TheInfoList

Humidity is the concentration of
water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous p ...
present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
,
dew Dew is in the form of droplets that appears on thin, exposed objects in the morning or evening due to . As the exposed surface cools by its heat, atmospheric moisture at a rate greater than that at which it can , resulting in the format ...

, or
fog Fog is a visible aerosol consisting of tiny water drop (liquid), droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus cloud, s ...

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 Warmer air can hold more moisture than colder air. The dew point is the temperature to which air must be cooled to become saturated with water vapor. It is assumed that air pressure and water content is constant. When cooled further, the air ...
. 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 of water per cubic metre of air at , but only of water per cubic metre of air at . 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 metre) or as mass of water vapor per mass of dry air (usually in grams per kilogram). 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 In mathematics, a ratio indicates how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8∶6, which is equivalent to ...

of water vapor mass to total moist air parcel mass. Humidity plays an important role for surface life. For animal life dependent on
perspiration Perspiration, also known as sweating, is the production of fluids secreted by the sweat gland Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin Skin is the layer of usually soft, fl ...
(
sweating Perspiration, also known as sweating, is the production of fluids secreted by the sweat glands in the skin of mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constituting the class (biology), cla ...

) to regulate internal body temperature, high humidity impairs heat exchange efficiency by reducing the rate of moisture
evaporation Evaporation is a type of that occurs on the of a as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other bas ...

from skin surfaces. This effect can be calculated using a heat index table, also known as a
humidex The humidex (short for humidity index) is an index number used by Canadian Canadians (french: Canadiens) are people identified with the country of Canada. This connection may be residential, legal, historical or cultural. For most Canadians, m ...
. 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. There is a very small difference described under "Enhancement factor" below, which can be neglected in many calculations unless high accuracy is required.

# Definitions

## Absolute humidity

Absolute humidity is the total mass of water vapor 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 per cubic metre when the air is saturated at . Absolute humidity is the mass of the water vapor $\left(m_\right)$, divided by the volume of the air and water vapor mixture $\left(V_ \right)$, which can be expressed as: :$AH = \frac.$ The absolute humidity changes as air
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

or
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with mass to change its velocity (e.g. moving fr ...

changes, if the volume is not fixed. This makes it unsuitable for
chemical engineering Chemical engineering is a certain type of engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline ...
calculations, e.g. in
drying Drying is a mass transfer Mass transfer is the net movement of mass from one location, usually meaning stream, phase, fraction or component, to another. Mass transfer occurs in many processes, such as absorption Absorption may refer to: Chemi ...

, where temperature can vary considerably. 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. Mass of water per unit volume as in the equation above is also defined as ''volumetric humidity''. Because of the potential confusion,
British Standard British Standards (BS) are the standards produced by the BSI Group which is incorporated under a royal charter (and which is formally designated as the Standards organization#National standards bodies, national standards body (NSB) for the UK). Th ...
BS 1339 British Standard BS 1339 (revised), Humidity and Dewpoint, Parts 1-3 (2002-2007) 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. The field concerned with the study of physical and thermodynamic properties of gas–vapor mixtures is named
psychrometrics Psychrometrics, psychrometry, and hygrometry are names for the field of engineering concerned with the physical and thermodynamic Thermodynamics is a branch of physics Physics is the natural science that studies matter, its Elementa ...
.

## Relative humidity

The relative humidity $\left(RH$ or $\phi\right)$ of an air-water mixture is defined as the ratio of the
partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas mix ...
of water vapor $\left(p_\right)$ in the mixture to the equilibrium vapor pressure of water $\left(p^*_\right)$ over a flat surface of pure water at a given temperature:Perry, R.H. and Green, D.W, ''
Perry's Chemical Engineers' Handbook ''Perry's Chemical Engineers' Handbook'' (also known as ''Perry's Handbook'', ''Perry's'', or ''The Chemical Engineer's Bible'') was first published in 1934 and the most current ninth edition was published in July 2018. It has been a source of che ...
'' (7th Edition),
McGraw-Hill McGraw Hill is an American learning company and one of the "big three" educational publishers that provides customized educational content, software, and services for pre-K Pre-kindergarten (also called Pre-K or PK) is a voluntary classroom ...
, , Eqn 12-7
::$\phi =$ In other words, relative humidity is the ratio of ''how much water vapour is in the air'' and ''how much water vapour the air could potentially contain'' at a given temperature. It varies with the temperature of the air: colder air can hold less vapour, so chilling some air can cause the water vapour to
condense Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

. Likewise, warming some air containing a fog may cause that fog to evaporate, as the air between the water droplets becomes more able to hold water vapour. So changing the temperature of air can change the relative humidity, even when the absolute humidity remains constant. 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 Warmer air can hold more moisture than colder air. The dew point is the temperature to which air must be cooled to become saturated with water vapor. It is assumed that air pressure and water content is constant. When cooled further, the air ...
. Relative humidity is normally expressed as a
percentage In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities a ...

