Water vapor
89547
223988963
2008-07-06T21:02:40Z
Rjstott
182
/* Lightning generation */ caterbility not the correct word
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! {{chembox header}} | Water vapor
|-
| [[Systematic name]]
| Water Vapor
|-
| [[Liquid State]]
| Water
|-
| [[Solid]] state
| Ice
|-
! {{chembox header}} | Properties<ref>Lide, David. ''<u> CRC Handbook of Chemistry and Physics</u>, 73rd ed''. 1992, CRC Press.</ref>
|-
| [[Melting point]]
| 0 °C
|-
| [[Boiling point]]
| 100 °C
|-
|[[individual gas constant]]
|461.5 J/(kg·K)
|-
|[[latent heat of evaporation]]
|2.27 MJ/kg
|-
|[[molecular weight]]
|18.02 g/mol
|-
|[[specific heat capacity]]
|1.84 kJ/(kg·K)
|}
'''Water vapor''' or '''water vapour''' (see [[American and British English spelling differences|spelling differences]]), also ''aqueous vapor'', is the [[gas]] phase of [[water (molecule)|water]]. Water [[vapor]] is one [[Phase (matter)|state]] of the [[water cycle]] within the [[hydrosphere]].<ref>Technically called the ''Hydrologic cycle'', from U.S. Geologic Survey. [http://ga.water.usgs.gov/edu/watercycle.html Water Cycle.] Retrieved on 2006-10-24.</ref> Water vapor can be produced from the [[evaporation]] of liquid [[water]] or from the [[sublimation (chemistry)|sublimation]] of [[ice]]. Under normal atmospheric conditions,<ref>Normal atmosphere means in the Earth's troposphere under a large variety of temperatures and pressures that are ''naturally'' occurring anywhere and at anytime.</ref> water vapor is continuously generated by evaporation and removed by [[condensation]].
==General properties of water vapor==
===Evaporation/sublimation===
Whenever a water molecule leaves a surface, it is said to have 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>Schroeder, David. ''<u>Thermal Physics</u>''. 2000, Addison Wesley Longman. p36</ref> The amount of water vapor in the air determines how fast each molecule will return back to the surface. When a net evaporation occurs, the body of water will undergo a net cooling directly related to the loss of water.<ref>This remains true as long as surface water exists, or water that is capable of being evaporated exists. Otherwise, with a net [[heat flux]] on the observed body when the water completely evaporates, ''then'' the temperature of the observed body begins to rise. ''(see [[Thermodynamics]])''</ref>
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.[http://www.grow.arizona.edu/Grow--GrowResources.php?ResourceId=208] The measurements range from under 30 to over 120 inches per year. Formulas for calculating the rate of evaporation from a water surface such as a swimming pool of can be found here[http://www.thermexcel.com/english/program/pool.htm] and here[http://www.rlmartin.com/rspec/whatis/equations.htm]
Evaporative cooling is restricted by [[Standard conditions for temperature and pressure|atmospheric conditions]]. [[humidity|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 gram per cubic metre in dry air to 30 grams per cubic metre (0.03 ounce per cubic foot) when the vapour is saturated at 30 °C.[http://www.britannica.com/eb/article-53259/climate#292984.hook]
(See also [http://www.tis-gdv.de/tis_e/misc/klima.htm Absolute Humidity table])
Another form of evaporation is [[Sublimation (chemistry)|sublimation]], by which water molecules become gaseous directly from 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.
=== Condensation ===
[[Image:Above the Clouds.jpg|thumb|right|200px|Clouds, formed by condensed water vapor.]]
Water vapor will only condense onto another surface when that surface is cooler than the temperature of the water vapor, 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>See [[Thermodynamics]], as it is a process of energy transfer. This should not be confused with precipitates falling onto a surface.</ref> The water molecule brings a parcel of heat with it. In turn, the temperature of the atmosphere drops slightly.<ref>The atmosphere is a heat bath, heat is transferred by molecular conduction.</ref> <ref>Schroeder, p19.</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.
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, the direct formation of ice from water vapor, is a type of condensation. [[Frost]] and [[snow]] are examples of [[Deposition (meteorology)|deposition]].
===Water vapor [[density]]===
Water vapor is lighter or less dense than dry air. At equivalent temperatures it is buoyant with respect to dry air.
====Water vapor and dry air density calculations at 0°C====
[[Image:dewpoint.jpg|right]]
The [[molecular mass]] or weight of water is 18.02g/mol, as calculated from the sum of the [[atomic masses]] of its constituent [[atoms]].
