TheAirhumid previous chapter has clarified how mixtures of ideal gases are to be treated. Definitions for characterising the composition of the mixture have been analysed, as has the irreversibility that occurs when different ideal gases are mixed. In this chapter, the focus is now on humid air, a technically relevant mixture that is used in a wide range of technical applications. For example, combustion plants are operated with ambient air, or fresh/humid air is used for air conditioning in buildings. Its components at atmospheric conditions are as follows: Dry air, i.e. a mixture of \(\text {N}_{2}\) and \(\text {O}_{2}\) and with a small amount of \(\text {CO}_{2}\) . This type of mixture has been treated in Chap. 19 . In most cases, these components are regarded as ideal gases, so that the principles of Chap. 19 can be easily applied. Water can occur in humid air as a liquid, i.e. as mist or rain, as a solid, i.e. as ice or snow, and as vapour. Chapter 18 has shown, that water as a real fluid can be subject to a phase change. Our everyday experience shows that this phase change takes place under atmospheric conditions such as rain or snow. It is therefore necessary to clarify how the mixture can be described thermodynamically when the vapour condenses and forms a liquid.