Unknown to Franklin but now clear to a growing roster of lightning researchers and astronomers is that along with bright thunderbolts, thunderstorms unleash sprays of X-rays and even intense bursts of gamma rays, a form of radiation normally associated with such cosmic spectacles as collapsing stars. The radiation in these invisible blasts can carry a million times as much energy as the radiation in visible lightning, but that energy dissipates quickly in all directions rather than remaining in a stiletto-like lightning bolt.
Dark lightning appears sometimes to compete with normal lightning as a way for thunderstorms to vent the electrical energy that gets pent up inside their roiling interiors, Dwyer says. Unlike with regular lightning, though, people struck by dark lightning, most likely while flying in an airplane, would not get hurt. But according to Dwyer’s calculations, they might receive in an instant the maximum safe lifetime dose of ionizing radiation — the kind that wreaks the most havoc on the human body.
The only way to determine whether an airplane had been struck by dark lightning, Dwyer says, “would be to use a radiation detector. Right in the middle of [a flash], a very brief bluish-purple glow around the plane might be perceptible. Inside an aircraft, a passenger would probably not be able to feel or hear much of anything, but the radiation dose could be significant.”
However, because there’s only about one dark lightning occurrence for every thousand visible flashes and because pilots take great pains to avoid thunderstorms, Dwyer says, the risk of injury is quite limited. No one knows for sure if anyone has ever been hit by dark lightning.
About 25 million visible thunderbolts hit the United States every year, killing about 30 people and many farm animals, says John Jensenius, a lightning safety specialist with the National Weather Service in Gray, Maine. Worldwide, thunderstorms produce about a billion or so lightning bolts annually.
The conditions for lightning occur when powerful updrafts in cumulonimbus clouds force water droplets and ice crystals to rub against one another, creating massive amounts of positive- and negative-charged particles. The updrafts cause these two types of charged particles to separate, with the top of the thundercloud usually becoming positively charged as the lower part becomes negatively charged.