Scientists have known for almost a decade that thunderstorms are capable of generating brief but powerful bursts of gamma-rays called terrestrial gamma-ray flashes, or TGFs. Because they can originate near the same altitudes at which commercial aircraft routinely fly, scientists have been trying to determine whether or not terrestrial gamma ray flashes present a radiation hazard to individuals in aircraft. In the middle of the storm, radiation doses could be roughly equal to a full-body CT scan, preliminary research suggests.

Source: Dark lightning: Are airplane passen...bursts of gamma-rays from thunderclouds?