It sounds like the beginning of Zombies on a plane—the movie starring Samuel L. Jackson that I want to see next year—but scientist have proven the existence of something called dark lightning, an invisible force that can expose unsuspecting airline passengers to sizable amounts of gamma ray radiation.
"Passengers flying through a thunderstorm when a Terrestrial Gamma Flash occurs would receive a significant dose of [gamma ray] radiation, comparable to a full body CT scan," says Professor Joseph Dwyer of the Florida Institute of Technology. These flashes are caused by dark lightning and amount to a sizable part of your lifetime radiation budget.
Talking to ABC Science, Professor Dwyer said that this "exotic type of discharge, which we've coined dark lightning, produces a lot of high energy electrons and their anti-matter counterparts called positrons. This generates lots of gamma rays, but not much visible light, which is why we call it dark lightning."
In a research paper published on the journal Geophysical Research Letters, Dwyer and his team demonstrate that "lightning-like events responsible for TGFs emit relatively little visible light and, thus, are inherently dark." The TGFs—which were first detected by NASA's Compton gamma ray telescope in 1994—may be caused by electron avalanches that retro-feed themselves within thunderclouds.
Fortunately, pilots try to avoid storms as much as they can. Unfortunately, sometimes that's not possible.