Editing Humidity (section)

== Effects ==
[[File:Acoustic guitar room at Guitar Center humidified to 50% relative humidity in winter.jpg|thumb|Hygrostat set to 50% relative humidity]]
[[File:Humidor innen.jpg|thumb|[[Humidor]], used to control humidity of cigars]]
Climate control refers to the control of temperature and relative humidity in buildings, vehicles and other enclosed spaces for the purpose of providing for human comfort, health and safety, and of meeting environmental requirements of machines, sensitive materials (for example, historic) and technical processes.

=== Climate ===
{{See also|Precipitation (meteorology)|Humid subtropical climate}}
[[File:Australia-9am-daily-humidity-average.png|thumb|Average humidity around Australia year-round at 9 am {{legend|#94a257|80–90%}}{{legend|#eee8b4|30–40%}}]]
While humidity itself is a climate variable, it also affects other climate variables. Environmental humidity is affected by winds and by rainfall.

The most humid cities on Earth are generally located closer to the equator, near coastal regions. Cities in parts of Asia and Oceania are among the most humid. [[Bangkok]], [[Ho Chi Minh City]], [[Kuala Lumpur]], [[Hong Kong]], [[Metro Manila|Manila]], [[Jakarta]], [[Naha]], [[Singapore]], [[Kaohsiung]] and [[Taipei]] have very high humidity most or all year round because of their proximity to water bodies and the equator and often overcast weather. 

Some places experience extreme humidity during their rainy seasons combined with warmth giving the feel of a lukewarm sauna, such as [[Kolkata]], [[Chennai]] and [[Kochi]] in India, and [[Lahore]] in Pakistan. [[Sukkur]] city located on the [[Indus River]] in Pakistan has some of the highest and most uncomfortable [[dew point]]s in the country, frequently exceeding {{convert|30|C|F|}} in the [[monsoon]] season.<ref>{{cite web|url=http://www.wunderground.com/history/airport/OPSK/2013/7/6/DailyHistory.html|title=Weather History for Sukkur, Pakistan – Weather Underground|access-date=2013-07-31|archive-date=2017-09-15|archive-url=https://web.archive.org/web/20170915114104/https://www.wunderground.com/history/airport/OPSK/2013/7/6/DailyHistory.html|url-status=live}}</ref>

High temperatures combine with the high dew point to create heat index in excess of {{convert|65|C|F|}}. [[Darwin, Australia|Darwin]] experiences an extremely humid wet season from December to April. [[Houston]], [[Miami]], [[San Diego]], [[Osaka]], [[Shanghai]], [[Shenzhen]] and [[Tokyo]] also have an extreme humid period in their summer months. During the South-west and North-east Monsoon seasons (respectively, late May to September and November to March), expect heavy rains and a relatively high humidity post-rainfall. 

Outside the monsoon seasons, humidity is high (in comparison to countries further from the Equator), but completely sunny days abound. In cooler places such as Northern Tasmania, Australia, high humidity is experienced all year due to the ocean between mainland Australia and Tasmania. In the summer the hot dry air is absorbed by this ocean and the temperature rarely climbs above {{convert|35|C|0|abbr=on}}.

=== Global climate ===
{{See also|Greenhouse effect}}
Humidity affects the [[energy budget]] and thereby influences temperatures in two major ways. First, water vapor in the atmosphere contains "latent" energy. During transpiration or evaporation, this [[latent heat]] is removed from surface liquid, cooling the Earth's surface. This is the biggest non-radiative cooling effect at the surface. It compensates for roughly 70% of the average net radiative warming at the surface.

Second, water vapor is the most abundant of all [[greenhouse gases]]. Water vapor, like a green lens that allows green light to pass through it but absorbs red light, is a "selective absorber". Like the other greenhouse gasses, water vapor is transparent to most solar energy. However, it absorbs the infrared energy emitted (radiated) upward by the Earth's surface, which is the reason that humid areas experience very little nocturnal cooling but dry desert regions cool considerably at night. This selective absorption causes the greenhouse effect. It raises the surface temperature substantially above its theoretical radiative equilibrium temperature with the sun, and water vapor is the cause of more of this warming than any other greenhouse gas.

