From AMS Glossary
(Also called aqueous vapor, moisture.) Water substance in vapor form; one of the most important of all constituents of the atmosphere.
Its amount varies widely in space and time due to the great variety of both "sources" of evaporation and "sinks" of condensation that provide active motivation to the hydrologic cycle. Approximately half of all of the atmospheric water vapor is found below 2-km altitude, and only a minute fraction of the total occurs above the tropopause. Water vapor is important not only as the raw material for cloud and rain and snow, but also as a vehicle for the transport of energy (latent heat) and as a regulator of planetary temperatures through absorption and emission of radiation, most significantly in the thermal infrared (the greenhouse effect). The amount of water vapor present in a given air sample may be measured in a number of different ways, involving such concepts as absolute humidity, mixing ratio, dewpoint, relative humidity, specific humidity, and vapor pressure.