Jupiter is a planet of clouds--clouds upon clouds down to the point where gravitational pressure condenses vapor into liquids. Jupiter is also a planet of storms, with the Great Red Spot the largest and longest-lived. But planetary scientists reviewing data from the Galileo orbiter mission have recently found something almost comfortingly familiar: anvil-shaped clouds that seem to be Jovian thunderheads. Researchers tracked these 600-mile-wide storms across the planet and discovered that they are associated with low-pressure systems just like those on Earth. The Galileo team also found that one of these clouds seems to extend 15 miles above and 30 miles below the surrounding layers of wispy ammonia clouds. In fact, the cloud base is so deep that it must be made primarily of water, and may even generate lightning flashes, says Don Banfield, a Cornell planetary scientist and Galileo team member. "Water is the only substance present in large enough quantities in Jupiter's atmosphere so that it could lead to electricity," says Banfield.