Relative humidity 223970 225930339 2008-07-16T01:57:16Z Vsmith 84417 /* A Common Misconception */ rmv duplication [[Image:Umidaderelativa.jpg|thumb|A [[hygrometer]] used to measure the humidity of air.]] '''Relative humidity''' is a term used to describe the amount of [[water vapor]] that exists in a gaseous mixture of air and water. == 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 prescribed temperature. Relative humidity is normally expressed as a percentage and is defined in the following manner: :::<math> RH = {p_{(H_2O)} \over p^*_{(H_2O)}} \times 100% </math> where: <math> RH_{\,_\,} </math> is the relative humidity of the mixture being considered; <math> {p_{(H_2O)}} </math> is the partial pressure of water vapor in the mixture; and <math> {p^*_{(H_2O)}} </math> is the saturated [[vapor pressure]] of water at the temperature of the mixture. At the conditions of pressure and temperature in the earth's atmosphere, gasses behave nearly like ideal gasses, so the vapor pressure of water vapor depends only on the temperature, and is independent of the presence or absence of air or any other gasses in the water vapor-other gasses mixture. == Estimating Relative Humidity == The relative humidity of an air-water mixture can be estimated if both the [[temperature]] (''T'') and the [[dew point temperature]] (''T<sub>d</sub>'') of the mixture are known. When both ''T'' and ''T<sub>d</sub>'' are expressed in degrees [[celsius]] then <ref> Perry, R.H. and Green, D.W, [[Perry's Chemical Engineers' Handbook]] (7th Edition), [[McGraw-Hill]], ISBN 0-07-049841-5 , Page 12-7 </ref>: :::<math> RH = {{e_p} \over {e_s}} \times 100% </math> where the partial pressure of water vapor in the mixture is estimated by <math> e_p </math> : :::<math> e_p = e^{(17.269 \times T_d) \over {(273.3 + T_d)}} </math> and the saturated [[vapor pressure]] of water at the temperature of the mixture is estimated by <math> e_s </math> : :::<math> e_s = e^{(17.269 \times T) \over {(273.3 + T)}} </math> Unfortunately enough, this equation does not work very well in practice, because the dew point temperature <math> T_d </math> can only be calculated when the RH is known. == A Common Misconception == It is important to note that the physical properties of air are not included in the definition of relative humidity. Often the notion of air holding water vapor is presented in discussions of relative humidity. This is misleading as [[air]] simply acts as a transporter of water vapour, not a holder of it. Relative humidity is wholly understood in terms of the physical properties of water alone and therefore is unrelated to the concept of ''air holding water''. [http://www.atmos.umd.edu/~stevenb/vapor/] [http://www.ems.psu.edu/~fraser/Bad/BadFAQ/BadCloudsFAQ.html] In fact, water vapor can be present in an airless volume and therefore the relative humidity of this volume can be readily calculated. This misconception is likely a result of the use of the word ''saturation'' which is often misused in definitions of relative humidity. In the present context the word ''saturation'' refers to the state of water vapor [http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html], not the [[solubility]] of one material in another. The thermophysical properties of water-air mixtures encountered at atmospheric conditions are reasonably approximated by assuming they behave as a mixture of [[ideal gas]]es. [http://www.nyu.edu/classes/tuckerman/honors.chem/lectures/lecture_8/node7.html]. For many practical purposes the assumption that both components (air and water) behave independently of each other is reasonable. Therefore the physical properties of the air-water 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. == 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 in a non-condensable phase other than air. [http://blowers.chee.arizona.edu/201project/GLsys.interrelatn.pg1.HTML] The relative humidity of an air-water system is dependent not only on the temperature but also on the absolute pressure of the system of interest. This is dependence demonstrated by considering the air-water system shown below. The system is closed (i.e. no matter enters or leaves the system). [[Image:Changes in Relative Humidity.png]] If the system at State A is isobariacally heated (heating with no change in system pressure) then the relative humidity of the system decreases because the saturated vapor pressure of water increases with increasing temperature. This is shown in State B If the system at State A is isothermally compressed (compressed with no change in system temperature) then the relative humidity of the system increases because the partial pressure of water in the system increases with increasing system pressure. This is shown in State C. 