As the atmosphere ascends near the center of a vortex, it expands and cools. Cooling condenses certain volatile materials into liquid droplets, which then fall as rain. As they fall, the raindrops collide with surrounding air molecules, stripping electrons from the molecules and thereby developing tiny electric charges in the air. The cumulative effect of countless falling raindrops can be an electric charge and resulting electric field so great that they ionize the molecules in the atmosphere, forming a conductive path and creating a surge of current and a flash of lightning. A single observation of Jupiter’s night side by Voyager 1 revealed several dozen lightning bolts within an interval of 3 minutes. Cassini has also imaged lightning flashes in Saturn’s atmosphere, and radio receivers on Voyager 1 picked up lightning static in the atmospheres of both Uranus and Neptune.