Absolute and Relative Humidity
The absolute humidity expresses the amount of water vapor present in the air. Absolute humidity is a ratio of mass of water vapor to the unit volume of total air. The composition of a unit volume of dry air, at standard pressure (101.3 kPa) and temperature (298.2 K), is: Nitrogen = 78.09%,
Oxygen = 20.95%, and Trace gases = 0.96%. Under such standard temperature and pressure conditions, the partial pressure of the water vapor at equilibrium between air and pure water is equal to 3.17 kPa and partial pressure of remaining components is 98.13 kPa. At this equilibrium condition, the mass exchange between water and air is not possible. If we express this equilibrium in terms of the total mass of water vapor per unit volume, the ratio is equal to 22.99 g/m3 , which is called the absolute humidity. Therefore, the maximum amount of water that can be vaporized into air is 22.99 g/m3 , under standard pressure and temperature conditions. Additional vaporization is only possible when either the temperature or the total air pressure is changed. The vapor pressure under the stated equilibrium condition is called saturated vapor pressure.
Relative humidity (RH) illustrates the state of thermodynamic equilibrium between air and water. RH is the ratio of partial pressure of water vapor to the saturated vapor pressure under the same equilibrium conditions. It can also be defined as the ratio of absolute humidity in equilibrium with solution to the absolute humidity in equilibrium with pure water at the same temperature. It is normally expressed in percentage. RH is an important material constant state variable in understanding the behavior of partly saturated soils under the varying dynamic climatic conditions.
The solubility of air in water is also an important aspect which is important to understand the limitations of some of the measuring systems used for measuring the unsaturated state variables viz. multi-step outflow technique, which is beyond the scope of the present lectures.