; 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 Warmer air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosphere (0.04402961% at April 2019 concentr ...

. 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 Supersaturation occurs with a chemical solution when the concentration of a solute exceeds the concentration specified by the value equilibrium solubility. Most commonly the term is applied to a solution of a solid in a liquid. A supersaturated sol ...
. 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. Relative humidity is an important
metric METRIC (Mapping EvapoTranspiration at high Resolution with Internalized Calibration) is a computer model Computer simulation is the process of mathematical modelling, performed on a computer, which is designed to predict the behaviour of or th ...
used in
weather forecasts Weather forecasting is the application of science and technology forecasting, to predict the conditions of the Earth's atmosphere, atmosphere for a given location and time. People have attempted to predict the weather informally for millennia an ...
and reports, as it is an indicator of the likelihood of
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
, dew, or fog. In hot summer
weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a p ...

, a rise in relative humidity increases the
apparent temperature Apparent temperature is the temperature equivalent perceived by humans, caused by the combined effects of air temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in ...
to
human Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A speci ...

s (and other
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

s) by hindering the
evaporation Evaporation is a type of that occurs on the of a as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other bas ...

of
perspiration Perspiration, also known as sweating, is the production of fluids secreted by the sweat gland Sweat glands, also known as sudoriferous or sudoriparous glands, , are small tubular structures of the skin Skin is the layer of usually soft, fl ...
from the skin. For example, according to the
Heat Index The heat index (HI) is an index that combines air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmospher ...
, a relative humidity of 75% at air temperature of would feel like 83.6 °F ±1.3 °F (28.7 °C ±0.7 °C).

## Relationship between absolute-, relative-humidity, and temperature

In the Earth's atmosphere at sea level:

## Specific humidity

''Specific humidity'' (or moisture content) is the ratio of the mass of water vapor to the total mass of the air parcel. Specific humidity is approximately equal to the
mixing ratio In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in th ...
, 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. As temperature decreases, the amount of water vapor needed to reach saturation also decreases. As the temperature of a parcel of air becomes lower it will eventually reach the point of saturation without adding or losing water mass.

## 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.

# Measurement

A device used to measure humidity of air is called a psychrometer or
hygrometer A hair tension dial hygrometer with a nonlinear scale. A hygrometer is an instrument used to measure the amount of water vapor (99.9839 °C) , - , Boiling point The boiling point of a substance is the temperature at which the vapor ...

. A humidistat is a humidity-triggered switch, often used to control a
dehumidifier A dehumidifier is an air conditioning Air conditioning (also A/C, air conditioner) is the process of removing heat and controlling the humidity Humidity is the concentration of water vapour (99.9839 °C) , - , Boiling point , ...

. The humidity of an air and water vapor mixture is determined through the use of psychrometric charts if both the dry bulb temperature (''T'') and the
wet bulb temperature The wet-bulb temperature (WBT) is the temperature read by a thermometer covered in water-soaked cloth ( wet-bulb thermometer) over which air is passed. At 100% relative humidity Humidity is the concentration of water vapor present in the air. ...
(''T''w) of the mixture are known. These quantities are readily estimated by using a sling
psychrometer A hygrometer is an instrument used to measure the amount of water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 ...

