Researchers have known for a while that if you pump small particles into the atmosphere you can get a thunderstorm, where lightning forms. Lightning forms in clouds with a mix of ice, liquid water, and a kind of vigorous up and down motion called updraft. As heavier ice particles—a sort of small hale that nephrologists, or cloud researchers, call graupel—move downward thanks to gravity, smaller, snowier particles move upward because of the updraft, bumping into each other and their respective electrical charges. Graupels generally carry a negative electrical charge, and the snowy floaters a positive one. So when they bump into each other, you get a bit of electrical discharge—the sparks we see as lightning.