The average molecular mass of [[air]] (Approx. 79% nitrogen, N<sub>2</sub>; 21% Oxygen, 0<sub>2</sub>) is 28.57g/mol at standard temperature and pressure ([[Standard conditions for temperature and pressure|STP]]).
Using [[Avogadro's Law]] and the [[ideal gas]] law, water vapor and air will have a [[molar volume]] of 22.414 litre/mol at STP. A molar mass of air and water vapour occupy the same volume
of 22.414 litres. The [[density]] (mass/volume) of water vapor is 0.804g/litre, which is significantly less than that of dry air at 1.27g/litre at STP.
Note that STP conditions include a temperature of 0°C, at which the ability of water to become vapor is very restricted. Its [[concentration]] in air is very low at 0°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°C. However the difference in densities between air and water vapour would still exist.
====Air and water vapor density interactions 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. 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 water vapor in the air increases, its buoyancy will become larger. This increase in buoyancy can have a signicant atmospheric impact, giving rise to powerful, moisture rich, upward air currents when the air temperature and sea temperature reaches 25°C or above. This phenomenon provides a significant motivating force for [[cyclonic]] and anticyclonic weather systems ([[tornados]] and hurricanes).
===Water vapour and respiration or breathing===
Water vapor's 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°C). The proportion of water vapor is significant enough to give rise to the stuffiness that can be experienced in humid jungle conditions or in poorly air conditioned buildings.
===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]]<ref>Abbreviated to Vapor pressure</ref>.
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:
{| background=#efefef align=center
|
|-
|<math>\log_{10} \left ( p \right )= </math>
|<math>-7.90298 (\frac{373.16}{T}-1) + 5.02808 \log_{10} \frac{373.16}{T} </math>
|-
|
|<math>- 1.3816 . 10^{-7} (10^{11.344 (1-\frac{T}{373.16})} -1) </math>
|-
|
|<math>+ 8.1328 . 10^{-3} (10^{-3.49149 (\frac{373.16}{T}-1)} -1) </math>
|-
|
|<math>+ \log_{10} \left ( 1013.246 \right )</math>
|}
:Where '''''T''''', temperature of the moist air, is given in units of [[kelvin]]s, and '''''p''''' is given in units of [[millibar]]s ([[hectopascal]]s).
The formula is valid from about −50 to 102 °C; however there are a very limited number of measurements of the vapor pressure of water over supercooled liquid water.<ref>A number of other formulas are listed and compared at [http://cires.colorado.edu/~voemel/vp.html CIRES].</ref>
Under adverse 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.
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.
==Water vapor in Earth's atmosphere==<!-- This section is linked from [[Creation science]] -->
Gaseous water represents a small but environmentally significant constituent of the [[Earth's atmosphere|atmosphere]]. Approximately 99.99% of it is contained in the [[troposphere]]. The [[condensation]] of water vapor to the liquid or ice phase is responsible for [[clouds]], [[rain]], [[snow]], and other [[precipitation]], all of which count among the most significant elements of what we experience as [[weather]]. Less obviously, the [[latent heat of vaporization]], which is released to the atmosphere whenever condensation occurs, is one of the most important terms in the atmospheric energy budget on both local and global scales. For example, latent heat release in atmospheric [[convection]] is directly responsible for powering destructive storms such as [[tropical cyclones]] and severe [[thunderstorms]]. Water vapor is also a potent [[greenhouse gas]]. Because the water vapor content of the atmosphere is expected to greatly increase in response to warmer temperatures, there is the potential for a [[water vapor feedback]] that could amplify the expected climate warming effect due to increased [[carbon dioxide]] alone. However, it is less clear how cloudiness would respond to a warming climate; depending on the nature of the response, clouds could either further amplify or partly mitigate the water vapor feedback.
[[Fog]] and clouds form through condensation around [[cloud condensation nuclei]]. In the absence of nuclei, condensation will only occur at much lower temperatures. Under persistent condensation or deposition, cloud droplets or snowflakes form, which [[precipitation (meteorology)|precipitate]] when they reach a critical mass.
[[Image:BAMS climate assess boulder water vapor 2002.gif|thumb|350px|Increasing water vapor at Boulder, Colorado.]]
The average residence time of water molecules in the [[troposphere]] is about 10 days. Water depleted by precipitation is replenished by evaporation from the seas, lakes, rivers and the transpiration of plants, and other biological and geological processes.
