NationMaster - Encyclopedia: Relative humidity

Relative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water. Image File history File linksMetadata Download high-resolution version (1627x2480, 2715 KB) Aparelho que mede a umidade relativa do ar. ... Image File history File linksMetadata Download high-resolution version (1627x2480, 2715 KB) Aparelho que mede a umidade relativa do ar. ... The interior of a Stevenson screen showing a motorized psychrometer Hygrometers are instruments used for measuring humidity. ...

1 Definition
2 A common misconception
3 Related concepts
4 Other important facts
5 References
6 See also
7 External links

Definition

The relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a given temperature. Relative humidity is expressed as a percentage and is calculated in the following manner: In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ... Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases. ...

$RH = {p_{(H_2O)} over p^*_{(H_2O)}} times 100%$

where:

$RH_{,_,}$ is the relative humidity of the mixture being considered;

${p_{(H_2O)}}$ is the partial pressure of water vapor in the mixture; and

${p^*_{(H_2O)}}$ is the saturation vapor pressure of water at the temperature of the mixture.

The saturation vapor pressure is the vapor pressure of water when air is saturated with water (having the maximum amount of water vapor that air can hold for a given temperature and pressure). ...

A common misconception

Often the concept of air holding water vapor is used in the description of relative humidity. Relative humidity is wholly understood in terms of the physical properties of water alone and therefore is unrelated to this concept. [1] This misconception is likely a result of the use of the word saturation which is often mis-used in definitions of relative humidity. In the present context saturation refers to the saturation state of water vapor [2], not the solubility of one material in another. The thermo-physical properties of water-air mixtures encountered at atmospheric conditions can be reasonably approximated by assuming that they behave like a mixture of ideal gases. [3]. For many practical purposes this assumption implies that both components (air and water) behave independently of each other and therefore the physical properties of the mixture can be estimated by considering the physical properties of each component separately. This is reflected in the definition of relative humidity - only the physical properties of water are considered when determining the relative humidity of a mixture. Solubility is a chemical property referring to the ability for a given substance, the solute, to dissolve in a solvent. ... An ideal gas or perfect gas is a hypothetical gas consisting of identical particles of zero volume, with no intermolecular forces. ...

Related concepts

The term relative humidity is reserved for systems of water vapor in air. The term relative saturation is used to describe the analogous property for systems consisting of a condensable phase other than water or a non-condensable phase other than air. [4]

The relative humidity of a system is dependent not only on the temperature but also on the absolute pressure of the system of interest. Therefore, a change in relative humidity can be explained by a change in system temperature, a change in the absolute pressure of the system, or change in both of these system properties.

Other important facts

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A gas in this context is referred to as saturated when the vapor pressure of water in the air is at the equilibrium vapor pressure for water vapor at the temperature of the gas and water vapor mixture; liquid water (and ice, at the appropriate temperature) will fail to lose mass through evaporation when exposed to saturated air. It may also correspond to the possibilility of dew or fog forming, within a space that lacks temperature differences among its portions, for instance in response to decreasing temperature. Fog consists of very minute droplets of liquid, primarily held aloft by isostatic motion. Dew on a spider web Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening. ... For other uses, see Fog (disambiguation). ...

The statement that relative humidity can never be above 100%, while a fairly good guide, is not absolutely accurate, without a more sophisticated definition of humidity than the one given here. An arguable exception is the Wilson cloud chamber which uses, in nuclear physics experiments, an extremely brief state of "supersaturation" to accomplish its function. Charles Thomson Rees Wilson (February 14, 1869 - November 15, 1959) was a Scottish physicist. ... Discovery of the positron in 1932 by Carl D. Anderson in a cloud chamber The cloud chamber, also known as the Wilson chamber, is used for detecting particles of ionizing radiation. ... The term supersaturation refers to a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances. ...

For a given dewpoint and its corresponding absolute humidity, the relative humidity will change inversely, albeit nonlinearly, with the temperature. This is because the partial pressure of water increases with temperature – the operative principle behind everything from hair dryers to dehumidifiers. The dew point or dewpoint of a given parcel of air is the temperature to which the parcel must be cooled, at constant barometric pressure, for the water vapor component to condense into water, called dew. ... Absolute humidity is the mass of water vapor in a given volume of air or gas, expressed by weight and usually measured in grams per cubic meter, though grains per cubic foot has also been used in the United States. ... For other uses, see Temperature (disambiguation). ... Categories: Stub | Hairdressing ... A dehumidifier is a household appliance that reduces the level of humidity in the air. ...

Due to the increasing potential for a higher water vapor partial pressure at higher air temperatures, the water content of air at sea level can get as high as 3% by mass at 30 °C (86 °F) compared to no more than about 0.5% by mass at 0 °C (32 °F). This explains the low levels (in the absence of measures to add moisture) of humidity in heated structures during winter, indicated by dry skin, itchy eyes, and persistence of static electric charges. Even with saturation (100% relative humidity) outdoors, heating of infiltrated outside air that comes indoors raises its moisture capacity, which lowers relative humidity and increases evaporation rates from moist surfaces indoors (including human bodies.) Winter is one of the four seasons of temperate zones. ... Beyond overall skin structure, refer below to: See-also. ... An itch (Latin: pruritus) is an uncomfortable sensation felt on an area of skin that causes a person or animal to desire to scratch that area. ... For other uses, see Eye (disambiguation). ... Static electricity is a class of phenomena involving the net charge present on an object; typically referring to charged object with voltages of sufficient magnitude to produce visible attraction, repulsion, and sparks. ...

