The link between the circulation and transfers of energy from the Earth's surface is thermodynamics. Thermodynamics deals with internal transformations of the energy of a system and exchanges of energy between that system and its environment. Here, we develop the principles of thermodynamics for a discrete system, namely an air parcel moving through the circulation. In Chap. 10, these principles are generalized to a continuum of such systems, which represents the atmosphere as a whole.

THERMODYNAMIC CONCEPTS

A thermodynamic system refers to a specified collection of matter (Fig. 2.1). Such a system is said to be “closed” if no mass is exchanged with its surroundings. Otherwise it is “open.” The air parcel that will serve as our system is, in principle, closed. In practice, however, mass can be exchanged with the surroundings through entrainment and mixing across the system's boundary, which is referred to as the control surface. In addition, trace constituents such as water vapor can be absorbed through diffusion across the control surface. Above the planetary boundary layer, such exchanges are slow compared with other processes that influence an air parcel. The system may therefore be treated as closed.

The thermodynamic state of a system is defined by the various properties characterizing it. In a strict sense, all of those properties must be specified to define the system's thermodynamic state. However, that requirement is simplified for many applications, as is discussed next.