Unlike most other greenhouse gases, however, water is not merely below its boiling point in all regions of the Earth, but below its freezing point at many altitudes.  As a condensible greenhouse gas, it [[Precipitation|precipitates]], with a much lower [[scale height]] and shorter atmospheric lifetime — weeks instead of decades.  Without other greenhouse gases, Earth's [[blackbody temperature]], below the freezing point of water, would cause water vapor to be removed from the atmosphere.<ref>{{cite web |url=http://www.saga.iao.ru/glossary/?catalog=9&sowa=All&term=@1621 |title=Blackbody Radiation |access-date=2015-01-11 |archive-date=2020-08-14 |archive-url=https://web.archive.org/web/20200814131004/http://www.saga.iao.ru/glossary/?catalog=9&sowa=All&term=@1621 |url-status=live }}</ref><ref>{{cite web |url=http://www.physics.rutgers.edu/~abragg/109/L14.html |title=Lecture notes |access-date=2015-01-11 |archive-date=2017-10-23 |archive-url=https://web.archive.org/web/20171023010911/http://www.physics.rutgers.edu/~abragg/109/L14.html |url-status=dead }}</ref><ref>{{cite web |url=http://storm.colorado.edu//~toohey/lecture8.html |title=Radiative Balance, Earth's Temperature, and Greenhouse Gases (lecture notes) |access-date=2015-01-11 |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304085052/http://storm.colorado.edu//~toohey/lecture8.html |url-status=live }}</ref>  Water vapor is thus a "slave" to the non-condensible greenhouse gases.<ref>{{cite web | url=https://www.e-education.psu.edu/geosc10/l12_p7.html | title=GEOSC 10 Optional Enrichment Article 1 | author=Alley, R. | date=2014 | access-date=2015-01-11 | archive-date=2018-09-08 | archive-url=https://web.archive.org/web/20180908115943/https://www.e-education.psu.edu/geosc10/l12_p7.html | url-status=dead }}</ref><ref>{{cite web|url=http://www.soest.hawaii.edu/MET/Faculty/businger/courses/notes101/28ModelingClimate.pdf |title=Lecture 28: Future Global Warming Modeling Climate Change |author=Businger, S. |url-status=dead |archive-url=https://web.archive.org/web/20150130212102/http://www.soest.hawaii.edu/MET/Faculty/businger/courses/notes101/28ModelingClimate.pdf |archive-date=2015-01-30 }}</ref><ref>{{cite web | url=http://www.astro.washington.edu/users/eschwiet/essays/greenhouse_ASTR555.pdf | title=Comparing the Greenhouse Effect on Earth, Mars, Venus, and Titan: Present Day and through Time | author=Schwieterman, E. | access-date=2015-01-11 | archive-date=2015-09-23 | archive-url=https://web.archive.org/web/20150923175539/http://www.astro.washington.edu/users/eschwiet/essays/greenhouse_ASTR555.pdf | url-status=dead }}</ref>

=== Animal and plant life ===
[[File:Tillandsia usneoides (14956541092).jpg|thumb|upright|''[[Tillandsia usneoides]]'' in Tropical house, Royal Botanic Gardens, Kew. It is growing where the climate is warm enough and has a relatively high average humidity.]]
Humidity is one of the fundamental [[abiotic factor]]s that defines any habitat (the tundra, wetlands, and the desert are a few examples), and is a determinant of which animals and plants can thrive in a given environment.<ref>C. Michael Hogan. 2010. [http://www.eoearth.org/article/Abiotic_factor?topic=49461 ''Abiotic factor''. Encyclopedia of Earth. eds Emily Monosson and C. Cleveland. National Council for Science and the Environment] {{webarchive |url=https://web.archive.org/web/20130608071757/http://www.eoearth.org/article/Abiotic_factor?topic=49461 |date=June 8, 2013 }}. Washington DC</ref>

The human body dissipates heat through perspiration and its evaporation. [[Heat convection]], to the surrounding air, and [[thermal radiation]] are the primary modes of heat transport from the body. Under conditions of high humidity, the rate of evaporation of sweat from the skin decreases. Also, if the atmosphere is as warm or warmer than the skin during times of high humidity, blood brought to the body surface cannot dissipate heat by conduction to the air. With so much blood going to the external surface of the body, less goes to the active muscles, the brain, and other internal organs. Physical strength declines, and fatigue occurs sooner than it would otherwise. Alertness and mental capacity also may be affected, resulting in ''heat stroke'' or [[hyperthermia]].

