Editing Humidity (section)

{{short description|Concentration of water vapour in the air}}
{{for|the Serbian film|Humidity (film)}}
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{{Humidity|img=Cloud forest mount kinabalu.jpg}}<!-- caption: Tropical forests often have high humidity -->

[[File:RH wiki.png|thumb|Global distribution of relative humidity at the surface averaged over the years 1981–2010 from the CHELSA-BIOCLIM+ data set<ref>{{cite web |url=https://essd.copernicus.org/preprints/essd-2022-212/essd-2022-212.pdf |title=Global climate-related predictors at kilometre resolution for the past and future |last1=Brun |first1=Philipp |last2=Zimmermann |first2=Niklaus E. |date=2022-06-27 |publisher=ESSD – Land/Biogeosciences and biodiversity |doi=10.5194/essd-2022-212 |doi-access=free |last3=Hari |first3=Chantal |last4=Pellissier |first4=Loïc |last5=Karger |first5=Dirk N. |url-status=live |archive-url=https://web.archive.org/web/20230108232418/https://essd.copernicus.org/preprints/essd-2022-212/essd-2022-212.pdf |archive-date= Jan 8, 2023 }}</ref>]]

'''Humidity''' is the concentration of [[water vapor]] present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye.<ref>{{cite web|title=What is water vapor? |website=WeatherQuestions.com |url=http://www.weatherquestions.com/What_is_water_vapor.htm|access-date=2012-08-28|archive-date=2019-03-11|archive-url=https://web.archive.org/web/20190311045520/http://www.weatherquestions.com/What_is_water_vapor.htm|url-status=live}}</ref> Humidity indicates the likelihood for [[precipitation (meteorology)|precipitation]], [[dew]], or [[fog]] to be present.

Humidity depends on the temperature and pressure of the system of interest. The same amount of water vapor results in higher relative humidity in cool air than warm air. A related parameter is the [[dew point]]. The amount of water vapor needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing water mass. The amount of water vapor contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 8&nbsp;g of water per cubic metre of air at {{cvt|8|°C}}, and 28&nbsp;g of water per cubic metre of air at {{cvt|30|°C}}

Three primary measurements of humidity are widely employed: absolute, relative, and specific. '''Absolute humidity''' is expressed as either mass of water vapor per volume of moist air (in grams per cubic meter)<ref>{{cite book|chapter-url=https://archive.org/details/treatiseonproduc00wyerrich/page/22|title=A Treatise on Producer-Gas and Gas-Producers|last=Wyer|first=Samuel S.|publisher=McGraw-Hill Book Company|year=1906|pages=23|chapter=Fundamental Physical Laws and Definitions}}</ref> or as mass of water vapor per mass of dry air (usually in grams per kilogram).<ref>Perry, R.H. and Green, D.W, (2007) ''[[Perry's Chemical Engineers' Handbook]]'' (8th Edition),  Section 12, Psychrometry, Evaporative Cooling and Solids Drying [[McGraw-Hill]], {{ISBN|978-0-07-151135-3}}</ref> '''Relative humidity''', often expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature. '''Specific humidity''' is the [[ratio]] of water vapor mass to total moist air parcel mass.

Humidity plays an important role for surface life. For animal life dependent on [[perspiration]] (sweating) to regulate internal body temperature, high humidity impairs heat exchange efficiency by reducing the rate of moisture [[evaporation]] from skin surfaces. This effect can be calculated using a [[heat index]] table, or alternatively using a similar  [[humidex]].

The notion of air "holding" water vapor or being "saturated" by it is often mentioned in connection with the concept of relative humidity. This, however, is misleading—the amount of water vapor that enters (or can enter) a given space at a given temperature is almost independent of the amount of air (nitrogen, oxygen, etc.) that is present. Indeed, a vacuum has approximately the same equilibrium capacity to hold water vapor as the same volume filled with air; both are given by the equilibrium vapor pressure of water at the given temperature.<ref name="Babin"/><ref name=psu.edu /> There is a very small difference described under "Enhancement factor" below, which can be neglected in many calculations unless great accuracy is required.