. There are several empirical formulas that can be used to estimate the equilibrium vapor pressure of water vapor as a function of temperature. The
Antoine equation The Antoine equation is a class of semi-empirical correlations describing the relation between vapor pressure and temperature for pure substances. The Antoine equation is derived from the Clausius–Clapeyron relation. The equation was presented in ...
is among the least complex of these, having only three parameters (''A'', ''B'', and ''C''). Other formulas, such as the
Goff–Gratch equationThe Goff–Gratch equation is one (arguably the first reliable in history) amongst many experimental correlation proposed to estimate the Vapour pressure of water, saturation water vapor pressure at a given temperature. Another similar equation base ...
and the Magnus–Tetens approximation, are more complicated but yield better accuracy. The
Arden Buck equationThe Arden Buck equations are a group of empirical correlations that relate the saturation vapor pressure to temperature for Vapour pressure of water, moist air. The curve fits have been optimized for more accuracy than the Goff–Gratch equation in t ...
is commonly encountered in the literature regarding this topic: :$e^*_w = \left\left(1.0007 + 3.46 \times 10^ P\right\right) \times 6.1121\, e^,$ where $T$ is the dry-bulb temperature expressed in degrees Celsius (°C), $P$ is the absolute pressure expressed in millibars, and $e^*_w$ is the equilibrium vapor pressure expressed in millibars. Buck has reported that the maximal relative error is less than 0.20% between when this particular form of the generalized formula is used to estimate the equilibrium vapor pressure of water. There are various devices used to measure and regulate humidity. Calibration standards for the most accurate measurement include the gravimetric hygrometer, chilled mirror hygrometer, and electrolytic hygrometer. The gravimetric method, while the most accurate, is very cumbersome. For fast and very accurate measurement the chilled mirror method is effective. For process on-line measurements, the most commonly used sensors nowadays are based on
capacitance Capacitance is the ratio of the amount of electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' ( ...
measurements to measure relative humidity, frequently with internal conversions to display absolute humidity as well. These are cheap, simple, generally accurate and relatively robust. All humidity sensors face problems in measuring dust-laden gas, such as exhaust streams from s. Humidity is also measured on a global scale using remotely placed
satellite In the context of spaceflight Spaceflight (or space flight) is an application of astronautics to fly spacecraft into or through outer space, either human spaceflight, with or uncrewed spaceflight, without humans on board. Most spaceflight ...

s. These satellites are able to detect the
concentration In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in t ...

of water in the
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere Planetary means relating to a planet A planet is an astronomical body orbiting a star or Stellar evo ...
at altitudes between . Satellites that can measure water vapor have sensors that are sensitive to
infrared radiation Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natur ...

. Water vapor specifically absorbs and re-radiates radiation in this spectral band. Satellite water vapor imagery plays an important role in monitoring climate conditions (like the formation of thunderstorms) and in the development of
weather forecasts Weather forecasting is the application of science and technology forecasting, to predict the conditions of the Earth's atmosphere, atmosphere for a given location and time. People have attempted to predict the weather informally for millennia an ...
.

# Air density and volume

Humidity depends on water vaporization and condensation, which, in turn, mainly depends on temperature. Therefore, when applying more pressure to a gas saturated with water, all components will initially decrease in volume approximately according to the
ideal gas law The ideal gas law, also called the general gas equation, is the equation of state In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the na ...

. However, some of the water will condense until returning to almost the same humidity as before, giving the resulting total volume deviating from what the ideal gas law predicted. Conversely, decreasing temperature would also make some water condense, again making the final volume deviate from predicted by the ideal gas law. Therefore, gas volume may alternatively be expressed as the dry volume, excluding the humidity content. This fraction more accurately follows the ideal gas law. On the contrary the saturated volume is the volume a gas mixture would have if humidity was added to it until saturation (or 100% relative humidity). Humid air is less dense than dry air because a molecule of water ( M ≈ 18 u) is less massive than either a molecule of
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

(M ≈ 28) or a molecule of
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

(M ≈ 32). About 78% of the molecules in dry air are nitrogen (N2). Another 21% of the molecules in dry air are oxygen (O2). The final 1% of dry air is a mixture of other gases. For any gas, at a given temperature and pressure, the number of molecules present in a particular volume is constant – see
ideal gas law The ideal gas law, also called the general gas equation, is the equation of state In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the na ...

. So when water molecules (vapor) are introduced into that volume of dry air, the number of air molecules in the volume must decrease by the same number, if the temperature and pressure remain constant. (The addition of water molecules, or any other molecules, to a gas, without removal of an equal number of other molecules, will necessarily require a change in temperature, pressure, or total volume; that is, a change in ''at least'' one of these three parameters. If temperature and pressure remain constant, the volume increases, and the dry air molecules that were displaced will initially move out into the additional volume, after which the mixture will eventually become uniform through diffusion.) Hence the mass per unit volume of the gas—its density—decreases.
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes the study of such topics a ...

discovered this phenomenon and wrote about it in his book ''
Opticks ''Opticks: or, A Treatise of the Reflexions, Refractions, Inflexions and Colours of Light'' is a book by English natural philosopher Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March Old Style and New Style dates, 1726/ ...

