High above your head, a particle accelerator may be forming. Under the right conditions, thunderstorms 40 kilometers above Earth may drive narrow beams of electrons to near light speeds, researchers will report tomorrow at the Royal Astronomical Society meeting in Glasgow, United Kingdom. The recipe for such strange weather: mix lightning with high-energy particles from space called cosmic rays. The cosmic rays knock electrons off of atmospheric air molecules, while the lightning forces the electrons upward along electric fields. These electron beams may only be a small fraction as powerful as those produced by the Large Hadron Collider, but they produce plenty of radio waves that the research team detected after seeing flickering, oddly-shaped lightning bolts called sprites (inset). Eventually, the accelerated electrons become trapped in the Earth's radiation belts, where they can interfere with satellite communications and navigation.