Aerosols tend to cause cooling of the Earth's surface immediately below them. The magnitude of this cooling effect depends on the size and composition of the aerosol particles, as well as the reflective properties of the underlying surface. It is thought that aerosol cooling may partially offset expected global warming that is attributed to increases in the amount of carbon dioxide from human activity. Indeed, if there were no aerosols in the atmosphere, there would be no clouds. As aerosol concentration increases within a cloud, the water in the cloud gets spread over many more particles, each of which is correspondingly smaller.
Smaller particles fall more slowly in the atmosphere and decrease the amount of rainfall.
In this way, changing aerosols in the atmosphere can change the frequency of cloud occurrence, cloud thickness, and rainfall amounts. Adding aerosols to a cloudy area may create smaller cloud droplets. Very little is known about the scale of these indirect effects and the circumstances under which they operate. We need frequent global observations of cloud and aerosol properties to help us gain a better understanding of how they interact with one another. For example, cigarette smoke particles are in the middle of this size range and typical cloud drops are 10 or more microns in diameter.