Paris - Once every 20 or 30 years, a superstorm greater than Earth breaks out on Saturn and whips around the ringed planet in a violent spectacle that rages for months on end.The storm can stretch hundreds of thousands of kilometres before fizzling out - some continue all the way around the planet until they meet their own tail.Dubbed "Great White Spots" after the tinge of their lightning-laced brew, the outbursts are so large they can be witnessed by telescopes from Earth.In the last century and a half, astronomers have observed six of these events, bemused by their titanic scale but puzzled why they occur so infrequently.Now a team has offered an explanation: the extraordinary behaviour of water vapour in the gas giant's atmosphere.As on Earth, Saturn's atmosphere consists of different layers, explain Cheng Li and Andrew Ingersoll of the California Institute of Technology, the authors of the report published on Monday in Nature Geoscience.For most of the time, the outer layer where clouds form is less dense than the sub-cloud layer that stretches all the way down to the centre of the gassy planet.Like oil floating on water, the less dense outer layer rests on top of a denser air mix of mainly hydrogen and helium, but also water molecules.In Saturn's case, the outer layer prevents the warmer air underneath from rising, cooling and condensing - the process required to create thunderstorms.This state of affairs lasts for decades at a time.During the very long calm before the storm, the outer atmosphere radiates heat into space and progressively cools until finally it becomes more dense then the lower layer.The balance between the layers becomes disrupted, and the warm air that had been kettled up below punches its way outward. The heavier water molecules in the roiling mix are then shed in massive storms until the original balance is restored, and calm returns."The time scale depends on how fast the planet can cool by radiating heat into space," Li said by email. "Because Saturn has a massive atmosphere, it takes decades to cool."Saturn and its gas giant neighbour Jupiter both sport massive storms. But rather than giant thunderstorms, the rain that falls in Jupiter's "Great Red Spot" is more similar to an Earthly drizzle, said Li.This is likely because Saturn has more water that can condense to form clouds than Jupiter.The two scientists tested their theory by developing simulation software similar to that used for weather forecasting on Earth, and compared the results to observations of the Cassini spacecraft orbiting Saturn.