Thermodynamics describes the macroscopic behavior of matter. In this chapter, we review the thermodynamics necessary for the understanding of cloud formation. We start by defining some fundamental terms, such as state variables. We discuss the physical laws that describe processes in dry air, like the ideal gas law, and introduce the concept of air parcels. Since thermodynamic processes that involve adiabatic changes can best be visualized with the help of thermodynamic charts, tephigrams are introduced.

Water is a unique substance because it exists in all three phases (ice, liquid water, and water vapor) in Earth’s atmosphere. The transitions between its different phases are crucial for the understanding of liquid and ice cloud formation and microphysical processes inside clouds. Relevant concepts such as the Gibbs free energy, to describe phase transitions are introduced. We end the chapter by extending the laws introduced for dry air to moist air, and discuss topics such as the water phase diagram, Clausius–Clapeyron equation and equilibrium water vapor pressure, to analyze how water vapor influences atmospheric processes and cloud formation.