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Water vapor

Water vapor, also aqueous vapour, is the gas phase of water. On the Earth, water vapor is one state of the water cycle within the hydrosphere. Water vapor can be produced from the evaporation of liquid water or from the sublimation of ice. Under normal atmospheric conditions, water vapor is continuously evaporating and condensing. Normally, water vapor is invisible to the naked eye.


General properties of water vapor


Property Quantity
individual gas constant 461.5 J/(kgK)
latent heat of evaporation 2.27 MJ/kg
molecular weight 18.02 g/mol
specific heat capacity 1.84 kJ/(kgK)

Whenever a water molecule leaves a surface, it is said to have evaporated. Each water molecule that becomes water vapor takes a parcel of heat with it. This process is called evaporative cooling. The amount of water vapor in the air will determine how fast each molecule will return back to the surface or not. So, when a net evaporation occurs, that body of water will undergo a net cooling directly related to the loss of water.

Evaporative cooling is restricted by atmospheric conditions. The amount of water vapor in the air is referred to as humidity. Measurement of the vapor content of air is accomplished with devices known as hygrometers. The measurements are expressed as specific humidity or percent relative humidity. The temperature of the atmosphere and the water surface determines the equilibrium vapor pressure, 100% relative humidity occurs when the partial pressure of water vapor is equal to the equilibrium vapor pressure. This is often referred to as complete saturation.

Another form of evaporation is sublimation, in which water molecules become gaseous from ice instead of liquid water. Under the same principle, when ice has a higher temperature than the surrounding atmosphere, sublimation occurs. It is sublimation that accounts for the slow, mid-winter disappearance of ice and snow at temperatures too low to cause melting.


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. The water molecule brings a parcel of heat with it. Which in turn, drops the temperature of the atmosphere slightly. 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 condensation in the air begins to form.

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 is a type of condensation. Frost and snow are examples of deposition (or sublimation). Deposition is the direct formation of ice from water vapor.

General Discussion

The amount of water vapor in an atmosphere exists due to 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. This quantity is expressed by;

p = { e^{77.345+0.0057T- {7235 \over T}} \over T^{8.2}} .
Where T, temperature of the moist air, is given in units of K, and p is given in units of Pa.

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 dispell 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.

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, and weather is modified by climate.

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.

The average residence time of water molecules in the troposphere is about 1 week. 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.

Radar and satellite imaging

Because water molecules absorb microwaves and other radio wave frequencies, water in the atmosphere attenuates radar signals. 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 tend to suffer the same effect.

Water vapor reflects radar 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 a gas.

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 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 a susbstance (or insulator) that decreases the ability of the cloud to discharge its electrical energy. Over a certain amount of time, if the cloud continues to generate more static electricity, the barrier that was created by the atmospheric water vapor will ultimately break down. This energy will be released to a locally, opposite 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.

See also, Van de Graaf 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 absorbs and reemits light from the sun. Knowing a comet's distance from the sun, astronomers may deduce a comet's water content from its brilliance. 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. 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.

See also

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