Water vapor
Water vapor (H2O) | |
---|---|
Invisible water vapor condenses to form
visible clouds of liquid rain droplets | |
Liquid state
|
Water
|
Solid state | Ice |
Properties[1] | |
Molecular formula | H2O |
Molar mass | 18.01528(33) g/mol |
Melting point | 0.00 °C (273.15 K)[2] |
Boiling point | 99.98 °C (373.13 K)[2] |
Specific gas constant
|
461.5 J/(kg·K) |
Heat of vaporization
|
2.27 MJ /kg
|
Heat capacity at 300 K | 1.864 kJ/(kg·K)[3]
|
Water vapor, water vapour or aqueous vapor is the
Being a component of Earth's hydrosphere and hydrologic cycle, it is particularly abundant in
Water vapor is a relatively common atmospheric constituent, present even in the
Properties
Evaporation
Whenever a water molecule leaves a surface and diffuses into a surrounding gas, it is said to have
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.
Evaporative cooling is restricted by
Sublimation
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
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.
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 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 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 (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 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 (degC) | 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 (temp dependant) | 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(II) 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 (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) |
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 (degC) | measurement uncertainty | typical measurement frequency | system cost | notes |
Impact on air density
Water vapor is lighter or less dense than dry air.[12][13] 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 g/L and water vapor at standard temperature has a vapor pressure of 0.6 kPa and the much lower density of 0.0048 g/L.
Calculations
Water vapor and dry air density calculations at 0 °C:
- The atoms.
- The average molar mass of air (approx. 78% nitrogen, N2; 21% oxygen, O2; 1% other gases) is 28.57 g/mol at standard temperature and pressure (STP).
- Obeying Avogadro's Law and the ideal gas law, moist airwill have a lower density than dry air. At max. saturation (i. e. rel. humidity = 100% at 0 °C) the density will go down to 28.51 g/mol.
- STP conditions imply 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 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 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 °C or above. This phenomenon provides a significant driving force for cyclonic and anticyclonic weather systems (typhoons and hurricanes).
Respiration and breathing
Water vapor is a by-product of 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 (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 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
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.
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
where T, temperature of the moist air, is given in units of
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. There are a number of other formulae which can be used.[15]
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.
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. 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
In Earth's atmosphere
Gaseous water represents a small but environmentally significant constituent of the
Water vapor is the "working medium" of the atmospheric thermodynamic engine which transforms heat energy from sun irradiation into mechanical energy in the form of winds. Transforming thermal energy into mechanical energy requires an upper and a lower temperature level, as well as a working medium which shuttles forth and back between both. The upper temperature level is given by the soil or water surface of the Earth, which absorbs the incoming sun radiation and warms up, evaporating water. The moist and warm air at the ground is lighter than its surroundings and rises up to the upper limit of the troposphere. There the water molecules radiate their thermal energy into outer space, cooling down the surrounding air. The upper atmosphere constitutes the lower temperature level of the atmospheric thermodynamic engine. The water vapor in the now cold air condenses out and falls down to the ground in the form of rain or snow. The now heavier cold and dry air sinks down to ground as well; the atmospheric thermodynamic engine thus establishes a vertical convection, which transports heat from the ground into the upper atmosphere, where the water molecules can radiate it to outer space. Due to the Earth's rotation and the resulting Coriolis forces, this vertical atmospheric convection is also converted into a horizontal convection, in the form of cyclones and anticyclones, which transport the water evaporated over the oceans into the interior of the continents, enabling vegetation to grow.[21]
