The relative humidity is defined as

• the mass of water vapour present in 1 m3 of dry air
• the mass of water vapour present in 1 kg of dry air
• the ratio of the actual mass of water vapour in a unit mass of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature and pressure.
• the ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure