Nucleation is the first of several microphysical processes we need to study. Cloud micro-physics, by contrast with the macrophysics treated in Part III, is concerned with the liquid and solid particles that constitute clouds. We next learn how cloud particles form in the first place, how they grow, and how they interact with each other. The microphysical processes are key to determining how the atmospheric aerosol affects the optical properties and life-times of clouds (crucial to the Earth's climate), how water vapor is ultimately turned into precipitation, and how lightning forms. This part focuses on individual particles and interactions with their immediate environments. Effects of particle populations in the context of whole clouds are treated later.

Clouds are the places in the atmosphere where water changes phase. Water vapor enters a cloud and is converted, by one mechanism or another, into liquid and/or solid phases of water. Nucleation is the process by which these condensed phases are initiated. An overview of the possible phase transformations is shown in Fig. 7.1. The three common states of matter are identified at the apexes of the phase triangle, while the names used to define the various transformations are given in italics along each arrow.

A new phase can form only if that new phase is thermodynamically favored. As we saw in Section 3.2, the molar free energy (chemical potential) of water in the new (daughter) phase must be lower than that of the original (parent) phase for the new phase to have a chance of surviving.