All natural air is humid; dry air must be artificially obtained. Humid air is the most important gaseous
mixture in science and engineering: from breathing to meteorology and air conditioning. The atmosphere
is a heterogeneous mixture of gases (the air, properly) and liquid and solid particles in suspension (clouds,
dust, microorganisms...).
Humid air can be modelled as a binary gas mixture of dry air and water vapour because none of the
components of dry air is highly soluble in liquid water, and dry-air composition can be considered
invariable. The two-phase binary mixture of water and air is not an ideal mixture in both phases, but for
the gaseous phase the ideal mixture model is very appropriate, and Raoult's law for the two-phase
equilibrium of water still valid. As for the liquid phase, Henry law can be applied, but, if one takes into
account that air scarcely dissolves in water (at 100 kPa and 288 K xLN2=10⋅10-6 and xLO2=10⋅10-6
,increasing with pressure and decreasing with temperature), one may neglect that and consider that
equilibrium is with pure water. Notice, by the way, that oxygen dissolves more than nitrogen in water.
When does air need to be considered humid? In many engineering problems, air does not change
composition or the changes are irrelevant; in those cases there is no need for humid air formulation and
air can be treated as a pure substance (from aircraft lifting to most heat transfer problems). But in some
cases the change in composition of the air may be crucial, either because condensation occurs or because
Humid air
air entrains water from some source, as from vital meteorological processes, to artificial air conditioning
and evaporative cooling.