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Encyclopedia > Water vapor
Water vapor
Systematic name Water Vapor
Liquid State Water
Solid state Ice
Properties[1]
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)

## General properties of water vapor GA_googleFillSlot("encyclopedia_square");

### Evaporation/sublimation

Whenever a water molecule leaves a surface, it is said to have evaporated. Each individual water molecule which transitions from a more associated (liquid) to 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.[4] 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.[5] For other uses, see Heat (disambiguation) In physics, heat, symbolized by Q, is energy transferred from one body or system to another due to a difference in temperature. ... Evaporative cooling is a system in which latent heat of evaporation is used to carry heat away from an object to cool it. ...

Evaporative cooling is restricted by atmospheric conditions. The amount of water vapor in the air is the humidity. The vapor content of air is measured with devices known as hygrometers. The measurements are 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. In chemistry and other sciences, STP or standard temperature and pressure is a standard set of conditions for experimental measurements, to enable comparisons to be made between sets of data. ... Humidity is the amount of water vapor in air. ... The interior of a Stevenson screen showing a motorized psychrometer Hygrometers are instruments used for measuring humidity. ... Specific humidity is a method of expressing the amount of aqueous vapor in air by using a ratio of aqueous vapor to dry air. ... This does not adequately cite its references or sources. ...

Another form of evaporation is sublimation, by which water molecules become gaseous directly from ice without first becoming liquid water. When ice has a higher temperature than the surrounding atmosphere, sublimation occurs. Sublimation accounts for the slow mid-winter disappearance of ice and snow at temperatures too low to cause melting. Water vapor also evaporates when heated. This article does not cite any references or sources. ...

### Condensation

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 water vapor equilibrium in air has been exceeded. When water vapor condenses onto a surface, a net warming occurs on that surface.[6] The water molecule brings a parcel of heat with it. In turn, the temperature of the atmosphere drops slightly.[7] [8] 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. Image File history File linksMetadata Size of this preview: 800 Ã— 480 pixel Image in higher resolution (2000 Ã— 1200 pixel, file size: 670 KB, MIME type: image/jpeg) Other versions en:Image:Cloud From Plane Window. ... Image File history File linksMetadata Size of this preview: 800 Ã— 480 pixel Image in higher resolution (2000 Ã— 1200 pixel, file size: 670 KB, MIME type: image/jpeg) Other versions en:Image:Cloud From Plane Window. ... The saturation vapor pressure is the vapor pressure of water when air is saturated with water (having the maximum amount of water vapor that air can hold for a given temperature and pressure). ... Aerosol pollution over Northern India and Bangladesh - NASA Cloud condensation nuclei or CCNs (also known as cloud seeds) are small particles (typically 0. ... Dew on a spider web The dew point (or dewpoint) is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. ...

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. Frost on black pipes Frost is a solid deposition of water vapor from saturated air. ... Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. ... Deposition is the process by which, in sub-freezing air, water vapor changes directly to ice without first becoming a liquid. ...

### Water vapor density

Water vapor is lighter or less dense than dry air. At equivalent temperatures it is buoyant with respect to dry air. Look up air in Wiktionary, the free dictionary. ...

#### Water vapor density calculation at 0°C

The molecular mass or weight of water is 18.02g/mol, as calculated from the sum of the atomic masses of its constituent atoms. Graph of atmospheric dewpoints across a range of temperatures. ... The molecular mass (abbreviated Mr) of a substance, formerly also called molecular weight and abbreviated as MW, is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of carbon-12). ... In reference to a certain isotope of a chemical element, atomic mass (though also called relative atomic mass and atomic weight) is the mass of one atom of the isotope expressed in units (atomic mass unit, amu) such that the carbon-12 isotope has an atomic mass of exactly 12. ... Properties For alternative meanings see atom (disambiguation). ...

The average molecular mass of air is 28.57g/mol at standard temperature and pressure (STP). Look up air in Wiktionary, the free dictionary. ... In chemistry and other sciences, STP or standard temperature and pressure is a standard set of conditions for experimental measurements, to enable comparisons to be made between sets of data. ...

