Chapter 5 - Water in the atmosphere

Abstract

Water vapour is the most important variable constituent of the atmosphere. For this reason, water vapour is normally treated as a separate constituent alongside the well-mixed gases (nitrogen, oxygen, and argon) that make up the rest of the bulk of the atmosphere. When water vapour cools down enough it will condense to a liquid or even freeze to a solid: it will experience a phase transition. In this chapter we examine the thermodynamics of phase transitions and derive its key result, the Clausius–Clapeyron equation, which describes the equilibrium pressure of the vapour at coexistence with the liquid. Although we use these results to understand the properties of water in the Earth's atmosphere, we can use them equally to understand freezing of ▪ in the Martian atmosphere or the formation of methane, ▪, clouds on Titan.