On August 3, 2014, showers and thunderstorms grew from an area of low pressure several hundred miles southwest of Manzanillo, Mexico. That tropical depression eventually strengthened into Hurricane Julio and passed north of Hawaii. The system stirred up surf, but was not otherwise notable—except that it happened to produce some very curious flashes of light.
The flashes—known as terrestrial gamma ray flashes (TGFs)—are some of the highest-energy light occurring naturally on Earth. From space, scientists have detected them amid the thunderstorms associated with tropical cyclones. The energy produced in a TGF can reach 100 mega-electron volts, or about as much radiation as 400 chest X rays.