An instrument built to study cosmic rays has turned out to be a powerful tool for investigating thunderstorms. Earlier observations with the GRAPES-3 muon telescope revealed that thunderclouds can develop electrical potential differences far greater than those ever measured directly. But they also uncovered a puzzle: the instrument detected many more thunderstorm events in the eastern part of its field of view than in the western part. A new study by the same team, published in the Journal of Cosmology and Astroparticle Physics, has now explained why.