Boundaries: Phase, Pressure, Temperature
Evaporation/Sublimation
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. One limitation is given by the capacity of the atmosphere to hold water vapor. 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. Pressure and temperature of the atmosphere determine the allowable saturation of water vapor, where 100% relative humidity refers to complete saturation.
Also, sublimation is another form of evaporation. However, 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.
Condensation
Water vapor will only condense onto another surface when that surface is cooler than the temperature of the water vapor, or when water vapor saturation has occured. 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 is an excellent example 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 pressure 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.
Under adverse conditions, such as 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 saturated with water vapor at the body temperature. In the cold it quickly condenses, thus showing up as a mist or appearring as steam.
Water Vapor in the Hydrosphere
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. Hence atmospheric water strongly influences, and is strongly influenced 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, 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 saturation: 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.
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, 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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