 | Water vapor: Encyclopedia II - Water vapor - General properties of water vapor
Water vapor - General properties of water vapor
Water vapor - 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. 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 - Condensation
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. In turn, the temperature of the atmosphere drops 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.
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 (abbreviated to Vapor 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 formulae exist for this quantity; the most used reference formula is the Goff-Gratch equation for the SVP over liquid water:
Where T, temperature of the moist air, is given in units of kelvins, and p is given in units of millibars (hectopascals).
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. A number of other formulae are listed and compared at [1].
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 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.
Supermarket buildings that utilise open chiller cabinets are able to significantly lower vapor pressure (lowering humidity). This practice delivers several benefits and other problems.
Other related archivesAtmospheric pressure, CAPE, CIN, Clouds, Convection, Dew point, Exhaled, Fog, Frost, Gibbs free energy, Gibbs phase rule, Heat index, Humidity, Lightning, Mars, Pot T, Precipitation, Sea surface temperature, Temperature, Theta-e, Van de Graaff generator, Visibility, Vorticity, Wind, Wind chill, absorb, air, atmosphere, boiling point, capacity, charge, climate, cloud condensation nuclei, cloud physics, clouds, comets, comparison of GOES-12 satellite images, condensing, conditions, deposition, dew point, discharge, equation of state, evaporation, evaporative cooling, fog, frost, gas, gas laws, greenhouse effect, greenhouse gas, heat, heat capacity, heat of vaporization, hectopascals, hydrosphere, hygrometers, ice, ideal gas, insulator, kelvins, kinetic theory of gases, latent heat, latent heat flux, lightning, microwave radiometer, microwaves, millibars, mist, permittivity, phase of matter, precipitate, radar, radio wave, reflect, refract, relative humidity, snow, specific humidity, state, static electricity, steam, sublimates, sublimation, sun, superheating, supersaturation, swamp cooler, thermodynamics, troposphere, vapor partial pressure, vapor pressure, water, water cycle, weather
 Adapted from the Wikipedia article "General properties of water vapor", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki |
