Black Lightning!

Or at least dark purple...

Thunderstorms contain ‘dark lightning,’ invisible pulses of powerful radiation

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.

Makes some kind of sense.  X-rays are formed when electrons accelerated by an electric field slam into matter and decelerated suddenly.  One thing thunderstorms have in abundance are electrons moving around in an electric field.  It would be interesting to find out what kind of situation sets up a condition where a lot of electrons can be accelerated or decelerated suddenly.

Kind of apropos, as we're having our first spring thunderstorm even as I type this: