Information about 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) |
Water vapor or water vapour (see spelling differences), also aqueous vapor, is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere.[2] Water vapor can be produced from the evaporation of liquid water or from the sublimation of ice. Under normal atmospheric conditions,[3] water vapor is continuously evaporating and condensing.
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 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]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.
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.
Condensation
)
Clouds, formed by condensed water vapor.
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.
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 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.
The average molecular mass of air is 28.57g/mol at standard temperature and pressure (STP).
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.
Thus the density of water vapor is 0.804g/l, which is significantly less than that of dry air, 1.27g/l 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 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.
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.
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.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].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:
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- Where T, temperature of the moist air, is given in units of kelvins, and p is given in units of millibars (hectopascals).
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.
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.
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.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.
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Increasing water vapor at Boulder, Colorado.
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 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]
Radar and satellite imaging
Because water molecules absorb microwaves and other radio wave frequencies, water in the atmosphere attenuates radar signals.[12] In addition, atmospheric water will reflect and refract signals to an extent that depends on whether it is vapor, liquid or solid.[13])
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]
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.
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.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.
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]
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 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.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]
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]
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.
See also
Meteorological data and variables |
|---|
| Atmospheric pressure Baroclinity Cloud Convection CAPE CIN Dew point Heat index Humidex Humidity Lifted index Lightning Pot T Precipitation Sea surface temperature Surface solar radiation Surface weather analysis Temperature Theta-e Visibility Vorticity Wind chill Water vapor Wind |
External links
- National Science Digital Library - Water Vapor
- : A lesson plan from the National Science Digital Library.
- psu.edu science misconceptions - Bad Clouds
- Calculate the condensation of your exhaled breath
- Water Vapor Myths: A Brief Tutorial
- AGU Water Vapor in the Climate System - 1995
Footnotes/References
1. ^ Lide, David. CRC Handbook of Chemistry and Physics, 73rd ed. 1992, CRC Press.
2. ^ Technically called the Hydrologic cycle, from U.S. Geologic Survey. Water Cycle. Retrieved on 2006-10-24.
3. ^ Normal atmosphere means in the Earth's troposphere under a large variety of temperatures and pressures that are naturally occurring anywhere and at anytime.
4. ^ Schroeder, David. Thermal Physics. 2000, Addison Wesley Longman. p36
5. ^ 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)
6. ^ See Thermodynamics, as it is a process of energy transfer. This should not be confused with precipitates falling onto a surface.
7. ^ The atmosphere is a heat bath, heat is transferred by molecular conduction.
8. ^ Schroeder, p19.
9. ^ Abbreviated to Vapor pressure
10. ^ A number of other formulas are listed and compared at CIRES.
11. ^ Sigurdsson, H. et al., (2000) Encyclopedia of Volcanoes, San Diego, Academic Press
12. ^ Skolnik, Merrill. Radar Handbook, 2nd ed. 1990, McGraw-Hill, Inc. p23.5
13. ^ See Bright band.
14. ^ More correctly stated, the attenuation of microwave signals due to water vapor is directly related to the frequency of the microwaves, see Skolnik.
15. ^ Skolnik, pp2.44-2.54.
16. ^ Shadowitz, Albert. The Electromagnetic Field. 1975, McGraw-Hill Book Company. pp165-171.
17. ^ 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.
18. ^ Shadowitz, p270.
19. ^ Shadowitz, pp172-173, 182.
20. ^ Shadowitz, pp414-416.
21. ^ Commonly referred as dielectric breakdown.
22. ^ 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.
23. ^ Shadowitz, p172.
24. ^ ANATOMY OF COMETS, Retrieved December 2006.
25. ^ Jakosky, Bruce, et al. "Water on Mars", April 2004, Physics Today, p71.
26. ^ "Europe probe detects Mars water ice", January 23, 2004, Cnn.com, retrieved August 2005.
27. ^ Lloyd, Robin. "Water Vapor, Possible Comets, Found Orbiting Star", 11 July 2001, Space.com. Retrieved December 15, 2006.
2. ^ Technically called the Hydrologic cycle, from U.S. Geologic Survey. Water Cycle. Retrieved on 2006-10-24.
3. ^ Normal atmosphere means in the Earth's troposphere under a large variety of temperatures and pressures that are naturally occurring anywhere and at anytime.
4. ^ Schroeder, David. Thermal Physics. 2000, Addison Wesley Longman. p36
5. ^ 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)
6. ^ See Thermodynamics, as it is a process of energy transfer. This should not be confused with precipitates falling onto a surface.
7. ^ The atmosphere is a heat bath, heat is transferred by molecular conduction.
8. ^ Schroeder, p19.
