Editing Humidity (section)

== Definitions ==
[[File:Antarctic Air Visits Paranal.jpg|thumb|[[Paranal Observatory]] on [[Cerro Paranal]] in the [[Atacama Desert]] is one of the driest places on Earth.<ref>{{cite news|title=Antarctic Air Visits Paranal|url=http://www.eso.org/public/images/potw1405a/|access-date=4 February 2014|newspaper=ESO Picture of the Week|archive-date=28 November 2020|archive-url=https://web.archive.org/web/20201128000636/https://eso.org/public/images/potw1405a/|url-status=live}}</ref> ]]

=== Absolute humidity ===

Absolute humidity is the total mass of water vapor (gas form of water) present in a given volume or mass of air. It does not take temperature into consideration. Absolute humidity in the atmosphere ranges from near zero to roughly {{cvt|30|g}} per cubic metre when the air is saturated at {{cvt|30|°C}}.<ref>{{cite web|title=Climate – Humidity indexes|url=https://www.britannica.com/science/climate-meteorology/Humidity-indexes|website=Encyclopaedia Britannica|access-date=15 February 2018|archive-date=16 November 2020|archive-url=https://web.archive.org/web/20201116162949/https://www.britannica.com/science/climate-meteorology/Humidity-indexes|url-status=live}}</ref><ref>{{cite web|title=Climate/humidity table|url=http://www.tis-gdv.de/tis_e/misc/klima.htm|website=Transport Information Service of the German Insurance Association|access-date=15 February 2018|archive-date=12 November 2020|archive-url=https://web.archive.org/web/20201112031304/https://www.tis-gdv.de/tis_e/misc/klima.htm/|url-status=live}}</ref>

Air is a gas, and its volume varies with pressure and temperature, per [[Boyle's law]].  ''Absolute humidity'' is defined as water mass per volume of air.  A given mass of air will grow or shrink as the temperature or pressure varies. So the absolute humidity of a mass of air will vary due to changes in temperature or pressure, even when the proportion of water in that mass of air (its ''[[#Specific humidity|specific humidity]]'') remains constant.  This makes the term ''absolute humidity'' as defined not ideal for some situations. 

Absolute humidity is the mass of the water vapor <math> (m_{\text{H}_2\text{O}}) </math>, divided by the volume of the air and water vapor mixture <math> (V_\text{net} )</math>, which can be expressed as:
<math display="block"> AH = \frac{m_{\text{H}_2\text{O}}}{V_\text{net}}. </math>  In the equation above, if the volume is not set, the absolute humidity varies with changes in air temperature or pressure. Because of this variability, use of the term ''absolute humidity'' as defined is inappropriate for computations in chemical engineering, such as drying, where temperature variations might be significant. As a result, absolute humidity in chemical engineering may refer to mass of water vapor per unit mass of dry air, also known as the ''humidity ratio'' or ''mass mixing ratio'' (see "specific humidity" below), which is better suited for heat and mass balance calculations.{{cn|date=December 2024}}  Mass of water per unit volume as in the equation above is also defined as ''volumetric humidity''. Because of the potential confusion, [[British Standard]] BS 1339<ref name="BS1339">British Standard BS 1339 (revised), Humidity and Dewpoint, Parts 1–3 (2002–2007)</ref> suggests avoiding the term "absolute humidity". Units should always be carefully checked.  Many humidity charts are given in g/kg or kg/kg, but any mass units may be used.

=== Relative humidity ===
Relative humidity is the ratio of ''how much water vapour is in the air'' to ''how much water vapour the air could potentially contain'' at a given temperature and pressure. 

If a sample of humid air at temperature T<sub>1</sub> contains water vapour with [[partial pressure]] P<sub>w</sub> the relative humidity RH is:<ref>Van Wylen and Sonntag (1976), ''Fundamentals of Classical Thermodynamics'', SI Version 2e, Equation 11.16. John Wiley & Sons</ref>
:<math>RH = \frac{P_w}{P_s}</math><br>
where P<sub>s</sub> is the saturation pressure of water at temperature T<sub>1</sub>.

Relative humidity varies with any change in the temperature or pressure of the air: colder air can contain less vapour, and water will tend to condense out of the air more at lower temperatures. So changing the temperature of air can change the relative humidity, even when the specific humidity remains constant. If two parcels of air have the same specific humidity and temperature but different pressures, the parcel at the higher pressure will have the higher relative humidity.

Cooling air increases the relative humidity. If the relative humidity rises to 100% (the [[dew point]]) and there is an available surface or particle, the water vapour will [[Condensation|condense]] into liquid or [[Deposition (phase transition)|deposit]] into ice. Likewise, warming air decreases the relative humidity. Warming some air containing a fog may cause that fog to [[evaporate]], as the droplets are prone to total evaporation due to the lowering [[partial pressure]] of water vapour in that air, as the temperature rises.

