The following text is from space.com which was written during the time of discovery of the first dark spot on Uranus:
During the past decade, many bright spots have been seen on Uranus in both red and near-infrared filters. But this is the first dark spot ever seen on the planet. A team led by Lawrence Sromovsky of the University of Wisconsin and including Kathy Rages of the SETI Institute, Heidi Hammel of the Space Science Institute (Boulder, CO), andPatrick Fry of U. Wisconsin, observed the dark spot on Aug. 23 using the Hubble Space Telescope (HST).
What was the nature of the bright spots? How were they formed? Was it some form of storm or vortex? I couldn't find any information anywhere regarding this.