Using Avogadro's Law and the ideal gas law, both water vapor and air will have a molar volume of 22.414 l/mol at STP. This article does not cite any references or sources. ... An ideal gas or perfect gas is a hypothetical gas consisting of identical particles of zero volume, with no intermolecular forces. ... In chemistry, the molar volume of a substance is the ratio of the volume of a sample of that substance to the amount of substance (usually in mole) in the sample. ...

Thus the density of water vapor is 0.804g/l, which is significantly less than that of dry air, 1.27g/l at STP. For other uses, see Density (disambiguation). ...

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 is the maximum concentration of water vapor expected for a given temperature or dew point. The concentration increases significantly with temperature, approaching 100% at 100°C. However, the ideal gas law could equally well be applied at 100°C, when the difference in density would still exist. For other uses, see Concentration (disambiguation). ... For other uses, see Temperature (disambiguation). ... Absolute humidity is the mass of water vapor in a given volume of air or gas, expressed by weight and usually measured in grams per cubic meter, though grains per cubic foot has also been used in the United States. ...

Water vapor's contribution to the total pressure increases as its concentration increases. Its partial pressure contribution to air pressure also increases, lowering the partial pressure contribution of the other atmospheric gases (Dalton's Law) as the total air pressure must remain constant. In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ... In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ...

#### Air water vapor 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 buoyant if placed in a larger region of dry air. In physics, buoyancy is the upward force on an object produced by the surrounding fluid (i. ...

### 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 partial pressure[9]. Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases. ...

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: The Goff-Gratch Equation is the international standard for the computation of saturation vapor pressure. ...

 $log_{10} left ( p right )=$ $-7.90298 (frac{373.16}{T}-1) + 5.02808 log_{10} frac{373.16}{T}$ $- 1.3816 . 10^{-7} (10^{11.344 (1-frac{T}{373.16})} -1)$ $+ 8.1328 . 10^{-3} (10^{-3.49149 (frac{373.16}{T}-1)} -1)$ $+ log_{10} left ( 1013.246 right )$
Where T, temperature of the moist air, is given in units of kelvins, and p is given in units of millibars (hectopascals).

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.[10] For other uses, see Kelvin (disambiguation). ... A millibar (mbar, also mb) is 1/1000th of a bar, a unit for measurement of pressure. ... The pascal (symbol Pa) is the SI unit of pressure. ...

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.

Exhaled 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. Exhalation (or expiration) is the movement of air out of the bronchial tubes, through the airways, to the external environment during breathing. ... Dramatic morning mist Mist is a phenomenon of a liquid in small droplets floating through air. ...

Controlling water vapor in air is a key concern in the 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. HVAC may also stand for High-voltage alternating current HVAC systems use ventilation air ducts installed throughout a building that supply conditioned air to a room through rectangular or round outlet vents, called diffusers; and ducts that remove air from return-air grilles Fire-resistance rated mechanical shaft with HVAC... Human thermal comfort is the state of mind that expresses satisfaction with the surrounding environment, according to ASHRAE Standard 55. ... Refrigeration is the process of removing heat from an enclosed space, or from a substance, and rejecting it elsewhere for the primary purpose of lowering the temperature of the enclosed space or substance and then maintaining that lower temperature. ...

## Water vapor in Earth's atmosphere

Gaseous water represents a small but environmentally significant constituent of the atmosphere. Most of it is contained in the troposphere. Besides accounting for most of Earth's natural greenhouse effect, which warms the planet, gaseous water also condenses to form clouds, which may act to warm or cool, depending on the circumstances. In general terms, atmospheric water strongly influences, and is strongly influenced by weather. â€œAirâ€ redirects here. ... Atmosphere diagram showing the mesosphere and other layers. ... Wikinews has related news: Scientists warn thawing Siberia may trigger global meltdown A schematic representation of the exchanges of energy between outer space, the Earths atmosphere, and the Earth surface. ...

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 precipitate when they reach a critical mass. For other uses, see Fog (disambiguation). ... Aerosol pollution over Northern India and Bangladesh - NASA Cloud condensation nuclei or CCNs (also known as cloud seeds) are small particles (typically 0. ...

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. Stratospheric water vapor 1% increase. ... Stratospheric water vapor 1% increase. ... Atmosphere diagram showing the mesosphere and other layers. ...

