Editing Water vapor (section)

== Properties ==

=== Evaporation ===
Whenever a water molecule leaves a surface and diffuses into a surrounding gas, it is said to have [[Evaporation|evaporated]]. 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]]. 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]].<ref>{{harvp|Schroeder|2000|p=36}}</ref> 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.<ref>{{cite web|url=http://www.grow.arizona.edu/Grow--GrowResources.php?ResourceId%3D208 |access-date=April 7, 2008 |url-status=dead |archive-url=https://web.archive.org/web/20080412215652/http://www.grow.arizona.edu/Grow--GrowResources.php?ResourceId=208 |archive-date=April 12, 2008 |title=Geotechnical, Rock and Water Resources Library - Grow Resource - Evaporation }}</ref> The measurements range from under 30 to over 120&nbsp;inches per year. Formulas can be used for calculating the rate of evaporation from a water surface such as a swimming pool.<ref>{{cite web|url=http://www.thermexcel.com/english/program/pool.htm|title=swimming, pool, calculation, evaporation, water, thermal, temperature, humidity, vapor, excel|access-date=February 26, 2016}}</ref><ref>{{cite web |url=http://www.rlmartin.com/rspec/whatis/equations.htm |archive-url=https://web.archive.org/web/20080324232701/http://www.rlmartin.com/rspec/whatis/equations.htm |archive-date=March 24, 2008 |title=Summary of Results of all Pool Evaporation Rate Studies |publisher=R. L. Martin & Associates}}</ref> In some countries, the evaporation rate far exceeds the [[Precipitation (meteorology)|precipitation]] rate.

Evaporative cooling is restricted by [[Standard temperature and pressure|atmospheric conditions]]. [[Humidity]] is the amount of water vapor in the air. The vapor content of air is measured with devices known as [[hygrometer]]s. 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&nbsp;grams per cubic metre in dry air to 30&nbsp;grams per cubic metre (0.03 ounce per cubic foot) when the vapor is saturated at 30&nbsp;°C.<ref>{{cite web|url=http://www.britannica.com/eb/article-53259/climate#292984.hook|title=climate - meteorology|website=Encyclopædia Britannica|access-date=February 26, 2016}}</ref>

{{multiple image
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 |image1    = Meteorite Recovery Antarctica (retouched).jpg
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 |caption1  = Recovery of [[meteorite]]s in Antarctica ([[ANSMET]])
 |caption2  = [[Electron micrograph]] of freeze-etched [[capillary]] tissue
}}

=== Sublimation ===
{{unreferenced-section|date=March 2024}}
[[Sublimation (phase transition)|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.<ref>{{Cite web |title=The USAP Portal: Science and Support in Antarctica - About the Continent |url=https://www.usap.gov/aboutthecontinent/#:~:text=Antarctica%20is%20the%20coldest,%20windiest,sq%20km%20(5.4%20million%20sq. |access-date=2024-06-20 |website=www.usap.gov}}</ref> As a result, there are large areas where [[Millennium|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 [[meteorite]]s 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 microscope|scanning electron microscopy]]. Typically the specimens are prepared by [[cryofixation]] and [[Electron microscope|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 bilayer]]s with very low degrees of distortion.

=== Condensation ===
[[File:Above the Clouds.jpg|thumb|Clouds, formed by condensed water vapor]]

Water vapor will only condense onto another surface when that surface is cooler than the [[dew point]] temperature, or when the [[saturation vapor pressure|water vapor equilibrium]] in air has been exceeded. When water vapor condenses onto a surface, a net warming occurs on that surface.<ref>{{Cite journal |last1=Held |first1=Isaac M. |last2=Soden |first2=Brian J. |title=Water vapor feedback and global warming |date=November 2000 |journal=Annual Review of Energy and the Environment|volume=25|issue=1|pages=441–475 |doi=10.1146/annurev.energy.25.1.441|issn=1056-3466}}</ref> The water molecule brings heat energy with it. In turn, the temperature of the atmosphere drops slightly.<ref>{{harvp|Schroeder|2000|p=19}}</ref> In the atmosphere, condensation produces clouds, fog and precipitation (usually only when facilitated by [[cloud condensation nuclei]]). 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 (meteorology)|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 process|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 vapour 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 [[hydrocarbon]]s 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 (certain [[polyurethane]] foams and [[cyanoacrylate]] glues cure with exposure to atmospheric humidity) 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 [[humidity indicator card|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 by water|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.

