Water vapor (H2O)
St Johns Fog.jpg
Invisible water vapor condenses to form
visible clouds of liquid rain droplets
Liquid state Water
Solid state Ice
Molecular formula H2O
Molar mass 18.01528(33) g/mol
Melting point 0.00 °C (273.15 K)[2]
Boiling point 99.98 °C (373.13 K)[2]
specific gas constant 461.5 J/(kg·K)
Heat of vaporization 2.27 MJ/kg
Heat capacity at 300 K 1.864 kJ/(kg·K)[3]

Water vapor, water vapour or aqueous vapor is the gaseous phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Water vapor is transparent, like most constituents of the atmosphere.[4] Under typical atmospheric conditions, water vapor is continuously generated by evaporation and removed by condensation. It is less dense than most of the other constituents of air and triggers convection currents that can lead to clouds.

Being a component of Earth's hydrosphere and hydrologic cycle, it is particularly abundant in Earth's atmosphere, where it acts as the most potent greenhouse gas, stronger than other gases such as carbon dioxide and methane. Use of water vapor, as steam, has been important to humans for cooking and as a major component in energy production and transport systems since the industrial revolution.

Water vapor is a relatively common atmospheric constituent, present even in the solar atmosphere as well as every planet in the Solar System and many astronomical objects including natural satellites, comets and even large asteroids. Likewise the detection of extrasolar water vapor would indicate a similar distribution in other planetary systems. Water vapor is significant in that it can be indirect evidence supporting the presence of extraterrestrial liquid water in the case of some planetary mass objects.