Sulfuric acid clouds circle the entire planet at a height of 25 to 37 miles above the surface. They contain tiny acidic aerosols that are about a hundred times thinner than human hair. Together the droplets resemble the air pollution in highly populated cities on Earth. “It’s like a haze that you find when you fly into, say, New Delhi or Beijing,” says Sanjay Limaye, a planetary scientist at the University of Wisconsin-Madison.