Domesticated plants and animals (e.g. lizards) require regular upkeep of humidity percent when grown in-home and container conditions, for optimal thriving environment.

=== Human comfort ===
Although humidity is an important factor for thermal comfort, humans are more sensitive to variations in temperature than they are to changes in relative humidity.{{sfn|Fanger|1970|p=48}} Humidity has a small effect on thermal comfort outdoors when air temperatures are low, a slightly more pronounced effect at moderate air temperatures, and a much stronger influence at higher air temperatures.{{sfn|Bröde|Fiala|Błażejczyk|Holmér|2011|pp=481–494}}

Humans are sensitive to humid air because the human body uses evaporative cooling as the primary mechanism to regulate temperature. Under humid conditions, the ''rate'' at which perspiration evaporates on the skin is lower than it would be under arid conditions. Because humans perceive the rate of heat transfer from the body rather than temperature itself, we feel warmer when the relative humidity is high than when it is low.

Humans can be comfortable within a wide range of humidities depending on the temperature—from 30 to 70%{{sfn|Gilmore|1972|p=99}}—but ideally not above the Absolute (60&nbsp;°F Dew Point),<ref>[https://events.rdmobile.com/Sessions/Details/1098952] {{Webarchive|url=https://web.archive.org/web/20210210104307/https://events.rdmobile.com/Sessions/Details/1098952|date=2021-02-10}} ASHRAE Std 62.1-2019</ref> between 40%<ref name=infoplease.com /> and 60%.<ref name=engineeringtoolbox.com />  In general, higher temperatures will require lower humidities to achieve thermal comfort compared to lower temperatures, with all other factors held constant. For example, with clothing level = 1, metabolic rate = 1.1, and air speed 0.1&nbsp;m/s, a change in air temperature and mean radiant temperature from 20&nbsp;°C to 24&nbsp;°C would lower the maximum acceptable relative humidity from 100% to 65% to maintain thermal comfort conditions. The [[Center for the Built Environment|CBE]] Thermal Comfort Tool can be used to demonstrate the effect of relative humidity for specific thermal comfort conditions and it can be used to demonstrate compliance with ASHRAE Standard 55–2017.{{sfn|Schiavon|Hoyt|Piccioli|2013|pp=321–334}}

Some people experience difficulty breathing in humid environments. Some cases may possibly be related to respiratory conditions such as asthma, while others may be the product of anxiety. Affected people will often [[hyperventilation|hyperventilate]] in response, causing sensations of numbness, faintness, and loss of [[Attentional control|concentration]], among others.<ref>{{cite web|url=https://www.lung.ca/news/expert-opinions/pollution/heat-and-humidity|title=Heat and humidity – the lung association|website=www.lung.ca|date=26 August 2014|access-date=14 March 2018|archive-date=24 October 2020|archive-url=https://web.archive.org/web/20201024090754/https://www.lung.ca/news/expert-opinions/pollution/heat-and-humidity|url-status=live}}</ref>

Very low humidity can create discomfort, respiratory problems, and aggravate allergies in some individuals. Low humidity causes tissue lining nasal passages to dry, crack and become more susceptible to penetration of [[rhinovirus]] cold viruses.<ref name=rochester.edu /> Extremely low (below 20{{Spaces|1|thin}}%) relative humidities may also cause eye irritation.{{sfn|Arundel|Sterling|Biggin|Sterling|1986|pp=351–361}}<ref name=watertreatmentservices.co.uk /> The use of a [[humidifier]] in homes, especially bedrooms, can help with these symptoms.<ref name=webmd.com /> Indoor relative humidities kept above 30% reduce the likelihood of the occupant's nasal passages drying out, especially in winter.{{sfn|Arundel|Sterling|Biggin|Sterling|1986|pp=351–361}}<ref name=dhhs.nh.gov /><ref name=doh.wa.gov />