Similarly, during summer in humid climates a great deal of liquid water condenses from air cooled in air conditioners. Warmer air is cooled below its dewpoint and the excess water vapor condenses. This phenomenon is the same as that which causes water droplets to form on the outside of a cup containing an ice-cold drink.

A useful rule of thumb is that the maximum absolute humidity doubles for every 20 °F (11.1 °C) increase in temperature. Thus, the relative humidity will drop by a factor of 2 for each 20 °F (11.1 °C) increase in temperature, assuming conservation of absolute moisture. For example, in the range of normal temperatures, air at 70 °F (21.1 °C) and 50% relative humidity will become saturated if cooled to 50°F (10 °C), its dewpoint and 40 °F (4.4 °C) air at 80% relative humidity warmed to 70 °F (21.1 °C) will have a relative humidity of only 10% and feel dry. By comparison, a relative humidity between 40% and 60% is considered healthy and comfortable in comfort controlled environments (ASHRAE Standard 55[5]). This latter example leads to a RH of perhaps 30%, not 10%. T rises from 40 °F (4.4 °C) to 60 °F (15.6 °C), RH falls from 80% to 40%, T rises from 60 °F (15.6 °C) to 70 °F (21.1 °C), RH falls perhaps a quarter, from 40% to 30% - not to 10%. Absolute humidity is the mass of water vapor in a given volume of air or gas, expressed by weight and usually measured in grams per cubic meter, though grains per cubic foot has also been used in the United States. ... The dew point or dewpoint of a given parcel of air is the temperature to which the parcel must be cooled, at constant barometric pressure, for the water vapor component to condense into water, called dew. ...

Water vapor is a lighter gas than air at the same temperature, so humid air will tend to rise by natural convection. This is a mechanism behind thunderstorms and other weather phenomena. Relative humidity is often mentioned in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, it also increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin as the relative humidity rises. This effect is calculated as the heat index or humidex. Convection in the most general terms refers to the internal movement of currents within fluids (i. ... A rolling thundercloud over Enschede, The Netherlands. ... For the geological process, see Weathering or Erosion. ... Modern weather predictions aid in timely evacuations and potentially save lives and property damage Weather map of Europe, 10 December 1887 Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. ... For other uses, see Summer (disambiguation). ... For the geological process, see Weathering or Erosion. ... The heat index (HI) or humidex is an index that combines air temperature and relative humidity to determine an apparent temperature â€” how hot it actually feels. ... This article is about modern humans. ... For other uses, see Animal (disambiguation). ... â€œVaporizationâ€ redirects here. ... Perspiration (also called sweating or sometimes transpiration) is the production and evaporation of a fluid, consisting primarily of water as well as a smaller amount of sodium chloride (the main constituent of table salt), that is excreted by the sweat glands in the skin of mammals. ... The heat index (HI) or humidex is an index that combines air temperature and relative humidity to determine an apparent temperature â€” how hot it actually feels. ... Heat Index (HI) is an index that combines air temperature and relative humidity to determine an apparent temperature — how hot it actually feels. ...

A device used to measure humidity is called a hygrometer, one used to regulate it is called a humidistat, or sometimes hygrostat. (These are analogous to a thermometer and thermostat for temperature, respectively.) The interior of a Stevenson screen showing a motorized psychrometer Hygrometers are instruments used for measuring humidity. ... A humidifier is a household appliance that increases the level of water vapor, or moisture, in a room. ... An analogy is a comparison between two different things, in order to highlight some form of similarity. ... A common mercury thermometer A thermometer is a device that measures temperature or temperature gradient, using a variety of different principles. ... Bi-metallic thermostat for buildings A thermostat is a device for regulating the temperature of a system so that the systems temperature is maintained near a desired setpoint temperature. ...

References

Himmelblau, David M. (1985454545). Basic Principles And Calculations In Chemical Engineering. Prentice Hall. ISBN 0-13-066572-X.
Perry, R.H. and Green, D.W (1997). Perry's Chemical Engineers' Handbook (7th Edition). McGraw-Hill. ISBN 0-07-049841-5.

Pearson can mean Pearson PLC the media conglomerate. ... Perrys Chemical Engineers Handbook was first published in 1934 and the seventh edition was published in 1997. ... The McGraw-Hill Companies, Inc. ...

External links

Glossary definition of psychrometric tables - National Snow and Ice Data Center *Bad Clouds FAQ, PSU.edu

Categories: Psychrometrics | Atmospheric thermodynamics | Physical quantity

Results from FactBites:

relative humidity: Definition and Much More from Answers.com (1034 words)
	Relative humidity is a term used to describe the quantity of water vapor that exists in a gaseous mixture of air and water.
	Relative humidity is defined as the ratio of the partial pressure of water vapor in a gaseous mixture of air and water to the saturated vapor pressure of water at a given temperature.
	Relative humidity is often mentioned in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog.

Howstuffworks "What is relative humidity and how does it affect how I feel outside?" (396 words)
	Absolute humidity is the mass of water vapor divided by the mass of dry air in a volume of air at a given temperature.
	Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature).
	This doesn't mean that the relative humidity must be 100 percent in order for it to rain -- it must be 100 percent where the clouds are forming, but the relative humidity near the ground could be much less.

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