Great storms on both planets feature swirling clouds, thermal convection, rain and strong rotating winds.

"Hurricanes on Earth are low pressure centers at the ground and high pressures at the top where the storms flatten out," said Cassini team member Andrew Ingersoll, of the California Institute of Technology. "Storms on Saturn could be like hurricanes if what we're seeing is the top of the clouds."

Saturn's storm frequency seems to be about the same as Earth's, and the fraction of planet occupied by storms also is similar. Not surprising, because Saturn is so much larger than Earth - nine Earths would fit across its equator - its storms are bigger. Hurricane Katrina stretched more than 380 kilometers (240 miles) in diameter, while two storms Cassini spotted in February 2002 each extend more than 1,000 kilometers (620 miles), about the size of Texas or France.

On Earth, hurricane winds can exceed 240 kilometers per hour (150 miles per hour), similar to the speed of the jet stream, just about the fastest wind on the planet. Though spinning furiously, hurricanes travel along at a much slower pace - eight to 32 kilometers (five to 20 miles) per hour, but Saturn's jet stream is much stronger.

"Saturn's a very windy place," Ingersoll said. "The jet stream on Saturn blows 10 times faster than on Earth, up to a thousand miles per hour."

Saturn's winds are like conveyor belts, between which storms appear to roll like ball bearings, he explained. "While we don't know the wind speeds within the storms, a good guess is that they are slower than the winds in the jet stream."

Saturnian storms are most distinguished from their terrestrial counterparts by the forces that drive them and the physical differences between the two planets.

The heat that drives hurricanes on Earth comes from the oceans, which provide vast reservoirs of solar energy. The oceans are also the source of moisture for convection, which draws energy from the ocean into the atmosphere and creates storm clouds and driving rainfall. Hurricanes quickly fade once they make landfall, once the plug is pulled on their power source.

The fuel for Saturn's storms is quite different. The interior of the planet acts like an ocean and stores energy, but the energy does not come from the Sun.

"Saturn makes it own heat, which it got when the pieces that made the planet crashed together during the violent history of the early solar system," Ingersoll said.

Saturn's atmosphere has all the ingredients necessary for hurricane-like storms, including heat and water vapor, he added, so there is no need for that first step in hurricane development where the ocean evaporates. Also, without a solid surface like Earth's ocean, Saturn's storms behave very differently.

"You'd think that when two storms merge, for example, that you'd get a bigger storm," Ingersoll said, "but they seem to stay the same size. They can also split apart. They may go on forever, merging and splitting."

Scientists will be able to study Saturn's storms more closely next year, when Cassini tours a region in the southern hemisphere that scientists call Storm Alley.

With the exception of a few storms, such as the dramatic Dragon Storm observed by Cassini last year, most of Saturn's storms are unnamed, unlike those on Earth. That may change, however, Ingersoll said, when scientists get to know them better.

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Cassini at JPL
Cassini Image Team

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