[[Air conditioning]] reduces discomfort by reducing not just temperature but humidity as well. Heating cold outdoor air can decrease relative humidity levels indoors to below 30%.<ref>{{cite web|title=Optimum Humidity Levels for Home|url=http://www.airbetter.org/optimum-humidity-levels-home/|website=AirBetter.org|date=3 August 2014|access-date=8 January 2017|archive-date=10 January 2020|archive-url=https://web.archive.org/web/20200110125649/https://www.airbetter.org/optimum-humidity-levels-home/|url-status=live}}</ref> According to [[ASHRAE 55|ASHRAE Standard 55-2017: Thermal Environmental Conditions for Human Occupancy]], indoor thermal comfort can be achieved through the [[PMV/PPD model|PMV]] method with relative humidities ranging from 0% to 100%, depending on the levels of the other factors contributing to thermal comfort.<ref name=ASHRAE55 /> However, the recommended range of indoor relative humidity in air conditioned buildings is generally 30–60%.{{sfn|Wolkoff|Kjaergaard|2007|pp=850–857}}<ref name=ASHRAE160 />

=== Human health ===
Higher humidity reduces the infectivity of aerosolized influenza virus. A study concluded, "Maintaining indoor relative humidity >40% will significantly reduce the infectivity of aerosolized virus."<ref>{{cite journal |last1=Noti |first1=John D. |last2=Blachere |first2=Francoise M. |last3=McMillen |first3=Cynthia M. |last4=Lindsley |first4=William G. |last5=Kashon |first5=Michael L. |last6=Slaughter |first6=Denzil R. |last7=Beezhold|first7=Donald H. |year=2013 |title=High Humidity Leads to Loss of Infectious Influenza Virus from Simulated Coughs |journal=PLOS ONE |volume=8 |issue=2 |pages=e57485 |bibcode=2013PLoSO...857485N |doi=10.1371/journal.pone.0057485 |pmid=23460865|pmc=3583861 |doi-access=free }}</ref>

Excess moisture in buildings expose occupants to fungal spores, cell fragments, or [[mycotoxin]]s.<ref name="pmid21196349">{{cite journal | vauthors=Park J, Cox-Ganser JM | title=Meta-Mold exposure and respiratory health in damp indoor environments | journal=[[Frontiers in Bioscience]] | volume=3 | issue=2 | pages=757–771 | year=2011 | doi = 10.2741/e284 | pmid=21196349| doi-access=free }}</ref> Infants in homes with [[mold]] have a much greater risk of developing [[asthma]] and [[allergic rhinitis]].<ref name="pmid21196349" /> More than half of adult workers in moldy/humid buildings develop nasal or sinus symptoms due to mold exposure.<ref name="pmid21196349" />

[[Mucociliary clearance]] in the [[respiratory tract]] is also hindered by low humidity. One study in dogs found that mucus transport was lower at an absolute humidity of 9&nbsp;g/m<sup>3</sup> than at 30&nbsp;g/m<sup>3</sup>.<ref name="MRM">{{cite journal |last1=Pieterse |first1=A |last2=Hanekom |first2=SD |title=Criteria for enhancing mucus transport: a systematic scoping review. |journal=Multidisciplinary Respiratory Medicine |date=2018 |volume=13 |pages=22 |doi=10.1186/s40248-018-0127-6 |pmid=29988934|pmc=6034335 |doi-access=free }}</ref>

Increased humidity can also lead to changes in [[Body water|total body water]] that usually leads to moderate weight gain, especially if one is acclimated to working or exercising in hot and humid weather.<ref>{{cite web|title=To what degree is a person's body weight affected by the ambient temperature and humidity? Do we conserve or release water as the climate changes?|url=https://www.scientificamerican.com/article/experts-body-wieght-ambient-temperature/|access-date=2021-06-09|website=Scientific American|language=en|archive-date=2021-06-09|archive-url=https://web.archive.org/web/20210609144808/https://www.scientificamerican.com/article/experts-body-wieght-ambient-temperature/|url-status=live}}</ref>

