Water vapor (British: Water vapour) is water in gaseous form. It arises either through evaporation of liquid water or sublimation from solid ice. By the reverse processes of condensation and deposition, vapor converts to water or ice. Above 100 °C (212 °F) and below sea-level pressure, most water molecules in a closed system of pure water assume the form of vapor.

Phase changes between vapor and liquid

Evaporation is the transformation of a liquid into a gas. Like any other liquid, water may evaporate gradually or violently. Its rate is fastest at temperatures near the boiling point. [See also cavitation.]

Violent evaporation (boiling) is the formation of pure water vapor at the boiling point of water. However, the white mist that emerges from a boiling kettle is not water vapor, but a mass of droplets suspended in the air. The droplets are formed when the very hot, pure water vapor meets the cold air, and cools below the boiling point. "Steam" may refer either to the droplets or to the hot vapor from which they condense.

Condensation is the formation of liquid water either from pure water vapor or water intermixed with air. In the atmosphere, condensation produces clouds, fog and rain, the latter usually only when facilitated by cloud condensation nuclei.

Phase changes between vapor and ice

Water molecules may become gaseous also by sublimation from ice or snow. It is sublimation that often accounts for the slow, mid-winter disappearance of ice and snow from fields and cities at temperatures too low to cause melting.

The accumulation of ice or snow from vapor is called deposition. During deposition, gaseous water molecules coalesce to form tiny stalagmite-like structures on cold surfaces where ice crystals can grow. The process typically occurs when vapor encounters ice or other cold surfaces.

Clouds and water 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 greenhouse effect, which warms the planet, gaseous water also condenses to form clouds, which may act to warm or cool, depending on the circumstances. Atmospheric water strongly influences climate as a result.

Fog and clouds form through condensation around condensation nuclei. In the absence of nuclei, condensation will only occur at lower temperatures. Under persistent condensation or deposition, raindrops 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, not to mention other biological and geological processes.

Measurement of the vapor content of air is accomplished with devices known generically as hygrometers, and measurements are expressed as specific humidity or percent relative humidity. One hundred percent relative humidity refers to the concentration of water molecules that will exist above a plane surface of water at equilibrium and at the same temperature and pressure as the air being measured.

The water vapor in humid air is liable to condense as dew onto surfaces. The "dew point" of an air parcel is the temperature to which it must cool before dew begins to form. Either particles or surface irregularities can nucleate this condensation. The formation of dew on a standard mirror, for example, is the means by which some devices measure humidity.

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.

This comparison of 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.

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, space scientists 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.

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See also fog, heat capacity, thermodynamics, boiling point, phase of matter, latent heat, heat of vaporization, kinetic theory of gases, ideal gas, gas laws, vapor pressure, deposition, frost, greenhouse gas, troposphere, air, latent heat flux, microwave radiometer, Gibbs free energy, Gibbs phase rule, equation of state.