Quran has told us 1400 years ago. As common as clouds are on Earth, the processes that produce them are not well understood. Scientists think particles of dust or pollen can serve as nuclei for water droplets, which in turn gather by the trillions into clouds. That would help explain how clouds form over urban areas: Fine particles called aerosols are emitted from the exhaust pipes of millions of vehicles and work their way into the atmosphere, where they are thought to attract water molecules.