''.

# Pressure dependence

The relative humidity of an air–water system is dependent not only on the temperature but also on the absolute pressure of the system of interest. This dependence is demonstrated by considering the air–water system shown below. The system is closed (i.e., no matter enters or leaves the system). If the system at State A is isobarically heated (heating with no change in system pressure), then the relative humidity of the system decreases because the equilibrium vapor pressure of water increases with increasing temperature. This is shown in State B. If the system at State A is isothermally compressed (compressed with no change in system temperature), then the relative humidity of the system increases because the partial pressure of water in the system increases with the volume reduction. This is shown in State C. Above 202.64 kPa, the RH would exceed 100% and water may begin to condense. If the pressure of State A was changed by simply adding more dry air, without changing the volume, the relative humidity would not change. Therefore, a change in relative humidity can be explained by a change in system temperature, a change in the volume of the system, or change in both of these system properties.

## Enhancement factor

The enhancement factor $\left(f_w\right)$ is defined as the ratio of the saturated vapor pressure of water in moist air $\left(e\text{'}_w\right)$ to the saturated vapor pressure of pure water: :$f_W = \frac.$ The enhancement factor is equal to unity for ideal gas systems. However, in real systems the interaction effects between gas molecules result in a small increase of the equilibrium vapor pressure of water in air relative to equilibrium vapor pressure of pure water vapor. Therefore, the enhancement factor is normally slightly greater than unity for real systems. The enhancement factor is commonly used to correct the equilibrium vapor pressure of water vapor when empirical relationships, such as those developed by Wexler, Goff, and Gratch, are used to estimate the properties of psychrometric systems. Buck has reported that, at sea level, the vapor pressure of water in saturated moist air amounts to an increase of approximately 0.5% over the equilibrium vapor pressure of pure water.

# Effects

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

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 Bangkok is the capital and most populous city of Thailand Thailand ( th, ประเทศไทย), historically known as Siam, () officially the Kingdom of Thailand, is a country in Southeast Asia Southeast Asia or Southeastern ...

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Ho Chi Minh City , population_density_km2 = 4292 , population_density_metro_km2 = 697.2 , population_demonym = Saigonese , demographics_type1 = List of ethnic groups in Vietnam, Ethnic groups , demographics1_footnotes = , demographics1_title ...

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Manila Manila ( , ; fil, Maynila, ), officially the City of Manila ( fil, Lungsod ng Maynila, ), is the capital Capital most commonly refers to: * Capital letter Letter case (or just case) is the distinction between the letters that are ...

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Jakarta Jakarta (; ), officially the Special Capital Region of Jakarta ( id, Daerah Khusus Ibukota Jakarta), is the capital Capital most commonly refers to: * Capital letter Letter case (or just case) is the distinction between the lette ...

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Kaohsiung Kaohsiung City (; Mandarin Chinese: ; Wade–Giles: ''Kao¹-hsiung²'') is a Special municipality (Taiwan), special municipality in southern Taiwan. It ranges from the coastal urban centre to the rural Yushan Range with an area of . Kao ...

and
Taipei Taipei (), officially Taipei City, is the Capital city, capital and a Special municipality (Taiwan), special municipality of the Taiwan, Republic of China (Taiwan). Located in Regions of Taiwan, Northern Taiwan, Taipei City is an enclave of t ...

have very high humidity most or all year round because of their proximity to water bodies and the
equator The Equator is a circle of latitude, about in circumference, that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the N ...

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 Kolkata ( or , ; also known as Calcutta , the official name until 2001) is the capital Capital most commonly refers to: * Capital letter Letter case (or just case) is the distinction between the letters that are in larger upperca ...
,
Chennai Chennai (, ), also known as Madras (List of renamed Indian cities and states#Tamil Nadu, the official name until 1996), is the capital city of the states and territories of India, Indian state of Tamil Nadu. The state's largest city in area ...

and
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in
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, and
Lahore Lahore (; pnb, ; ; ur, ; ) is the capital of the Pakistani province of Punjab Punjab (; ; ; ; also as Panjāb or Panj-Āb) is a geopolitical, cultural, and in , specifically in the northern part of the , comprising areas of east ...

in
Pakistan Pakistan, . Pronounced variably in English as , , , and . officially the Islamic Republic of Pakistan, is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a popul ...

