What scientists have only just discovered is the process behind the burst of gamma-rays. Thunderstorms are apparently capable of producing a rare kind of electrical breakdown (it involves electrons and positrons - antimatter, I mean, of course). The electrons and positrons incite an explosive, if unstable, growth in the number of high-energy particles, ultimately emitting the terrestrial gamma ray flashes while discharging the thundercloud. This process - this flash - moves even faster than regular lightning, and somewhat poetically is referred to as "dark lightning". Very little visible light can be seen from this occurrence, even if radiation levels are spiking.
Passengers can definitely get a dose of that gamma radiation if their luck runs bad. If you're flying through the top of the storm, a nearby strike of dark lightning will produce the radioactive equivalent to about 10 chest x-rays - or the same amount that an average person would experience within a year's time. More disturbingly, if flying near the middle of the storms, the radiation dose could be at least 10 times larger - comparable to some of the largest doses received during medical procedures, roughly equal to a full-body CT scan.
After hearing of this research, I have two reactions: a) I wonder why a superhero hasn't shown up with the name of "Dark Lightning", and b) Well, I guess now I have an excuse for hating flying.
[Edited to Add:] If you fly too much and get an overdose of gamma radiation, statistically, you will not gain superpowers.