9. ^ Abbreviated to Vapor pressure
10. ^ A number of other formulas are listed and compared at CIRES.
11. ^ Sigurdsson, H. et al., (2000) Encyclopedia of Volcanoes, San Diego, Academic Press
12. ^ Skolnik, Merrill. Radar Handbook, 2nd ed. 1990, McGraw-Hill, Inc. p23.5
13. ^ See Bright band.
14. ^ More correctly stated, the attenuation of microwave signals due to water vapor is directly related to the frequency of the microwaves, see Skolnik.
15. ^ Skolnik, pp2.44-2.54.
16. ^ Shadowitz, Albert. The Electromagnetic Field. 1975, McGraw-Hill Book Company. pp165-171.
17. ^ 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.
18. ^ Shadowitz, p270.
19. ^ Shadowitz, pp172-173, 182.
20. ^ Shadowitz, pp414-416.
21. ^ Commonly referred as dielectric breakdown.
22. ^ 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.
23. ^ Shadowitz, p172.
24. ^ ANATOMY OF COMETS, Retrieved December 2006.
25. ^ Jakosky, Bruce, et al. "Water on Mars", April 2004, Physics Today, p71.
26. ^ "Europe probe detects Mars water ice", January 23, 2004, Cnn.com, retrieved August 2005.
27. ^ Lloyd, Robin. "Water Vapor, Possible Comets, Found Orbiting Star", 11 July 2001, Space.com. Retrieved December 15, 2006.
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boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid.[1][2][3][4]
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Gas is one of the four major states of matter, consisting of freely moving atoms or molecules without a definite shape. Compared to the solid and liquid states of matter a gas has lower density and a lower viscosity.
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Water (H2O, HOH) is the most abundant molecule on Earth's surface, composing of about 70% of the Earth's surface as liquid and solid state in addition to being found in the atmosphere as a vapor.
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Vapor or vapour (see spelling differences) is the gas phase component of another state of matter (e.g. liquid or solid) which does not completely fill its container. It is distinguished from the pure gas phase by the presence of the same substance in another state of matter.
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water cycle.]]
The Earth's water is always in movement, and the water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth.
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The Earth's water is always in movement, and the water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth.
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water cycle, a key process of the hydrosphere.]]
A hydrosphere (Greek hydro means "water") in physical geography describes the collective mass of water found on, under, and over the surface of a planet.
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A hydrosphere (Greek hydro means "water") in physical geography describes the collective mass of water found on, under, and over the surface of a planet.
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Evaporation is the process by which molecules in a liquid state (e.g. water) spontaneously become gaseous (e.g. water vapor), without being heated to boiling point. It is the opposite of condensation.
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Water is a common chemical substance that is essential to all known forms of life.[1] In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor.
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Sublimation of an element or compound is a transition from the solid to gas phase with no intermediate liquid stage. Sublimation is a phase transition that occurs at temperatures and pressures below the triple point (see phase diagram).
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Condensation is the change in matter of a substance to a denser phase, such as a gas (or vapor) to a liquid.[1] Condensation commonly occurs when a vapor is cooled to a liquid, but can also occur if a vapor is compressed (i.e.
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Evaporative cooling is a physical phenomenon in which evaporation of a liquid, typically into surrounding air, cools an object or a liquid in contact with it. Latent heat describes the amount of heat that is needed to evaporate the liquid; this heat comes from the liquid itself and
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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.
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Humidity is the amount of water vapor in a sample of air compared to the maximum amount of water vapor the air can hold at any specific temperature. Absolute humidity, relative humidity and specific humidity are different ways to express the water content in a parcel of air.
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Hygrometers are instruments used for measuring humidity. A simple form of a hygrometer is specifically known as a "psychrometer" and consists of two thermometers, one of which includes a dry bulb and the other of which includes a bulb that is kept wet to measure
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Humidity is the amount of water vapor in a sample of air compared to the maximum amount of water vapor the air can hold at any specific temperature. Absolute humidity, relative humidity and specific humidity are different ways to express the water content in a parcel of air.
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Relative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water.
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Definition
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Sublimation of an element or compound is a transition from the solid to gas phase with no intermediate liquid stage. Sublimation is a phase transition that occurs at temperatures and pressures below the triple point (see phase diagram).
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The saturation vapor pressure is the static pressure of a vapor when the vapor phase of some material is in equilibrium with the liquid phase of that same material. The saturation vapor pressure of any material is solely dependent on the temperature of that material.
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Cloud condensation nuclei or CCNs (also known as cloud seeds) are small particles (typically 0.0002 mm, or 1/100 th the size of a cloud droplet [1] ) about which cloud droplets coalesce.
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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. The condensed water is called dew. The dew point is a saturation point.
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