{| class="wikitable sortable" style="text-align: center; font-size: 85%; width: auto; table-layout: fixed;"
|-
! style="width:12em" |
! medium
! temperature range (degC)
! measurement [[Measurement uncertainty|uncertainty]]
! typical measurement frequency
! system cost
! notes
|-
! style="text-align:left;"| [[Hygrometer#Sling psychrometer|Sling psychrometer]]
| air
| −10 to 50
| low to moderate
| hourly
| low
|
|-
! style="text-align:left;"| Satellite-based spectroscopy
| air
| −80 to 60
| low
|
| very high
|
|-
! style="text-align:left;"| [[Hygrometer#Capacitive|Capacitive]] sensor
| air/gases
| −40 to 50
| moderate
| 2 to 0.05&nbsp;Hz
| medium
| prone to becoming saturated/contaminated over time
|-
! style="text-align:left;"| Warmed capacitive sensor
| air/gases
| −15 to 50
| moderate to low
| 2 to 0.05&nbsp;Hz (temp dependant)
| medium to high
| prone to becoming saturated/contaminated over time
|-
! style="text-align:left;"| [[hygrometer#Resistive|Resistive]] sensor
| air/gases
| −10 to 50
| moderate
| 60 seconds
| medium
| prone to contamination
|-
! style="text-align:left;"| Lithium chloride [[dewcell]]
| air
| −30 to 50
| moderate
| continuous
| medium
| see [[dewcell]]
|-
! style="text-align:left;"| [[Cobalt(II) chloride]]
| air/gases
| 0 to 50
| high
| 5 minutes
| very low
| often used in [[Humidity indicator card]]
|-
! style="text-align:left;"| [[Absorption spectroscopy]]
| air/gases
|
| moderate
|
| high
|
|-
! style="text-align:left;"| Aluminum oxide
| air/gases
|
| moderate
|
| medium
| see [[Moisture analysis]]
|-
! style="text-align:left;"| Silicon oxide
| air/gases
|
| moderate
|
| medium
| see [[Moisture analysis]]
|-
! style="text-align:left;"| Piezoelectric sorption
| air/gases
|
| moderate
|
| medium
| see [[Moisture analysis]]
|-
! style="text-align:left;"| Electrolytic
| air/gases
|
| moderate
|
| medium
| see [[Moisture analysis]]
|-
! style="text-align:left;"| [[Hygrometer#Hair tension hygrometers|Hair tension]]
| air
| 0 to 40
| high
| continuous
| low to medium
| Affected by temperature. Adversely affected by prolonged high concentrations
|-
! style="text-align:left;"| Nephelometer
| air/other gases
|
| low
|
| very high
|
|-
! style="text-align:left;"| [[Goldbeater's skin]] (Cow Peritoneum)
| air
| −20 to 30
| moderate (with corrections)
| slow, slower at lower temperatures
| low
| ref:WMO Guide to Meteorological Instruments and Methods of Observation No. 8 2006, (pages 1.12–1)
|-
! style="text-align:left;"| Lyman-alpha
|
|
|
| high frequency
| high
| http://amsglossary.allenpress.com/glossary/search?id=lyman-alpha-hygrometer1 Requires frequent calibration
|-
! style="text-align:left;"| [[hygrometer#Gravimetric|Gravimetric]] Hygrometer
|
|
| very low
|
| very high
| often called primary source, national independent standards developed in US, UK, EU & Japan
|- class="sortbottom"
!
! medium
! temperature range (degC)
! measurement [[Measurement uncertainty|uncertainty]]
! typical measurement frequency
! system cost
! notes
|}

