Our atmosphere is basically a two-component system. One component is dry air and the other is water existing in vapor and possibly one of the condensed phases (liquid water or ice). According to Dalton's law, in a mixture of ideal gases each gas can be assumed to behave as if the other gases were absent. As such, in a mixture of dry air, water vapor, and a condensed phase, the “water” system (water vapor + condensed phase) can be treated as being independent of the dry air. In this case, the concepts developed in the previous chapter (for the one-component heterogeneous system “water”) are valid for the two-component heterogeneous system “dry air + water”. We will call the system consisting of dry air and water vapor “moist air” and it can be unsaturated or saturated with water vapor. Since liquid water is absent, moist air is a two-component system with one phase present. As such, according to equation (6.1) we need three state variables to specify the system's state. Usually these variables are taken to be pressure, temperature, and a new variable (to be defined soon) called mixing ratio. If the condensed phase is present and in equilibrium with the vapor phase, then two variables are needed (typically temperature and pressure).

For clarity in our notation we will use the subscripts d, w, v to indicate dry air, liquid water, and water vapor, respectively. The only exception will be for the vapor pressure, which we will denote simply as e. Variables with no subscripts will correspond to a mixture of dry air and water vapor.