First, the changing weather patterns are in some sense cyclic. The fact that Jupiter may be reverting to the appearance it presented at the time of Pioneers 10 and 11 is not surprising. From Earth-based studies, astronomers have found that the face of Jupiter often goes through a major change every few years. The transition is very rapid, but the planet maintains its new “look” for some time—until the next major transition. At the time of the Pioneer flybys, “The Red Spot was prominent, but it was surrounded by intense cloud. There was no visible structure at all in the south tropical zone—it was totally bland. You couldn’t see the turbulent area to the west,” explained Garry Hunt. “And, I believe that the buildup of cloud that we’re seeing to the east of the Red Spot is the beginning of the transition that will produce the Pioneer look.” There is even more order underlying Jupiter’s changing atmosphere. This order is revealed in part in the alternating belts and zones. It is believed that the cloud-covered zones are regions of rising air, and the belts are regions of descending air. The internal energy of Jupiter provides the power to maintain this pattern of slow vertical circulation. In addition, there are horizontal or zonal flows that are much more regular than the changing cloud patterns. [Illustration: The three large white ovals are the longest-lived features in Jupiter’s atmosphere, after the Great Red Spot. Like the Red Spot, they are anticyclonic, or high-pressure, regions. During Voyager 2 encounter, one of the ovals was just south of the Red Spot. This picture shows the other two ovals as they looked in early July. The clouds show very similar internal structures. To the east of each of them, recirculating currents are clearly seen. [P-21754C]] [Illustration: In this frame, a similar structure is seen to the west of the cloud.] [Illustration: A sequence of pictures of Jupiter, taken once per rotation (about ten hours), can be used to construct a time-lapse movie of the circulation of the Jovian atmosphere. These frames are from the Voyager 1 “Blue Movie” of the Great Red Spot region. Every odd Jovian rotation is shown, so the 24 frames correspond to 48 Jupiter days, or about 20 Earth days. White spots can be seen entering the Red Spot from the upper right and being carried around by its six-day rotation until they are ejected toward the lower right. Above the Red Spot, the flow is toward the right; below it, toward the left. The rotation of the Spot is counterclockwise, or anticyclonic. [260-449]]