.
Sukkur Sukkur (; ) is a city in the Pakistan Pakistan, . Pronounced variably in English as , , , and . officially the Islamic Republic of Pakistan, is a country in South Asia. It is the world's List of countries and dependencies by population ...

city located on the
Indus River The Indus ( ) is a transboundary river A transboundary river is a river that crosses at least one political border, either a border within a nation or an international boundary. Bangladesh has the highest number of these rivers, including t ...

in Pakistan has some of the highest and most uncomfortable
dew point Warmer air can hold more moisture than colder air. The dew point is the temperature to which air must be cooled to become saturated with water vapor. It is assumed that air pressure and water content is constant. When cooled further, the air ...
s in the country, frequently exceeding in the
Monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology ...

season. High temperatures combine with the high dew point to create heat index in excess of .
Darwin Darwin most often refers to: * Charles Darwin (1809–1882), English naturalist and writer, best known as the originator of the theory of biological evolution by natural selection * Darwin, Northern Territory, a capital city in Australia * Darwin ( ...

experiences an extremely humid wet season from December to April.
Houston Houston ( ) is the List of cities in Texas by population, most populous city in Texas, List of United States cities by population, fourth-most populous city in the United States, most populous city in the Southern United States, as well as th ...

,
Miami Miami (), officially the City of Miami, is a coast, coastal metropolis located in Miami-Dade County, Florida, Miami-Dade County in southeastern Florida, United States. With a population of 467,963 as of the 2020 United States census, 2020 censu ...

,
San Diego San Diego ( , ; ) is a city in the U.S. state of California on the coast of the Pacific Ocean and immediately adjacent to the Mexico–United States border, Mexican border. With a 2020 population of 1,386,932, San Diego is the List of United ...

,
Osaka is a designated city in the Kansai region The or the , lies in the southern-central region of Japan , image_flag = Flag of Japan.svg , alt_flag = Centered deep red circle on a white rectangle , ...

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Shanghai Shanghai (, , Standard Chinese, Standard Mandarin pronunciation: ) is one of the four Direct-administered municipalities of China, direct-administered municipalities of the China, People's Republic of China. The city is located on the sou ...

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Shenzhen Shenzhen (; ; ), also known as Sham Chun, is a major sub-provincial A sub-provincial division () (or deputy-provincial divisions) in China China, officially the People's Republic of China (PRC), is a country in East Asia. It is the Lis ...

and
Tokyo Tokyo (Japanese language, Japanese: , ''Tōkyō'' ), historically known in the west as Tokio and officially the Tokyo Metropolis (, ''Tōkyō-to''), is capital of Japan, the capital and most populous Prefectures of Japan, prefecture of Japan ...

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 .

## Global climate

Humidity affects the
energy budget An energy budget is a balance sheet of energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the or ...
and thereby influences temperatures in two major ways. First, water vapor in the atmosphere contains "latent" energy. During transpiration or evaporation, this
latent heat Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system A thermodynamic system is a body of matter In classical physics and general chemistry, matter is any su ...
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 A greenhouse gas (GHG or GhG) is a gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics ...

. 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
precipitates Precipitation is the process of conversion of a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All ever ...

, with a much lower
scale height In various scientific contexts, a scale height, usually denoted by the capital letter ''H'', is a distance over which a quantity decreases by a factor of e (the base of natural logarithms The natural logarithm of a number is its logarithm to ...
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. Water vapor is thus a "slave" to the non-condensible greenhouse gases.

## Animal and plant life

Humidity is one of the fundamental abiotic factors 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. The human body dissipates heat through perspiration and its evaporation.
Heat convection Convection (or convective heat transfer) is the transfer of heat from one place to another due to the movement of fluid. Although often discussed as a distinct method of heat transfer, convective heat transfer involves the combined processes of ...
, to the surrounding air, and
thermal radiation Thermal radiation is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space an ...
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 as or warmer than the skin during times of high humidity,
blood Blood is a body fluid Body fluids, bodily fluids, or biofluids are liquid A liquid is a nearly incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow (isochoric process, isochoric flow) refers t ...

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
muscle Skeletal muscles (commonly referred to as muscles) are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly cat ...

s, the
brain A brain is an organ Organ may refer to: Biology * Organ (anatomy) An organ is a group of Tissue (biology), tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tis ...