=== Impact on air density ===
Water vapor is lighter or less [[Density of air|dense than dry air]].<ref>{{cite news| url=https://www.washingtonpost.com/blogs/capital-weather-gang/wp/2013/08/05/why-dry-air-is-heavier-than-humid-air/| title=Why dry air is heavier than humid air| newspaper=The Washington Post| date=August 5, 2013| access-date=28 December 2014| author=Williams, Jack}}</ref><ref>{{cite web| url=http://www.wwrf.org/humidity101.htm| archive-url=https://archive.today/20130416080406/http://www.wwrf.org/humidity101.htm| url-status=dead| archive-date=16 April 2013| title=Humidity 101| publisher=World Water rescue Foundation| access-date=28 December 2014}}</ref> At equivalent temperatures it is buoyant with respect to dry air, whereby the density of dry air at [[standard temperature and pressure]] (273.15 K, 101.325 kPa) is 1.27&nbsp;g/L and water vapor at standard temperature has a [[vapor pressure]] of 0.6 kPa and the much lower density of 0.0048&nbsp;g/L.

==== Calculations ====
[[File:dewpoint.jpg|right|frameless|upright=1.15]]

Water vapor and dry air density calculations at 0&nbsp;°C:
* The [[molar mass]] of water is {{nowrap|18.02 g/mol}}, as calculated from the sum of the [[atomic mass]]es of its constituent [[atoms]].
* The average molar mass of air (approx. 78% nitrogen, N<sub>2</sub>; 21% oxygen, O<sub>2</sub>; 1% other gases) is {{nowrap|28.57 g/mol}} at standard temperature and pressure ([[Standard temperature and pressure|STP]]).
* Obeying [[Avogadro's Law]] and the [[ideal gas law]], [[Humidity|moist air]] will have a lower density than dry air. At max. saturation (i. e. rel. humidity = 100% at 0&nbsp;°C) the density will go down to 28.51 g/mol.
* STP conditions imply a temperature of 0&nbsp;°C, at which the ability of water to become vapor is very restricted. Its [[concentration]] in air is very low at 0&nbsp;°C. The red line on the chart to the right is the maximum concentration of water vapor expected for a given temperature. The water vapor concentration increases significantly as the temperature rises, approaching 100% ([[steam]], pure water vapor) at 100&nbsp;°C. However the difference in densities between air and water vapor would still exist (0.598 vs. 1.27 g/L).

==== At equal temperatures ====
At the same temperature, a column of dry air will be denser or heavier than a column of air containing any water vapor, the molar mass of diatomic [[nitrogen]] and diatomic [[oxygen]] both being greater than the molar mass of water. Thus, any volume of dry air will sink if placed in a larger volume of moist air. Also, a volume of moist air will rise or be [[Buoyancy|buoyant]] if placed in a larger region of dry air. As the temperature rises the proportion of water vapor in the air increases, and its buoyancy will increase. The increase in buoyancy can have a significant atmospheric impact, giving rise to powerful, moisture rich, upward air currents when the air temperature and sea temperature reaches 25&nbsp;°C or above. This phenomenon provides a significant driving force for [[Cyclone|cyclonic]] and [[Anticyclone|anticyclonic]] weather systems (typhoons and hurricanes).

=== Respiration and breathing ===
Water vapor is a by-product of [[respiration (physiology)|respiration]] in plants and animals. Its contribution to the pressure, increases as its concentration increases. Its [[partial pressure]] contribution to air pressure increases, lowering the partial pressure contribution of the other atmospheric gases [[partial pressure|(Dalton's Law)]]. The total air pressure must remain constant. The presence of water vapor in the air naturally dilutes or displaces the other air components as its concentration increases.

