Before embarking on a discussion of humidity and humidification, a number of key physical principles must first be defined:

Water vapour consists of water molecules in their gaseous form below its critical temperature, the temperature above which the water vapour cannot be liquefied no matter what pressure it is placed under.

Gaseous water molecules have kinetic energy and exert a pressure known as the vapour pressure.

The saturated vapour pressure (SVP) is the maximum pressure exerted by gas molecules at equilibrium at a given temperature.

Humidity is the amount of water vapour present in a sample of gas. It can be expressed in two ways:

As an absolute humidity, the mass of water vapour present per unit volume of gas (g/m3or mg/litre).

As a ratio known as the relative humidity, the ratio of water vapour present in the gas relative to the maximum water vapour the gas could contain (%). Relative humidity is more commonly used and is more easily measured.

The amount of water vapour a given volume of air can hold increases as its temperature increases, and therefore the relative humidity is affected by temperature. At a relative humidity of 100%, air is said to be fully saturated with water vapour. As the temperature of a gas decreases, the maximum amount of water vapour it can contain also decreases, increasing the relative humidity above 100% and causing condensation to occur. The temperature at which condensation occurs is known as the dew point (°C).