, and other internal
organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma Parenchyma () is the bulk of functional ...
.
Physical strength Physical strength is the measure of a human Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organ ...
declines, and
fatigue Fatigue describes a state of tiredness that does not resolve with rest or sleep. In general usage, fatigue is synonymous with extreme tiredness or exhaustion that normally follows prolonged physical or mental activity. When it does not resolve ...
occurs sooner than it would otherwise. Alertness and mental capacity also may be affected, resulting in ''heat stroke'' or
hyperthermia Hyperthermia, also known simply as overheating, is a condition where an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat In thermodynamics, heat is ...
.

## 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. 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. 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%—but ideally not above the Absolute (60°F Dew Point), between 40% and 60%. 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 m/s, a change in air temperature and mean radiant temperature from 20 °C to 24 °C would lower the maximum acceptable relative humidity from 100% to 65% to maintain thermal comfort conditions. The
CBE The Most Excellent Order of the British Empire is a British order of chivalry An order of chivalry, order of knighthood, chivalric order, or equestrian order is an order of knights typically founded during or inspired by the original Catho ...
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. Some people experience difficulty breathing in humid environments. Some cases may possibly be related to respiratory conditions such as
asthma Asthma is a long-term Long-Term Capital Management L.P. (LTCM) was a hedge fund''A financial History of the United States Volume II: 1970–2001'', Jerry W. Markham, Chapter 5: "Bank Consolidation", M. E. Sharpe, Inc., 2002 based in Greenwich, ...

, while others may be the product of
anxiety Anxiety is an emotion Emotions are mental state, psychological states brought on by neurophysiology, neurophysiological changes, variously associated with thoughts, feelings, behavioural responses, and a degree of pleasure or suffering, disp ...

. Sufferers will often hyperventilate in response, causing sensations of
numbness Hypoesthesia or numbness is a common side effect of various medical conditions which manifests as a reduced sense of touch The somatosensory system is a part of the sensory nervous system The sensory nervous system is a part of the nervous ...
,
faintness Syncope, also known as fainting, is a loss of consciousness , an English Paracelsian physician Consciousness, at its simplest, is "sentience or awareness of internal and external existence". Despite millennia of analyses, definitions, expl ...
, and loss of
concentration In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in t ...
, among others. 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 The rhinovirus (from the grc, ῥίς, rhis "nose", , romanized: "of the nose", and the la, vīrus) is the most common viral infectious agent in humans and is the predominant cause of the common cold The common cold, also known simply a ...
cold viruses. Extremely low (below 20%) relative humidities may also cause eye irritation. The use of a
humidifier A humidifier is a device, primarily an electrical appliance A home appliance, also referred to as a domestic appliance, an electric appliance or a household appliance, is a machine which assists in household A household consists of one or sev ...
in homes, especially bedrooms, can help with these symptoms. Indoor relative humidities should be kept above 30% to reduce the likelihood of the occupant's nasal passages drying out, especially in winter.
Air conditioning Air conditioning (also A/C, air conditioner) is the process of removing heat and controlling the humidity Humidity is the concentration of water vapour (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 ...
reduces discomfort by reducing not just temperature but humidity as well. Heating cold outdoor air can decrease relative humidity levels indoors to below 30%. According to ASHRAE Standard 55-2017: Thermal Environmental Conditions for Human Occupancy, indoor thermal comfort can be achieved through the PMV method with relative humidities ranging from 0% to 100%, depending on the levels of the other factors contributing to thermal comfort. However, the recommended range of indoor relative humidity in air conditioned buildings is generally 30–60%.

## 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."
Mucociliary clearance Mucociliary clearance (MCC), mucociliary transport, or the mucociliary escalator, describes the self-clearing mechanism of the airways in the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biol ...
in the
respiratory tract The respiratory tract is the subdivision of the respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system A biological system is a complex network Network and networking may refer to: ...

is also hindered by low humidity. One study in dogs found that mucus transport was lower at an absolute humidity of 9 g water/m3 than at 30 g water/m3. Increased humidity can also lead to changes in
total body water In physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology Biology is the natural science that studies life and living organisms, including their anatomy, physica ...
that usually leads to moderate weight gain, especially if one is acclimated to working or exercising in hot and humid weather.

