Basic definitions

Dew point temperature

Cooling the temperature of the air so it is saturated with water vapour (i.e. 100% relative humidity) gives the dew point temperature. At this point the surrounding air cannot hold more humidity in gaseous form.

For practical reasons, referred to as water when above 0 °C (+32 °F) and ice when below 0 °C (+32 °F).

Expressed in degrees Celsius (°C) or degrees Fahrenheit (°F).

Water vapour pressure

The pressure exerted by the water vapour contained in any mixture of gases. The total pressure exerted by the gas mixture is the sum of the pressures exerted by its components – including the water vapour. Water vapour pressure varies in direct proportion to the total gas.

Expressed in units of pressure such as pascal (Pa), millibar (mbar) or millimetre of mercury (mmHg).

Saturation water vapour pressure

The maximum pressure of water vapour that can exist in a gas at a given temperature.

Expressed in units of pressure as partial water vapour pressure.

Parts per million by volume

The ratio of the water vapour pressure to the total gas pressure. Expressed as parts per million by volume (ppm(v)) or volume per million (vpm).

Parts per million by weight

Modified figure according to the ratio of the molecular weight of water vapour to the molecular weight of the carrier gas mixture. Expressed as parts per million weight (ppm(w)).


Quantity representation of the concentration of water vapour (the gaseous state of water) in the air. Water can be solid (ice), liquid (water) or gas (vapour). Humidity is influenced by the temperature and pressure of the measured environment.

Absolute humidity

The mass of water vapour in a unit volume of moist gas at a given temperature and pressure.

Measured and expressed as grams per cubic metre (g/m³), milligrams per litre (mg/L), milligrams per cubic metre (mg/m³) or pounds of water per million cubic feet of gas (lb/MMSCF).

Relative humidity

The ratio of actual water vapour pressure in the gas to the saturation water vapour pressure at the same temperature.