About once every Saturn year—29.5 of our years—a mysterious great white spot erupts in the planet's atmosphere that can outshine the planet’s brilliant rings. This image shows the last outbreak, which began as a spot in the north in late 2010 that then spread into a band bigger than Earth. Now, planetary scientists writing online today in Nature Geoscience propose that these periodic superstorms arise from water. Water vapor is heavier than Saturn's dominant gases, hydrogen and helium, so as rain or snow ferries water from the upper atmosphere to the lower, the lower layer becomes denser than the air above it, a stable configuration that the scientists calculate keeps a lid on rising warm air for decades. During that time, however, the upper atmosphere gradually cools by radiating warmth into space and gets so cold that it becomes denser than the air below. Then, the warm moist air below finally rushes upward to trigger a rash of thunderstorms so enormous that observers marvel at them through backyard telescopes.