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From NASA/ ESA, June 23, 2016: "Pancake-shaped clouds not only appear in the children's book "Cloudy With a Chance of Meatballs," but also 3 billion miles away on the gaseous planet Neptune. When they appeared in July 2015, witnessed by amateur astronomers and the largest telescopes, scientists suspected that these clouds were bright companions to an unseen, dark vortex. The dark vortex is a high-pressure system where the flow of ambient air is perturbed and diverted upward over the vortex. This forms huge, lens-shaped clouds, that resemble clouds that sometimes form over mountains on Earth.
When NASA's Voyager 2 spacecraft flew by Neptune in 1989, astronomers were surprised to see such a gaping, dark hole at southern latitudes in the giant planet's cyan-colored atmosphere. The dark spot later disappeared. But the Hubble Space Telescope captured a new northern dark spot of comparable size in 1994. Hubble captured the appearance of a new dark spot on May 16, 2016. The spot would span the width of the continental United States."
