the ground carved out by the lightning, rising strings of silicon nanospheres form a spinning ring, rather like a cigarette smoke ring. The ring coalesces into a ball of white-hot metal particles, floating through the air like a spherical fog. The ball’s glow is stoked as the silicon particles react with oxygen in the air, keeping the sphere burning bright for seconds or minutes.

Lit from within, the ball drifts through the air, a ghostly apparition. Eventually, like a fire burning down to ash, the ball runs out of silicon to burn, and disappears in a quiet poof. But if its temperature has risen high enough, ball lightning may also go out violently, with a loud bang.