Clouds are collections of trillions of rain droplets suspended in the atmosphere. These droplets consist of water or ice crystals that condense around a nucleus—usually a dust or other small particle, such as pollen, volcanic ash, mineral flakes, or other organic or inorganic materials.

To form, clouds require air that has cooled to the point where the air is saturated with moisture or ice crystals. In other words, when the humidity reaches 100 percent. Each cloud droplet is only a few micrometers in diameter, and it take a million such droplets to form a single rain drop.