Water vapor

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. 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 and fog.
Being 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. 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.
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 can also be indirect evidence supporting the presence of extraterrestrial liquid water in the case of some planetary mass objects.
Water vapor, which reacts to temperature changes, is referred to as a "feedback", because it amplifies the effect of forces that initially cause the warming. Therefore, it is a greenhouse gas.

Properties

Evaporation

Whenever a water molecule leaves a surface and diffuses into a surrounding gas, it is said to have evaporated. Each individual water molecule which transitions between a more associated and a less associated state does so through the absorption or release of kinetic energy. 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. Liquid water that becomes water vapor takes a parcel of heat with it, in a process called evaporative cooling. The amount of water vapor in the air determines how frequently molecules will return to the surface. When a net evaporation occurs, the body of water will undergo a net cooling directly related to the loss of water.
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. Others do likewise around the world. The US data is collected and compiled into an annual evaporation map. The measurements range from under 30 to over 120 inches per year. Formulas can be used for calculating the rate of evaporation from a water surface such as a swimming pool. In some countries, the evaporation rate far exceeds the precipitation rate.
Evaporative cooling is restricted by atmospheric conditions. Humidity is the amount of water vapor in the air. The vapor content of air is measured with devices known as hygrometers. The measurements are usually expressed as specific humidity or percent relative humidity. The temperatures of the atmosphere and the water surface determine the equilibrium vapor pressure; 100% relative humidity occurs when the partial pressure of water vapor is equal to the equilibrium vapor pressure. This condition is often referred to as complete saturation. Humidity ranges from 0 grams per cubic metre in dry air to 30 grams per cubic metre when the vapor is saturated at 30 °C.

Sublimation

is the process by which water molecules directly leave the surface of ice without first becoming liquid water. Sublimation accounts for the slow mid-winter disappearance of ice and snow at temperatures too low to cause melting. Antarctica shows this effect to a unique degree because it is by far the continent with the lowest rate of precipitation on Earth. As a result, there are large areas where millennial layers of snow have sublimed, leaving behind whatever non-volatile materials they had contained. This is extremely valuable to certain scientific disciplines, a dramatic example being the collection of meteorites that are left exposed in unparalleled numbers and excellent states of preservation.
Sublimation is important in the preparation of certain classes of biological specimens for scanning electron microscopy. Typically the specimens are prepared by cryofixation and freeze-fracture, after which the broken surface is freeze-etched, being eroded by exposure to vacuum until it shows the required level of detail. This technique can display protein molecules, organelle structures and lipid bilayers with very low degrees of distortion.

Condensation

Water vapor will only condense onto another surface when that surface is cooler than the dew point temperature, or when the water vapor equilibrium in air has been exceeded. When water vapor condenses onto a surface, a net warming occurs on that surface. The water molecule brings heat energy with it. In turn, the temperature of the atmosphere drops slightly. In the atmosphere, condensation produces clouds, fog and precipitation. The dew point of an air parcel is the temperature to which it must cool before water vapor in the air begins to condense. Condensation in the atmosphere forms cloud droplets.
Also, a net condensation of water vapor occurs on surfaces when the temperature of the surface is at or below the dew point temperature of the atmosphere. Deposition is a phase transition separate from condensation which leads to the direct formation of ice from water vapor. Frost and snow are examples of deposition.
There are several mechanisms of cooling by which condensation occurs:
1) Direct loss of heat by conduction or radiation.
2) Cooling from the drop in air pressure which occurs with uplift of air, also known as adiabatic cooling.
Air can be lifted by mountains, which deflect the air upward, by convection, and by cold and warm fronts.
3) Advective cooling - cooling due to horizontal movement of air.

Importance and uses

Provides water for plants and animals: Water vapor gets converted to rain and snow that serve as a natural source of water for plants and animals.
Controls evaporation: Excess water vapor in the air decreases the rate of evaporation.
Determines climatic conditions: Excess water vapor in the air produces rain, fog, snow etc. Hence, it determines climatic conditions.
Chemical reactions

A number of chemical reactions have water as a product. If the reactions take place at temperatures higher than the dew point of the surrounding air the water will be formed as vapor and increase the local humidity, if below the dew point local condensation will occur. Typical reactions that result in water formation are the burning of hydrogen or hydrocarbons in air or other oxygen containing gas mixtures, or as a result of reactions with oxidizers.
In a similar fashion other chemical or physical reactions can take place in the presence of water vapor resulting in new chemicals forming such as rust on iron or steel, polymerization occurring or forms changing such as where anhydrous chemicals may absorb enough vapor to form a crystalline structure or alter an existing one, sometimes resulting in characteristic color changes that can be used for measurement.

Measurement

Measuring the quantity of water vapor in a medium can be done directly or remotely with varying degrees of accuracy. Remote methods such electromagnetic absorption are possible from satellites above planetary atmospheres. Direct methods may use electronic transducers, moistened thermometers or hygroscopic materials measuring changes in physical properties or dimensions.

	medium	temperature range	measurement uncertainty	typical measurement frequency	system cost	notes
Sling psychrometer	air	−10 to 50	low to moderate	hourly	low
Satellite-based spectroscopy	air	−80 to 60	low		very high
Capacitive sensor	air/gases	−40 to 50	moderate	2 to 0.05 Hz	medium	prone to becoming saturated/contaminated over time
Warmed capacitive sensor	air/gases	−15 to 50	moderate to low	2 to 0.05 Hz	medium to high	prone to becoming saturated/contaminated over time
Resistive sensor	air/gases	−10 to 50	moderate	60 seconds	medium	prone to contamination
Lithium chloride dewcell	air	−30 to 50	moderate	continuous	medium	see dewcell
Cobalt chloride	air/gases	0 to 50	high	5 minutes	very low	often used in Humidity indicator card
Absorption spectroscopy	air/gases		moderate		high
Aluminum oxide	air/gases		moderate		medium	see Moisture analysis
Silicon oxide	air/gases		moderate		medium	see Moisture analysis
Piezoelectric sorption	air/gases		moderate		medium	see Moisture analysis
Electrolytic	air/gases		moderate		medium	see Moisture analysis
Hair tension	air	0 to 40	high	continuous	low to medium	Affected by temperature. Adversely affected by prolonged high concentrations
Nephelometer	air/other gases		low		very high
Goldbeater's skin	air	−20 to 30	moderate	slow, slower at lower temperatures	low	ref:WMO Guide to Meteorological Instruments and Methods of Observation No. 8 2006,
Lyman-alpha				high frequency	high	http://amsglossary.allenpress.com/glossary/search?id=lyman-alpha-hygrometer1 Requires frequent calibration
Gravimetric Hygrometer			very low		very high	often called primary source, national independent standards developed in US, UK, EU & Japan
	medium	temperature range	measurement uncertainty	typical measurement frequency	system cost	notes