Cassini's imaging cameras obtained the data for the top mosaic on April 17, 2008, and the data for the bottom mosaic on Aug. 11, 2011. The great northern storm was first detected by Cassini on Dec. 5, 2010. It developed a bright head of clouds that moved swiftly westward and a clockwise-spinning vortex that drifted much more slowly. By early July 2011, the head of the storm had churned its way all around the planet, collided with the vortex and disappeared. The thunder and lightning declined around that time and stopped entirely by Aug. 28, 2011.