Thermodynamics is defined as the study of equilibrium states of a system which has been subjected to some energy transformation. More specifically, thermodynamics is concerned with transformations of heat into mechanical work and of mechanical work into heat.

A system is a specific sample of matter. In the atmosphere a parcel of air is a system. A system is called open when it exchanges matter and energy with its surroundings (Figure 1.1). In the atmosphere all systems are more or less open. A closed system is a system that does not exchange matter with its surroundings. In this case, the system is always composed of the same pointmasses (a point-mass refers to a very small object, for example a molecule). Obviously, the mathematical treatment of closed systems is not as involved as the one for open systems, which are extremely hard to handle. Because of that, in atmospheric thermodynamics, we assume that most systems are closed. This assumption is justified when the interactions associated with open systems can be neglected. This is approximately true in the following cases. (a) The system is large enough to ignore mixing with its surroundings at the boundaries. For example, a large cumulonimbus cloud may be considered as a closed system but a small cumulus may not. (b) The system is part of a larger homogeneous system. In this case mixing does not significantly change its composition. A system is called isolated when it exchanges neither matter nor energy with its surroundings.

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