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. Specific humidity is a method of expressing the amount of aqueous vapor in air by using a ratio of aqueous vapor to dry air. ... This does not adequately cite its references or sources. ...

The abundance of gases emitted by volcanoes 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.[11] Cleveland Volcano in the Aleutian Islands of Alaska photographed from the International Space Station For other uses, see Volcano (disambiguation). ... Volcanic gases include a variety of substances given off by active (or, at times, by dormant) volcanos. ... This article is about volcanoes in geology. ...

MODIS/Terra global mean atmospheric water vapor

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[14] 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.[15] 3D (left and center) and 2D (right) representations of the terpenoid molecule atisane. ...

A 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. These two images for this satellite image comparisons come from the GOES12 satellite. ...

### 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 charge as found in Earth's atmosphere. But the ability, or capacity, of clouds to hold massive amounts of electrical energy is directly related to the amount of water vapor present in the local system. Lightning over Oradea in Romania For information on lightning precautions, see Lightning safety. ... Cloud physics describes the area of study of physical processes that lead to the formation, growth and precipitation of clouds. ... Electric charge is a fundamental conserved property of some subatomic particles, which determines their electromagnetic interaction. ... Capacitance is a measure of the amount of electric charge stored (or separated) for a given electric potential. ...

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[16] work hand in hand to produce the megawatt outputs of lightning. Permittivity is a physical quantity that describes how an electric field affects and is affected by a dielectric medium and is determined by the ability of a material to polarize in response to an applied electric field, and thereby to cancel, partially, the field inside the material. ...

After a cloud, for instance, has started its way to becoming a lightning generator, atmospheric water vapor acts as a substance (or insulator[17] [18] ) that decreases the ability of the cloud to discharge its electrical energy. Over a certain amount of time, if the cloud continues to generate and store[19] more static electricity[20], the barrier that was created by the atmospheric water vapor will ultimately break down[21] from the stored electrical potential energy. This energy will be released to a locally, opposite[22] 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.[23] This article or section is in need of attention from an expert on the subject. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Static electricity is a class of phenomena involving the net charge present on an object; typically referring to charged object with voltages of sufficient magnitude to produce visible attraction, repulsion, and sparks. ...

## Extraterrestrial water vapor

The brilliance of comet tails comes largely from water vapor. On approach to the sun, the ice many comets carry 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.[24] Bright tails in cold and distant comets suggests carbon monoxide sublimation. Sol redirects here. ... Comet Hale-Bopp Comet West For other uses, see Comet (disambiguation). ... This article does not cite any references or sources. ...

Scientists studying Mars hypothesize that if water moves about the planet, it does so as vapor.[25] 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.[26] Mars is the fourth planet from the Sun in the solar system, named after the Roman god of war (the counterpart of the Greek Ares), on account of its blood red color as viewed in the night sky. ...

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."[27] STAR is an acronym for: Organizations Society of Ticket Agents and Retailers], the self-regulatory body for the entertainment ticket industry in the UK. Society for Telescopy, Astronomy, and Radio, a non-profit New Jersey astronomy club. ... This article is about the American space agency. ...

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. HD 209458 b is an extrasolar planet that orbits the Sun-like star HD 209458 in the constellation Pegasus, some 150 light-years from Earths solar system, with evidence of water vapor. ...

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 Intellicast.com - Water Vapor - United States (65 words) Home » US » Satellite » Water Vapor The Water Vapor image shows areas of moist and dry air at mid-levels of the atmosphere (about 12,000 feet). Your use of this site constitutes your acceptance of the Terms of Use and Privacy Policy
 AGU Web Site: Water Vapor in the Climate System. A Special Report. (4752 words) Water vapor is water in the gaseous phase. Water vapor is constantly cycling through the atmosphere, evaporating from the surface, condensing to form clouds blown by the winds, and subsequently returning to the Earth as precipitation. As is shown in Figure 3, almost all water vapor in the atmosphere originates at the surface of the Earth, where water evaporates from the ocean and the continents owing to the Sun's radiation, and is transpired by plants and respired by animals into the atmosphere.
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