A small, bright feature that suddenly appeared in Saturn's northern hemisphere late last year and grew to a diameter approaching Earth's in a matter of weeks is a thunderstorm that's still raging. The day it was first observed in early December, the cloud measured only 2500 kilometers across, about the distance from Boston to Dallas. Three weeks later (shown on 24 December 2010), the storm system measured 17,000 km across and sported a tail that eventually stretched around the planet. Instruments on board the Cassini probe detected bursts of radio waves generated by lightning flashes that, at their peak, occurred at least 10 times per second, an international team of researchers reports today in Nature. Thermal energy brought up from lower layers of the atmosphere and released by the storm rivals that emitted by the entire planet in quiet times, the scientists note. Such storms, called "Great White Spots" due to their size and brightness, can be seen by Earthbound astronomers and occur on average every 30 years or so—approximately the length of a year on Saturn—but for some unknown reason this year's storm has appeared much earlier in the Saturnian spring than normal.
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