One of the most obvious examples of this mutual influence is the formation of clouds. It is commonly thought that clouds form when the air is saturated with water vapor, but the real process is much more complicated. Studies show that without the help of condensation nuclei or ice nuclei, small particles in the air that promote the formation of cloud droplets or ice crystals respectively, it would require several hundred percent relative humidity to initiate the condensational process.
Different chemicals have different efficiencies in achieving nucleation.
Hence the chemical type of particles may have great influence on cloud and precipitation formation. Once formed, clouds will have significant impacts on the atmosphere within which they form. These are the cloud feedback mechanisms. For example, the formation of a thick cloud shields the underlying surface from solar radiation, resulting in a local cooling. Formation of widespread persistent cloud cover may cause a significant reduction of solar heating on the surface and influence the global climate process.