Water vapor

Water vapor is the gas phase of water. It arises either through evaporation of liquid water or sublimation from solid ice. By the reverse processes of condensation and deposition, the vapor converts to water or ice. Although two or more phases may coexist in equilibrium, above 100 °C (212 °F) and below sea-level pressure, most water molecules assume the vapor phase.

Besides the pure gas, "water vapor" also refers to water molecules in air and in other gas mixtures. Humid air is sometimes said to "contain" water vapor in some percentage, for example.

Phase changes of water vapor

Any time two phases of a substance are in contact, phase change occurs in both directions simultaneously, though one direction may dominate. In recognition of this, the four phase changes involving water vapor will be treated in groups based on the other phase involved.

With the liquid phase

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 low humidities and temperatures near the boiling point.

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 from water vapor. In everyday settings, it occurs mostly from water vapor in air. The SVP of water in air varies with the temperature, so a change in air temperature may cause water to precipitate. If the water condenses onto a surface, dew results; if it forms suspended droplets, fog results.

With the solid (ice) phase

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. Like gradual evaporation, sublimation is fastest at low humidities and temperatures near the melting point. Sublimation is more gradual than evaporation.

The reverse of sublimation is deposition, in which gaseous water molecules coalesce to form tiny stalagmite-like structures on ice surfaces. The process occurs when vapor remains in the vicinty of the ice from which it sublimates, so that molecules of gas continuously exchange with the molecules of ice.

Clouds, precipitation and other phenomena 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 cloud condensation nuclei (CCN). In the absence of CCN, condensation will only occur at lower temperatures. Under persistent condensation or deposition, raindrops or snowflakes form, which precipitate when they reach a critical size.

The 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.

The vapor content of air is typically measured with a hygrometer and expressed as 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. This condition also describes the "dew point," at which water vapor condenses in the presence of CCN.

Radar and satellite imaging

Because water molecules absorb microwaves and other radio wave frequencies, water in the atmosphere attenuates radar signals. The signals lose strength progressively the farther they travel through the troposphere. Different frequencies attenuate at different rates, such that air in effect is opaque to some and transparent to others.

This comparison of satellite images shows the distribution of atmospheric water vapor relative to the oceans, clouds and continents of the Earth. The vapor is distributed unevenly around the equator.

Water vapor in space

The brilliance of comet tails comes largely from water vapor. On approach to the sun, the ice that many comets carry sublimates to vapor. 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.

External links

National Science Digital Library - Water Vapor
Measuring Water Vapor : A lesson plan from the National Science Digital Library.