Atmospheric aerosols, including clouds, exhibit large fluctuations in space and time in which their concentrations in number and mass vary greatly. Fluctuations affect properties such as rainout and nucleation of new aerosol important to weather and climate. Processes which lead to these fluctuations include localized emissions and mixing of different air masses. Here we briefly survey these processes, and how well they are understood, concentrating on their origins from condensation, particularly as it affects clouds.

In Table 1, we outline possible consequences of fluctuations. Drastic changes in the physics or threats to life can result, so that it is necessary to take them into account in atmospheric science. An example of nucleation of a new sulphuric acid aerosol as a result of lack of uniformity in the mixing of a volcanic plume with air has been found by Clement and Mather (2006).