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 == [[Image:Relative_Humidity.png]] 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 possibility 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 (in other words, the droplets fall through the air at terminal velocity, but as they are very small, this terminal velocity is very small too, so it doesn't look to us like they are falling and they seem to be being held aloft). The statement that relative humidity (RH%) 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. 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 &ndash; the operative principle behind everything from [[hair dryer]]s to [[dehumidifier]]s. 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]], [[itch]]y [[eye]]s, and persistence of [[static electricity|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.) 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&nbsp;°F (11.1&nbsp;°C) increase in temperature. Thus, the relative humidity will drop by a factor of 2 for each 20&nbsp;°F (11.1&nbsp;°C) increase in temperature, assuming conservation of absolute moisture. For example, in the range of normal temperatures, air at 70&nbsp;°F (21.1&nbsp;°C) and 50% relative humidity will become saturated if cooled to 50°F (10&nbsp;°C), its [[dewpoint]] and 40&nbsp;°F (4.4&nbsp;°C) air at 80% relative humidity warmed to 70&nbsp;°F (21.1&nbsp;°C) will have a relative humidity of only 29% and feel dry. By comparison, a relative humidity between 40% and 60% is considered healthy and comfortable in comfort controlled environments (ASHRAE Standard 55[http://en.wikipedia.org/wiki/Thermal_comfort]). 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 forecasting|weather forecasts]] and reports, as it is an indicator of the likelihood of [[precipitation (meteorology)|precipitation]], dew, or fog. In hot [[summer]] [[weather]], it also increases the [[heat index|apparent temperature]] to [[human]]s (and other [[animal]]s) by hindering the [[evaporation]] of [[perspiration]] from the skin as the relative humidity rises. This effect is calculated as the [[heat index]] or [[humidex]]. 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.) ==References== <references/> * {{cite book | last = Himmelblau | first = David M. | authorlink = | title = Basic Principles And Calculations In Chemical Engineering | publisher = [[Prentice Hall]] | year = 1985454545 | doi = | id = ISBN 0-13-066572-X }} * {{cite book | last = Perry, R.H. and Green, D.W | first = | authorlink = | title = [[Perry's Chemical Engineers' Handbook]] (7th Edition) | publisher = [[McGraw-Hill]] | year = 1997 | doi = | id = ISBN 0-07-049841-5 }} == See also == *[[Humidity]] *[[Humidity#Absolute humidity|Absolute Humidity]] *[[Humidity#Specific humidity|Specific Humidity]] *[[Concentration]] *[[Heat index]] *[[Dew point]] *[[Dew point depression]] *[[Psychrometrics]] *[[Humidity indicator]] *[[Hygrometer]] ==External links== *[http://nsidc.org/arcticmet/glossary/psychrometric_tables.html Glossary definition of psychrometric tables] - National Snow and Ice Data Center *[http://www.ems.psu.edu/~fraser/Bad/BadFAQ/BadCloudsFAQ.html Bad Clouds FAQ, PSU.edu] [[Category:Psychrometrics]] [[Category:atmospheric thermodynamics]] [[Category:Physical quantity|Humidity]] [[ar:رطوبة نسبية]] [[cs:Vlhkost vzduchu#Relativn.C3.AD_.28pom.C4.9Brn.C3.A1.29_vlhkost_vzduchu]] [[et:Suhteline niiskus]] [[fa:رطوبت نسبی]] [[fr:Humidité relative]] [[id:Kelembaban relatif]] [[io:Relativa humideso]] [[it:Umidità relativa]] [[lt:Santykinis drėgnumas]] [[nl:Relatieve luchtvochtigheid]] [[pl:Wilgotność względna]] [[pt:Umidade relativa]] [[ru:Относительная влажность]] [[simple:Relative humidity]] [[th:ความชื้นสัมพัทธ์]] [[uk:Вологість відносна]] [[zh:相對濕度]]