Water in Earth's atmosphere is not merely below its boiling point (100 °C), but
In the absence of other greenhouse gases, Earth's water vapor would condense to the surface;[34][35][36] this has likely happened, possibly more than once. Scientists thus distinguish between non-condensable (driving) and condensable (driven) greenhouse gases, i.e., the above water vapor feedback.[37][20][19]
Atmospheric concentration of water vapour is highly variable between locations and times, from 10 ppmv in the coldest air to 5% (50 000 ppmv) in humid tropical air,[38] and can be measured with a combination of land observations, weather balloons and satellites.[39] The water content of the atmosphere as a whole is constantly depleted by precipitation. At the same time it is constantly replenished by evaporation, most prominently from oceans, lakes, rivers, and moist earth. Other sources of atmospheric water include combustion, respiration, volcanic eruptions, the transpiration of plants, and various other biological and geological processes. At any given time there is about 1.29 x 1016 litres (3.4 x 1015 gal.) of water in the atmosphere. The atmosphere holds 1 part in 2500 of the fresh water, and 1 part in 100,000 of the total water on Earth.[40] The mean global content of water vapor in the atmosphere is roughly sufficient to cover the surface of the planet with a layer of liquid water about 25 mm deep.[41][42][43] The mean annual precipitation for the planet is about 1 metre, a comparison which implies a rapid turnover of water in the air – on average, the residence time of a water molecule in the troposphere is about 9 to 10 days.[43]
Global mean water vapour is about 0.25% of the atmosphere by mass and also varies seasonally, in terms of contribution to atmospheric pressure between 2.62 hPa in July and 2.33 hPa in December.
Episodes of surface geothermal activity, such as volcanic eruptions and geysers, release variable amounts of water vapor into the atmosphere. Such eruptions may be large in human terms, and major explosive eruptions may inject exceptionally large masses of water exceptionally high into the atmosphere, but as a percentage of total atmospheric water, the role of such processes is trivial. The relative concentrations of the various gases emitted by volcanoes varies considerably according to the site and according to the particular event at any one site. However, water vapor is consistently the commonest volcanic gas; as a rule, it comprises more than 60% of total emissions during a subaerial eruption.[49]
Atmospheric water vapor content is expressed using various measures. These include vapor pressure,
Radar and satellite imaging
Because water molecules absorb microwaves and other radio wave frequencies, water in the atmosphere attenuates radar signals.[50] In addition, atmospheric water will reflect and refract signals to an extent that depends on whether it is vapor, liquid or solid.
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 to a lesser 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.[51]
A comparison of
As water vapor absorbs light in the visible spectral range, its absorption can be used in spectroscopic applications (such as DOAS) to determine the amount of water vapor in the atmosphere. This is done operationally, e.g. from the Global Ozone Monitoring Experiment (GOME) spectrometers on ERS (GOME) and MetOp (GOME-2).[53] The weaker water vapor absorption lines in the blue spectral range and further into the UV up to its dissociation limit around 243 nm are mostly based on quantum mechanical calculations[54] and are only partly confirmed by experiments.[55]
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. The ability 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. Permittivity and capacitance work hand in hand to produce the megawatt outputs of lightning.[56]
After a cloud, for instance, has started its way to becoming a lightning generator, atmospheric water vapor acts as a substance (or
Extraterrestrial
Water vapor is common in the
Geological formations such as
The brilliance of comet tails comes largely from water vapor. On approach to the Sun, the ice many comets carry sublimes to vapor. Knowing a comet's distance from the sun, astronomers may deduce the comet's water content from its brilliance.[66]
Water vapor has also been confirmed outside the Solar System. Spectroscopic analysis of
See also
- Air density
- Atmospheric river
- Boiling point
- Condensation in aerosol dynamics
- Deposition
- Earth's atmosphere
- Eddy covariance
- Equation of state
- Evaporative cooler
- Fog
- Frost
- Gas laws
- Gibbs free energy
- Gibbs phase rule
- Greenhouse gas
- Heat capacity
- Heat of vaporization
- Humidity
- Hygrometer
- Ideal gas
- Kinetic theory of gases
- Latent heat
- Latent heat flux
- Microwave radiometer
- Phase of matter
- Saturation vapor density
- Steam
- Sublimation
- Superheating
- Supersaturation
- Thermodynamics
- Troposphere
- Vapor pressure
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