The task ahead is the formulation of prognostic and diagnostic equations to describe the future development of the atmospheric thermodynamic state. The variables describing this state are also called the state variables of the thermodynamic system. These quantities may be the pressure, temperature and density of the air as well as the concentrations of the water substance in its different phases, that is water vapor, liquid water, and ice. As will be later seen, other choices of state variables are also possible.

The laws of atmospheric motion, as far as they are required in this book, are considered to be known. If necessary, additional details will be given at the appropriate places. A suitable reference book for our purposes is Dynamics of the Atmosphere by the present authors, which from now on will be abbreviated by DA. Of course, any other suitable textbook on atmospheric dynamics might be just as satisfactory.

Description of the atmospheric thermodynamic state

The physical object to be investigated is called the thermodynamic system, anything interacting with the system is defined as the surroundings. The thermodynamic state defines all properties of the system that have to be determined from measurements or from calculations.