Measurements of vapor concentration are expressed as [[specific humidity]] or percent [[relative humidity]]. The annual mean global concentration of water vapor would yield about 25 mm of liquid water over the entire surface of the Earth if it were to instantly condense. However, the mean annual precipitation for the planet is about 1 meter, which indicates a rapid turnover of water in the air.
The abundance of gases emitted by [[volcano]]es varies considerably from volcano to volcano. However, water vapor is consistently the most common [[volcanic gas]], normally comprising more than 60% of total emissions during a subaerial [[volcanic eruption]].<ref>Sigurdsson, H. et al., (2000) ''Encyclopedia of Volcanoes'', San Diego, Academic Press</ref>
===Radar and satellite imaging===
[[Image:Atmospheric Water Vapor Mean.2005.030.jpg|thumb|[[MODIS]]/[[Terra (satellite)|Terra]] global mean atmospheric water vapor ]]
Because water molecules [[Absorption (electromagnetic radiation)|absorb]] [[microwave]]s and other [[radio wave]] frequencies, water in the atmosphere attenuates [[radar]] signals.<ref>Skolnik, Merrill. ''<u>Radar Handbook</u>, 2nd ed''. 1990, McGraw-Hill, Inc. p23.5</ref> In addition, atmospheric water will [[Reflection (physics)|reflect]] and [[refraction|refract]] signals to an extent that depends on whether it is vapor, liquid or solid.<ref>See Bright band.</ref>
Generally, radar signals lose strength progressively the farther they travel through the troposphere. Different frequencies attenuate at different rates, such that some components of air are opaque to some frequencies and transparent to others. Radio waves used for broadcasting and other communication experience the same effect.
Water vapor ''reflects'' radar<ref>More correctly stated, the attenuation of microwave signals due to ''water vapor'' is <u>directly</u> related to the frequency of the microwaves, see ''Skolnik''.</ref> to a less extent than do water's other two phases. In the form of drops and ice crystals, water acts as a prism, which it does not do as an individual [[molecule]]; however, the existence of water vapor in the atmosphere causes the atmosphere to act as a giant prism.<ref>Skolnik, pp2.44-2.54.</ref>
A [[Satellite Image Comparison|comparison of GOES-12 satellite images]] shows the distribution of atmospheric water vapor relative to the oceans, clouds and continents of the Earth. Vapor surrounds the planet but is unevenly distributed.
===Lightning generation===
Water vapor plays a key role in [[lightning]] production in the atmosphere. From [[cloud physics]], usually, clouds are the real generators of static [[electric charge|charge]] as found in Earth's atmosphere. But the ability, or capability of clouds to hold massive amounts of electrical energy is directly related to the amount of water vapor present in the local system.
The amount of water vapor directly controls the [[permittivity]] of the air. During times of low humidity, static discharge is quick and easy. During times of higher humidity, fewer static discharges occur. However, permittivity and capacitance<ref>Shadowitz, Albert. ''<u>The Electromagnetic Field</u>''. 1975, McGraw-Hill Book Company. pp165-171.</ref> work hand in hand to produce the megawatt outputs of lightning.
After a cloud, for instance, has started its way to becoming a lightning generator, atmospheric water vapor acts as a substance (or [[electrical insulation|insulator]]<ref>The term ''insulator'' is used to roughly describe the electrical properties of a gas mixture. Here, the dipole water molecules increase the reactance (impedance) and lower the permittivity of the air as humidity rises in the localized parcel of air.</ref> <ref>Shadowitz, p270.</ref> ) that decreases the ability of the cloud to [[electrostatic discharge|discharge]] its electrical energy. Over a certain amount of time, if the cloud continues to generate ''and'' store<ref>Shadowitz, pp172-173, 182.</ref> more [[static electricity]]<ref>Shadowitz, pp414-416.</ref>, the barrier that was created by the atmospheric water vapor will ultimately break down<ref>Commonly referred as ''[[dielectric]]'' breakdown.</ref> from the stored electrical potential energy. This energy will be released to a locally, opposite<ref>The term ''opposite charge'' in ESD and in E&M, may also include the case of largely differing electrical potentials of the same charge. This is normally called [[Voltage]] or potential difference.</ref> charged region in the form of lightning. The strength of each discharge is directly related to the atmospheric permittivity, capacitance, and the source's charge generating ability.<ref>Shadowitz, p172.</ref>
''See also,'' [[Van de Graaff generator]].
===Extraterrestrial water vapor===
The brilliance of comet tails comes largely from water vapor. On approach to the [[sun]], the ice many [[comet]]s carry [[Sublimation (chemistry)|sublimates]] to vapor, which reflects light from the sun. Knowing a comet's distance from the sun, astronomers may deduce a comet's water content from its brilliance.<ref>[http://www.il-st-acad-sci.org/planets/comets3.html ANATOMY OF COMETS], Retrieved December 2006.</ref> Bright tails in cold and distant comets suggests carbon monoxide sublimation.
