Characteristics
The dark, elliptically-shaped spot (with initial dimensions of 13,000 × 6,600 km, or 8,100 × 4,100 mi) was about the same size as Earth, and was similar in general appearance to Jupiter's Great Red Spot. Around the Great Dark Spot, winds were measured blowing up to 2,400 kilometers (1,500 mi) an hour, the fastest in the Solar System. The Great Dark Spot is thought to represent a hole in the methane cloud deck of Neptune. The spot was observed at different times with different sizes and shapes.
The Great Dark Spot generated large white clouds at or just below the tropopause layer similar to high-altitude cirrus clouds found on Earth. Unlike the clouds on Earth, however, which are composed of crystals of ice, Neptune's cirrus clouds are made up of crystals of frozen methane. And while cirrus clouds usually form and then disperse within a period of a few hours, the clouds in the Great Dark Spot were still present after 36 hours, or two rotations of the planet.
Neptune's dark spots are thought to occur in the troposphere at lower altitudes than the brighter upper cloud deck features. As they are stable features that can persist for several months, they are thought to be vortex structures.
Read more about this topic: Great Dark Spot