Relative humidity only considers the invisible water vapour. Mists, clouds, fogs and aerosols of water do not count towards the measure of relative humidity of the air, although their presence is an indication that a body of air may be close to the dew point.

Relative humidity is normally expressed as a percentage; a higher percentage means that the air–water mixture is more humid. At 100% relative humidity, the air is saturated and is at its dew point. In the absence of a foreign body on which droplets or crystals can [[nucleate]], the relative humidity can exceed 100%, in which case the air is said to be [[supersaturated]]. Introduction of some particles or a surface to a body of air above 100% relative humidity will allow condensation or ice to form on those nuclei, thereby removing some of the vapour and lowering the humidity.

In a scientific notion, the relative humidity (<math>RH</math> or <math>\phi</math>) of an air-water mixture is defined as the ratio of the [[partial pressure]] of water vapor (<math>p</math>) in air to the [[saturation vapor pressure]] (<math>p_s</math>) of water at the same temperature, usually expressed as a percentage:<ref name="Perry">Perry, R. H. and Green, D. W, ''[[Perry's Chemical Engineers' Handbook]]'' (8th Edition), [[McGraw-Hill]], {{ISBN|0-07-142294-3}}, pp. 12–14</ref><ref>{{cite book |last=Lide |first=David |date=2005 |title=CRC Handbook of Chemistry and Physics |edition=85 |url=https://archive.org/details/crchandbookofche81lide/page/15 |publisher=CRC Press |pages=[https://archive.org/details/crchandbookofche81lide/page/15 15–25] |isbn=0-8493-0485-7 }}</ref><ref name="Babin" />
<math display="block"> \phi = 100\% \cdot p/p_s</math>

Relative humidity is an important [[indicator (statistics)|metric]] used 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, a rise in relative humidity increases the [[apparent temperature]] to humans (and other animals) by hindering the [[evaporation]] of [[perspiration]] from the skin. For example, according to the ''[[heat index]]'', a relative humidity of 75% at air temperature of {{convert|80.0|F}} would feel like {{cvt|83.6|±|1.3|F}}.<ref>{{cite web |url=http://www.srh.noaa.gov/images/ffc/pdf/ta_htindx.PDF |first=Lans P. |last=Rothfusz |title=The Heat Index 'Equation' (or, More Than You Ever Wanted to Know About Heat Index) |publisher=Scientific Services Division (NWS Southern Region Headquarters) |date=1 July 1990 |access-date=2022-11-06 |url-status=dead |archive-url=https://web.archive.org/web/20111201083223/http://www.srh.noaa.gov/images/ffc/pdf/ta_htindx.PDF |archive-date=2011-12-01 }}</ref><ref name="Steadman1979">{{cite journal|last1=Steadman|first1=R. G.|title=The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science|journal=Journal of Applied Meteorology|volume=18|issue=7|year=1979|pages=861–873|issn=0021-8952|doi=10.1175/1520-0450(1979)018<0861:TAOSPI>2.0.CO;2|bibcode=1979JApMe..18..861S|doi-access=free}}</ref>

Because wood changes shape with changes in humidity, relative humidity is used to evaluate moisture content and size changes in wood, such as making allowances for seasonal movement in wood floors.

=== Specific humidity ===
<!-- This section is linked from [[Relative humidity]] -->
''Specific humidity'' (or moisture content) is the ratio of the mass of water vapor to the total mass of the air parcel.<ref>{{cite web|last1=Seidel|first1=Dian|title=What is atmospheric humidity and how is it measured?|url=http://www.arl.noaa.gov/faq_c1.php|website=National Oceanic and Atmospheric Administration|access-date=3 March 2017|archive-url=https://web.archive.org/web/20171018232823/http://www.arl.noaa.gov/faq_c1.php|archive-date=18 October 2017|url-status=dead}}</ref> Specific humidity is approximately equal to the [[mixing ratio]], which is defined as the ratio of the mass of water vapor in an air parcel to the mass of ''dry'' air for the same parcel.  It is typically represented with the symbol ω, and is commonly used in [[HVAC]] system design.{{cn|date=December 2024}}

=== 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.<ref>{{cite web|url=http://blowers.chee.arizona.edu/201project/GLsys.interrelatn.pg1.HTML|title=Vapor-Liquid/Solid System, 201 Class Page|publisher=University of Arizona|archive-url=https://web.archive.org/web/20060508100622/http://blowers.chee.arizona.edu/201project/GLsys.interrelatn.pg1.HTML |archive-date=May 8, 2006|url-status=dead}}</ref>