This can have an effect on respiration. In very warm air (35&nbsp;°C) the proportion of water vapor is large enough to give rise to the stuffiness that can be experienced in humid jungle conditions or in poorly ventilated buildings.

=== Lifting gas ===
Water vapor has lower density than that of [[air]] and is therefore [[buoyant]] in air but has lower vapor pressure than that of air. When water vapor is used as a [[lifting gas]] by a [[thermal airship]] the water vapor is heated to form steam so that its vapor pressure is greater than the surrounding air pressure in order to maintain the shape of a theoretical "steam balloon", which yields approximately 60% the lift of helium and twice that of hot air.<ref>{{cite web |last=Goodey |first=Thomas J. |title=Steam Balloons and Steam Airships |url=http://www.flyingkettle.com/jbfa.htm |access-date=August 26, 2010 |archive-date=August 30, 2010 |archive-url=https://web.archive.org/web/20100830180350/http://www.flyingkettle.com/jbfa.htm |url-status=dead }}</ref>

=== General discussion ===
The amount of water vapor in an atmosphere is constrained by the restrictions of partial pressures and temperature. Dew point temperature and relative humidity act as guidelines for the process of water vapor in the [[water cycle]]. Energy input, such as sunlight, can trigger more evaporation on an ocean surface or more sublimation on a chunk of ice on top of a mountain. The ''balance'' between condensation and evaporation gives the quantity called [[vapor pressure|vapor partial pressure]].

The maximum partial pressure (''saturation pressure'') of water vapor in air varies with temperature of the air and water vapor mixture. A variety of empirical formulas exist for this quantity; the most used reference formula is the [[Goff-Gratch equation]] for the SVP over liquid water below zero degrees Celsius:

:<math>\begin{align}
\log_{10} \left( p \right) =
& -7.90298 \left( \frac{373.16}{T}-1 \right) + 5.02808 \log_{10} \frac{373.16}{T} \\
& - 1.3816 \times 10^{-7} \left( 10^{11.344 \left( 1-\frac{T}{373.16} \right)} -1 \right) \\
& + 8.1328 \times 10^{-3} \left( 10^{-3.49149 \left( \frac{373.16}{T}-1 \right)} -1 \right) \\
& + \log_{10} \left( 1013.246 \right)
\end{align}</math>

where {{mvar|T}}, temperature of the moist air, is given in units of [[kelvin]], and {{mvar|p}} is given in units of [[millibar]]s ([[hectopascal]]s).

The formula is valid from about −50 to 102&nbsp;°C; however there are a very limited number of measurements of the vapor pressure of water over supercooled liquid water. There are a number of other formulae which can be used.<ref>{{cite web|url=http://cires.colorado.edu/~voemel/vp.html|title=Water Vapor Pressure Formulations|access-date=February 26, 2016}}</ref>

Under certain conditions, such as when the boiling temperature of water is reached, a net evaporation will always occur during standard atmospheric conditions regardless of the percent of relative humidity. This immediate process will dispel massive amounts of water vapor into a cooler atmosphere.

[[Exhalation|Exhale]]d air is almost fully at equilibrium with water vapor at the body temperature. In the cold air the exhaled vapor quickly condenses, thus showing up as a fog or [[mist]] of water droplets and as condensation or frost on surfaces. Forcibly condensing these water droplets from exhaled breath is the basis of [[exhaled breath condensate]], an evolving medical diagnostic test.

Controlling water vapor in air is a key concern in the [[HVAC|heating, ventilating, and air-conditioning]] (HVAC) industry. [[Thermal comfort]] depends on the moist air conditions. Non-human comfort situations are called [[refrigeration]], and also are affected by water vapor. For example, many food stores, like supermarkets, utilize open chiller cabinets, or ''food cases'', which can significantly lower the water vapor pressure (lowering humidity). This practice delivers several benefits as well as problems.