Despite its massive size and rapid rotation, despite stunning peculiarities like its stripes and bands and Great Red Spot, despite the fact that it's mostly ammonia without oceans or continents to influence it from below, the atmosphere of Jupiter behaves a lot like the Earth's.
White cirrus clouds swirl at the highest altitudes, parts of its skies are blue and lightning flickers above the thunderclouds. Most of the air currents in the northern hemisphere blow toward the east and in the southern hemisphers toward the west, just like they do on Earth.
Jet streams cross the planet at hundreds of miles an hour, giant thunderstorms pop suddenly into view along the equator. Reions of dry air appear at the same latitudes one would find Africa's Sahara and Australia's Great Sandy Desert.
"I think a lot of sound atmospheric concepts are going to emerge from studying Jupiter," said the California Institute of Technology's Dr. Andrew Ingersoll, a meteorologist analyzing Jupiter photographs taken by the Voyager spacecraft that flew by the planet March 5. "There are surprising similarities between the Earth and Jupiter."
One of the most striking similarities is the abundance of jet streams in the upper atmosphere, clearly the source of much of the changing weather on both planets.
There are as many as 20 jet streams circling Jupiter at speeds up to 350 miles on hour, piling air up in certain places and pulling it out of others. Scientists suspect that Jupiter's jet streams are at least one reason for the enormous white-colored ovals seen in the photographs of the planet. The ovals may be little more than gigantic high pressure systems that march around the planet and produce weather.
Just south of the Great Red Spot lies one of the largest white ovals on Jupiter, still trapped betwen two jet streams moving in opposite directions ever since it was first spotted by telescopes on Earth in 1939. West of the oval appears to be a "wake," left by the motion of the faster-moving west winds as they blow by.
Scientists suspect that one reason the ovals have the shape they do is that the jet stream trapping them above the clouds have different speeds, forcing gas at the outer edge of the oval to follow a circular path. One oval lies below a jet stream going west at 120 miles an hour.
The biggest oval of all is the Great Red Spot, three times the size of the Earth and trapped in its southerly latitude for at least 300 years. Whatever lies just west of the red spot is enormous, looking like huge bundles of cooked spaghetti. It could be a wake. It could be air that is trying to move east and is blocked by the red spot.
"On the west side of all high pressure systems on Earth is a blocked low pressure system," said Dr. Garry Hunt, University College of London meteorologist who is also analyzing Voyager photographs. "Why is that interesting? Because is relates to my droughts in Europe and your very peculiar weather over here."
Scientists had thought the red spot was a permanent hurricane drawing energy from deep inside the planet to the tops of the clouds, where it looks like a huge eye that measures 12,000 miles from top to bottom.
But there are no signs of spreading. A movie sequence taken by Voyager as it approached Jupiter shows a large white blob moving around the red spot and making a full revolution of the red spot every six days. The blob did change shape or direction and it did not slow down.
"It just went round and round," Cal Tech's Ingersoll said. "If there were any spreading at the top of the red spot the blob would have moved away from the red spot."
The red spot may not be a hurricane; instead it may be a passive storm, like a giant eddy of air driven by the temperature differences of the jet streams above and below it.
Much smaller eddies of air are formed on Earth when jet streams wander off the straight and narrow. Similar eddies are formed in the Earth's oceans when warm water currents like the Gulf Stream suddenly veer off course and run into a mass of cold water.
"What we see in the photographs is meteorological in orgin," London's Hunt said, "It means we don't have to invoke mountains on Jupiter to explain the red spot's existence."
While the red spot may be nothing more than an enormous eddy, its size alone might be the reason the southern hemisphere on Jupiter is so different from its northern hemisphere. The Voyager photos clearly show a north that is more placid and stable than the south.
If there are similarities between the Earth and Jupiter, there are also dramatic differences. Take the size of Jupiter's features. Even up close, Voyager did not see a single feature smaller than 50 miles across. As many as a dozen of the white ovals seen by Voyager are half the size of Earth.
And while the planet undergoes rapid change, many of Jupiter's features, such as the red spot, go unchanged. And the huge white oval beneath the spot has been there for 40 years. Why don't these "storms" break up the way storms dissipate on Earth?
The answer may lie in the fact that there are no land masses on Jupiter and no rapid changes in temperature below the clouds to steal energy from the storms. There also is no friction on Jupiter from the intersection of rock, water and gas to break things up.
There is another major difference which helps to explain the performance of some of Jupiter's features. The jet streams on Earth often wander as far north and south as 20 degrees, taking heat from the tropics. Not so on Jupiter, where the 20 jet streams rarely deviate in their easterly and westerly motions.
The reason could be that Jupiter supplies its own heat to the atmosphere from its slow but continuous contraction. Even toay, the planet radiates twice as much heat as it receives from the sun. Its internal heat may be deflected polewards before it even reaches the clouds, thus making temperatures more or less the same all over the planet.
That would give the jet streams little reason to wander north or south in search of warmer or colder air. That would mean the jet streams act as permanent barriers to the movement of other features, which may be why the Great Red Spot has not moved north or south in 300 years.