In the US, the National Weather Service measures the actual rate of evaporation from a standardized "pan" open water surface outdoors, at various locations nationwide. When a net evaporation occurs, the body of water will undergo a net cooling directly related to the loss of water. The amount of water vapor in the air determines how frequently molecules will return to the surface. Liquid water that becomes water vapor takes a parcel of heat with it, in a process called evaporative cooling. The aggregate measurement of this kinetic energy transfer is defined as thermal energy and occurs only when there is differential in the temperature of the water molecules. Each individual water molecule which transitions between a more associated (liquid) and a less associated (vapor/gas) state does so through the absorption or release of kinetic energy. Whenever a water molecule leaves a surface and diffuses into a surrounding gas, it is said to have evaporated. Water vapor can also be indirect evidence supporting the presence of extraterrestrial liquid water in the case of some planetary mass objects. Likewise the detection of extrasolar water vapor would indicate a similar distribution in other planetary systems. 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. Use of water vapor, as steam, has been important for cooking, and as a major component in energy production and transport systems since the industrial revolution. It is less dense than most of the other constituents of air and triggers convection currents that can lead to clouds and fog.īeing a component of Earth's hydrosphere and hydrologic cycle, it is particularly abundant in Earth's atmosphere, where it acts as a greenhouse gas and warming feedback, contributing more to total greenhouse effect than non-condensable gases such as carbon dioxide and methane. Under typical atmospheric conditions, water vapor is continuously generated by evaporation and removed by condensation. Water vapor is transparent, like most constituents of the atmosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. It is one state of water within the hydrosphere. Water vapor, water vapour or aqueous vapor is the gaseous phase of water.
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