Editing Humidity (section)

=== 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.