Scientists studying [[Mars (planet)|Mars]] hypothesize that if water moves about the planet, it does so as vapor.<ref>Jakosky, Bruce, et al. ''"Water on Mars"'', April 2004, Physics Today, p71.</ref> Most of the water on Mars appears to exist as ice at the northern pole. During Mars' summer, this ice sublimates, perhaps enabling massive seasonal storms to convey significant amounts of water toward the equator.<ref>''"Europe probe detects Mars water ice"'', [[January 23]], [[2004]], [http://www.cnn.com/2004/TECH/space/01/23/mars.water.ice/index.html Cnn.com], retrieved August 2005.</ref>
A star called CW Leonis was found to have a ring of vast quantities of water vapor circling the aging, massive [[star]]. A [[NASA]] satellite designed to study chemicals in interstellar gas clouds, made the discovery with an onboard spectrometer. Most likely, "the water vapor was vaporized from the surfaces of orbiting comets."<ref>Lloyd, Robin. ''"Water Vapor, Possible Comets, Found Orbiting Star"'', [[11 July]] [[2001]], [http://www.space.com/searchforlife/swas_water_010711.html Space.com]. Retrieved [[December 15]], [[2006]].</ref>
Spectroscopic analysis of [[HD 209458 b]], an extrasolar planet in the constellation Pegasus, provides the first evidence of atmospheric water vapor beyond the Solar System.
==Scientific Discrepancies, Confounding factors and limits of knowledge==
Since water vapor is very common, it has been studied and written about from many perspectives. As working knowledge has grown and developed within apparently unrelated fields several discrepancies in understanding may be encountered. These discrepancies often arise from an inability to rigidly determine either a volumetric or gravimetric basis of study; and/or use of constants inappropriate for the conditions being observed.
Many scientific studies view water vapor as a [[Confounding variable]] (preventing ''[[Ceteris paribus]]'', also 'lurking variable') due to its complex nature; this becomes especially true when the study observes significant variation in water vapor quantities, over time and/or location.
It is for the reasons above that this remains a particularly tricky and sometimes controversial factor in many fields of science, whether storage of foods or ancient artefacts, thermodynamics or climate change.
==See also==
{{commons|Water vapor|Water vapor}}
{{portalpar|Water|Drinking water.jpg}}
{| border="0" cellpadding="5" cellspacing="0"
|-
|
* [[Earth's atmosphere|air]]
* [[boiling point]]
* [[Condensation in aerosol dynamics]]
* [[Deposition (meteorology)|deposition]]
* [[equation of state]]
* [[Evaporative cooler]]
* [[fog]]
* [[frost]]
* [[gas laws]]
* [[Gibbs free energy]]
||
* [[Gibbs phase rule]]
* [[greenhouse gas]]
* [[heat capacity]]
* [[heat of vaporization]]
* [[ideal gas]]
* [[kinetic theory of gases]]
* [[latent heat flux]]
* [[latent heat]]
||
* [[microwave radiometer]]
* [[phase (matter)|phase of matter]]
* [[steam]]
* [[superheating]]
* [[supersaturation]]
* [[thermodynamics]]
* [[troposphere]]
* [[vapor pressure]]
|}
{{Meteorological variables}}
==External links==
*[http://www.nsdl.arm.gov/Library/glossary.shtml#water_vapor National Science Digital Library - Water Vapor]
*[http://avc.comm.nsdlib.org/cgi-bin/wiki_grade_interface.pl?Measuring_Water_Vapor Measuring Water Vapor] : A lesson plan from the National Science Digital Library.
*[http://www.ems.psu.edu/~fraser/Bad/BadClouds.html psu.edu science misconceptions - Bad Clouds]
*[http://www.sciencebits.com/exhalecondense Calculate the condensation of your exhaled breath]
*[http://www.atmos.umd.edu/~stevenb/vapor/ Water Vapor Myths: A Brief Tutorial]
*[http://www.agu.org/sci_soc/mockler.html AGU Water Vapor in the Climate System - 1995]
==Footnotes/References==
{{reflist}}
[[Category:greenhouse gases]]
[[Category:atmospheric thermodynamics]]
[[Category:Forms of water]]
[[Category:Water in gas]]
[[Category:Psychrometrics]]
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[[es:Vapor de agua]]
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