=== Building construction ===
[[File:Primary efflorescence decades later 001.png|thumb|Effects of high humidity level in a building structure ([[efflorescence|primary efflorescence]])]]
Common construction methods often produce building enclosures with a poor thermal boundary, requiring an [[Building insulation|insulation]] and air barrier system designed to retain indoor environmental conditions while resisting external environmental conditions.<ref>{{cite web|url=https://www.iea.org/reports/all-countries-targeted-for-zero-carbon-ready-codes-for-new-buildings-by-2030-2|title=Free publications|access-date=2013-12-23|archive-date=2022-10-09|archive-url=https://web.archive.org/web/20221009013832/https://www.iea.org/reports/all-countries-targeted-for-zero-carbon-ready-codes-for-new-buildings-by-2030-2|url-status=live}}</ref>  The energy-efficient, heavily sealed architecture introduced in the 20th century also sealed off the movement of moisture, and this has resulted in a secondary problem of [[condensation]] forming in and around walls, which encourages the development of mold and mildew. Additionally, buildings with foundations not properly sealed will allow water to flow through the walls due to [[capillary action]] of pores found in masonry products. Solutions for energy-efficient buildings that avoid condensation are a current topic of architecture.

For climate control in buildings using [[HVAC]] systems, the key is to maintain the relative humidity at a comfortable range—low enough to be comfortable but high enough to avoid problems associated with very dry air.

When the temperature is high and the relative humidity is low, evaporation of water is rapid; soil dries, wet clothes hung on a line or rack dry quickly, and perspiration readily evaporates from the skin. Wooden furniture can shrink, causing the paint that covers these surfaces to fracture.

When the temperature is low and the relative humidity is high, evaporation of water is slow. When relative humidity approaches 100%, condensation can occur on surfaces, leading to problems with mold, corrosion, decay, and other moisture-related deterioration. Condensation can pose a safety risk as it can promote the growth of mold and wood rot as well as possibly freezing emergency exits shut.

Certain production and technical processes and treatments in factories, laboratories, hospitals, and other facilities require specific relative humidity levels to be maintained using humidifiers, [[dehumidifier]]s and associated control systems.

=== Vehicles ===
The basic principles for buildings, above, also apply to vehicles. In addition, there may be safety considerations. For instance, high humidity inside a vehicle can lead to problems of condensation, such as misting of windshields and [[Short circuit|shorting]] of electrical components. In vehicles and [[pressure vessel]]s such as pressurized airliners, submersibles and spacecraft, these considerations may be critical to safety, and complex [[environmental control system]]s including equipment to maintain pressure are needed.

=== Aviation ===
Airliners operate with low internal relative humidity, often under 20%,<ref name=aviatoratlas.com>{{cite web |title=Airplane Humidity |work=Aviator Atlas |date=5 April 2020 |access-date=11 September 2020 |url= https://www.aviatoratlas.com/posts/airplane-humidity }}</ref> especially on long flights. The low humidity is a consequence of drawing in the very cold air with a low absolute humidity, which is found at airliner cruising altitudes. Subsequent warming of this air lowers its relative humidity. This causes discomfort such as sore eyes, dry skin, and drying out of mucosa, but humidifiers are not employed to raise it to comfortable mid-range levels because the volume of water required to be carried on board can be a significant weight penalty. As airliners descend from colder altitudes into warmer air, perhaps even flying through clouds a few thousand feet above the ground, the ambient relative humidity can increase dramatically. 

Some of this moist air is usually drawn into the pressurized aircraft cabin and into other non-pressurized areas of the aircraft and condenses on the cold aircraft skin. Liquid water can usually be seen running along the aircraft skin, both on the inside and outside of the cabin. Because of the drastic changes in relative humidity inside the vehicle, components must be qualified to operate in those environments. The recommended environmental qualifications for most commercial aircraft components is listed in [[DO-160|RTCA DO-160]].

Cold, humid air can promote the formation of ice, which is a danger to aircraft as it affects the wing profile and increases weight. Naturally aspirated internal combustion engines have a further danger of ice forming inside the [[carburetor]]. Aviation weather reports ([[METAR]]s) therefore include an indication of relative humidity, usually in the form of the [[dew point]].

