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Humidity
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== Other important facts == {{refimprovesect|date=January 2025}} [[File:Relative Humidity.png|thumb|upright=1.5]] At 100% relative humidity, air is [[saturated fluid|saturated]] and at its [[dew point]]: the water vapor pressure would permit neither [[evaporation]] of nearby liquid water nor [[condensation]] to grow the nearby water; neither [[Sublimation (phase transition)|sublimation]] of nearby ice nor [[Deposition (phase transition)|deposition]] to grow the nearby ice. Relative humidity can exceed 100%, in which case the air is [[supersaturated]]. Cloud formation requires supersaturated air. [[Cloud condensation nuclei]] lower the level of supersaturation required to form fogs and clouds – in the absence of nuclei around which droplets or ice can form, a higher level of supersaturation is required for these droplets or ice crystals to form spontaneously. In the [[Wilson cloud chamber]], which is used in nuclear physics experiments, a state of supersaturation is created within the chamber, and moving subatomic particles act as condensation nuclei so trails of fog show the paths of those particles. For a given dew point and its corresponding [[absolute humidity]], the relative humidity will change inversely, albeit nonlinearly, with the temperature. This is because the vapor pressure of water increases with temperature—the operative principle behind everything from hair dryers 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 {{convert|30|C}} compared to no more than about 0.5% by mass at {{convert|0|C}}. This explains the low levels (in the absence of measures to add moisture) of humidity in heated structures during winter, resulting in dry skin, itchy eyes, 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 and household plants. 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 dew point, 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 {{convert|20|F-change}} increase in temperature. Thus, the relative humidity will drop by a factor of 2 for each {{convert|20|F-change}} increase in temperature, assuming conservation of absolute moisture. For example, in the range of normal temperatures, air at {{convert|68|F}} and 50% relative humidity will become saturated if cooled to {{convert|50|F}}, its dew point, and {{convert|41|F}} air at 80% relative humidity warmed to {{convert|68|F}} will have a relative humidity of only 29% and feel dry. By comparison, thermal comfort standard [[ASHRAE 55]] requires systems designed to control humidity to maintain a dew point of {{convert|16.8|C}} though no lower humidity limit is established.<ref name=ASHRAE55 /> Water vapor is a lighter gas than other gaseous components of 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 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]]. 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 field concerned with the study of physical and thermodynamic properties of gas–vapor mixtures is named [[psychrometrics]]. === Relationship between absolute humidity, relative humidity, and temperature === {| class="wikitable" |+ Absolute humidity of [[Atmosphere of Earth|Earth's atmosphere]] at [[sea level]] in g/m{{sup|3}} (oz/cu. yd)<ref>{{cite web |title=Climate/humidity table |website=Transport Informations Service |publisher=German Insurance Association |url=https://www.tis-gdv.de/tis_e/misc/klima-htm/ |access-date=2021-06-17 |archive-date=2021-06-24 |archive-url=https://web.archive.org/web/20210624195928/https://www.tis-gdv.de/tis_e/misc/klima-htm/ |url-status=live }}</ref><ref>{{cite web |title=Absolute Humidity Table |url=http://mercury.pr.erau.edu/~draut129/InterestingStuff/AbsoluteHumidityTable.pdf |access-date=2021-06-17 |website=mercury.pr.erau.edu |archive-date=2021-06-24 |archive-url=https://web.archive.org/web/20210624195457/http://mercury.pr.erau.edu/~draut129/InterestingStuff/AbsoluteHumidityTable.pdf |url-status=dead }}</ref> |- !rowspan="2" scope="col" | Temperature !colspan="11" scope="colgroup" | Relative humidity |- ! scope="col" | 0% !! scope="col" | 10% !! scope="col" | 20% !! scope="col" | 30% !! scope="col" | 40% !! scope="col" | 50% !! scope="col" | 60% !! scope="col" | 70% !! scope="col" | 80% !! scope="col" | 90% !! scope="col" | 100% |- | {{convert|50|C|F}} | {{convert|0|g/m3|oz/cuyd|abbr=values}} | {{convert|8.3|g/m3|oz/cuyd|abbr=values}} | {{convert|16.6|g/m3|oz/cuyd|abbr=values}} | {{convert|24.9|g/m3|oz/cuyd|abbr=values}} | {{convert|33.2|g/m3|oz/cuyd|abbr=values}} | {{convert|41.5|g/m3|oz/cuyd|abbr=values}} | {{convert|49.8|g/m3|oz/cuyd|abbr=values}} | {{convert|58.1|g/m3|oz/cuyd|abbr=values}} | {{convert|66.4|g/m3|oz/cuyd|abbr=values}} | {{convert|74.7|g/m3|oz/cuyd|abbr=values}} | {{convert|83.0|g/m3|oz/cuyd|abbr=values}} |- | 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