An immense storm sighted by amateur astronomers recently in the clouds above Saturn has stretched to a length of 74,600 miles -- about the diameter of the giant ringed planet and more than nine times Earth's diameter.

The storm, which earlier appeared as a white spot, has combined with smaller disturbances to form a swath across the entire visible face of Saturn at its equator, according to Reta Beebe, a planetary astronomer at New Mexico State University. In the photograph above, taken by Scott Murrell of the university's observatory, it shows up as a white band just above Saturn's rings.

The storm apparently is caused by an upwelling of hot gas from deep within Saturn's atmosphere that broke through the planet's outer cloak of ammonia ice clouds. As the rising gaseous material cooled and expanded, it formed new ammonia ice crystals that stand out against the yellowish color of the older ammonia clouds below it, Beebe said.

Winds in the equatorial region where the storm formed are blowing eastward at 900 mph, she said. The storm travels around the planet -- the second largest in the solar system -- once every 10 hours and 15 minutes, or 24 minutes less than it takes the planet to rotate once.

Such storms have been seen before on Saturn, but this is the largest in 57 years. The most famous and longest-lived planetary storm in the solar system is the Great Red Spot of Jupiter. By studying such storms, scientists hope to gain insights into planetary weather patterns.

Next month, if all goes well, the orbiting Hubble Space Telescope will be trained on Saturn's storm, providing considerably more detail. Because Saturn appears near the horizon, observers on Earth must "look through the maximum thickness of atmosphere," Beebe said. "So there's an enormous advantage to looking from space."

The storm could prove tricky to catch, not only because its rapid rotation means it is out of Earth's view sporadically, but also because, as Beebe put it, "Saturn is being very uncooperative." The planet will move behind the sun in late November, and when it emerges again in February, the storm will almost surely have died.