Pilots must take humidity into account when calculating takeoff distances, because high humidity requires longer runways and will decrease climb performance.

Density altitude is the altitude relative to the standard atmosphere conditions (International Standard Atmosphere) at which the air density would be equal to the indicated air density at the place of observation, or, in other words, the height when measured in terms of the density of the air rather than the distance from the ground. "Density Altitude" is the pressure altitude adjusted for non-standard temperature.

An increase in temperature, and, to a much lesser degree, humidity, will cause an increase in density altitude. Thus, in hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude.

=== Electronics ===
[[File:Silica gel - bag.jpg|thumb|upright|[[Desiccant]] bag ([[silica gel]]), commonly included in packages containing electronic products to control humidity]]

Electronic devices are often rated to operate only under certain humidity conditions (e.g., 10% to 90%). The optimal humidity for electronic devices is 30% to 65%. At the top end of the range, moisture may increase the conductivity of permeable [[Insulator (electrical)|insulators]] leading to malfunction. Too low humidity may make materials brittle. A particular danger to electronic items, regardless of the stated operating humidity range, is [[condensation]]. When an electronic item is moved from a cold place (e.g., garage, car, shed, air conditioned space in the tropics) to a warm humid place (house, outside tropics), condensation may coat circuit boards and other insulators, leading to [[short circuit]] inside the equipment. Such short circuits may cause substantial permanent damage if the equipment is powered on before the condensation has [[evaporated]]. A similar condensation effect can often be observed when a person wearing glasses comes in from the cold (i.e. the glasses become foggy).<ref>{{cite web|title=Fogging Glasses|url=http://www.newton.dep.anl.gov/askasci/gen01/gen01333.htm|access-date=2012-08-08|archive-date=2015-02-26|archive-url=https://web.archive.org/web/20150226032245/http://www.newton.dep.anl.gov/askasci/gen01/gen01333.htm|url-status=dead}}</ref> 

It is advisable to allow electronic equipment to acclimatise for several hours, after being brought in from the cold, before powering on. Some electronic devices can detect such a change and indicate, when plugged in and usually with a small droplet symbol, that they cannot be used until the risk from condensation has passed. In situations where time is critical, increasing air flow through the device's internals, such as removing the side panel from a PC case and directing a fan to blow into the case, will reduce significantly the time needed to acclimatise to the new environment.

In contrast, a very low humidity level favors the build-up of [[static electricity]], which may result in spontaneous shutdown of computers when discharges occur. Apart from spurious erratic function, electrostatic discharges can cause dielectric breakdown in [[solid-state device]]s, resulting in irreversible damage. [[Data center]]s often monitor relative humidity levels for these reasons.

=== Industry ===
High humidity can often have a negative effect on the capacity of chemical plants and refineries that use furnaces as part of a certain processes (e.g., [[steam reforming]], wet [[sulfuric acid]] processes). For example, because humidity reduces ambient oxygen concentrations (dry air is typically 20.9% oxygen, but at 100% relative humidity the air is 20.4% oxygen), flue gas fans must intake air at a higher rate than would otherwise be required to maintain the same firing rate.<ref>{{cite web|url=http://www.industrialcontrolsonline.com/training/online/everything-you-need-know-about-combustion-chemistry-analysis|title=Everything You Need to Know About Combustion Chemistry & Analysis – Industrial Controls|access-date=2015-01-23|archive-date=2020-01-10|archive-url=https://web.archive.org/web/20200110151007/http://www.industrialcontrolsonline.com/training/online/everything-you-need-know-about-combustion-chemistry-analysis|url-status=live}}</ref>

=== Baking ===
High humidity in the oven, represented by an elevated [[wet-bulb temperature]], increases the [[thermal conductivity]] of the air around the baked item, leading to a quicker baking process or even burning. Conversely, low humidity slows the baking process down.<ref>{{cite web | url=http://www.scienceofcooking.com/why_is_humidity_important_in_cooking.htm | title=Why is humidity important in cooking? | access-date=2018-08-28 | archive-date=2020-11-12 | archive-url=https://web.archive.org/web/20201112023810/https://www.scienceofcooking.com/why_is_humidity_important_in_cooking.htm | url-status=live }}</ref>