For a Cloud to appear, it takes more than water vapor. Water won’t condense into droplets, or nucleate, without a surface to do so on, and this often takes the form of particles floating around the atmosphere so tiny as to be invisible. Called aerosols, these particles play an important role in everything from the pace of climate change to the water cycle because they influence how clouds form and grow.

Natural aerosols come from any number of places: Everything from compounds emitted by