## Building construction

Common construction methods often produce building enclosures with a poor thermal boundary, requiring an insulation and air barrier system designed to retain indoor environmental conditions while resisting external environmental conditions. 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 Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

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 Capillary action (sometimes capillarity, capillary motion, capillary effect, or wicking) is the ability of a liquid A liquid is a nearly incompressible In fluid mechanics Fluid mechanics is the branch of physics concerned with the ...
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 Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal rad ...

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 A mold () or mould () is a fungus A fungus (plural The plural (sometimes abbreviated An abbreviation (from Latin ''brevis'', meaning ''short'') is a shortened form of a word or phrase, by any method. It may consist of a group of ...
, 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 A dehumidifier is an air conditioning Air conditioning (also A/C, air conditioner) is the process of removing heat and controlling the humidity Humidity is the concentration of water vapour (99.9839 °C) , - , Boiling point , ...

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 A vehicle (from la, vehiculum) is a machine A machine is any physical system with ordered structural and functional properties. It may represent human-made or naturally occurring device molecular machine A molecular machine, nanite, or ...

can lead to problems of condensation, such as misting of
windshield The windshield (North American English North American English (NAmE, NAE) is the most generalized variety Variety may refer to: Science and technology Mathematics * Algebraic variety, the set of solutions of a system of polynomial eq ...

s and
shorting In finance Finance is the study of financial institutions, financial markets and how they operate within the financial system. It is concerned with the creation and management of money and investments. Savers and investors have money avail ...

of electrical components. In vehicles and
pressure vessel A pressure vessel is a container designed to hold gases or liquids at a pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics is the natural science that studies matter, its Elementary particle, fundamen ...
s such as pressurized
airliner An airliner is a type of aircraft for transporting passengers and air cargo. Such aircraft are most often operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an ai ...

s,
submersible A submersible is a small watercraft Watercraft, also known as water vessels or waterborne vessels, are vehicles A vehicle (from la, vehiculum) is a machine A machine is any physical system with ordered structural and functional ...

s and
spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite alt=, A full-size model of the Earth observation satellite ERS 2 ">ERS_2.html" ;"title="Earth observation satellite ERS 2">Earth obse ...

, these considerations may be critical to safety, and complex environmental control systems including equipment to maintain
pressure Pressure (symbol: ''p'' or ''P'') is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with mass to change its velocity (e.g. moving fr ...

are needed.

## Aviation

Airliners operate with low internal relative humidity, often under 20%, 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 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. Carburetor engines have a further danger of ice forming inside the
carburetor A carburetor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American En ...

. Aviation weather reports (
METAR METAR is a format for reporting weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur ...
s) therefore include an indication of relative humidity, usually in the form of the
dew point Warmer air can hold more moisture than colder air. The dew point is the temperature to which air must be cooled to become saturated with water vapor. It is assumed that air pressure and water content is constant. When cooled further, the air ...
. 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

Electronic devices are often rated to operate only under certain humidity conditions (e.g., 10% to 90%). At the top end of the range, moisture may increase the conductivity of permeable
insulators Insulator may refer to: * Insulator (electricity) An electrical insulator is a material in which the electron does not flow freely or the atom of the insulator have tightly bound electrons whose internal electric charge Electric charge is t ...
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 Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

. 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 A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit An electrical network is an interconnection of electrical component An electronic component is any basic discrete device or physical entity in an electron ...

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). 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 Static electricity is an imbalance of electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonl ...

, 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 devices, resulting in irreversible damage.
Data center A data center (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American Eng ...
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.

## Baking

High humidity in the oven, represented by an elevated
wet-bulb temperature The wet-bulb temperature (WBT) is the temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in all matter, which is the source of the occurrence of heat, a flow of ene ...
, increases the
thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

of the air around the baked item, leading to a quicker baking process or even burning. Conversely, low humidity slows the baking process down.

# Other important facts

At 100% relative humidity, air is saturated and at its
dew point Warmer air can hold more moisture than colder air. The dew point is the temperature to which air must be cooled to become saturated with water vapor. It is assumed that air pressure and water content is constant. When cooled further, the air ...
: the water vapor pressure would permit neither
evaporation Evaporation is a type of that occurs on the of a as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other bas ...

of nearby liquid water nor
condensation Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

to grow the nearby water; neither sublimation of nearby ice nor
deposition Deposition may refer to: * Deposition (law), taking testimony outside of court * List of deposed politicians, Deposition (politics), the removal of a person of authority from political power * Deposition (university), a widespread initiation ritual ...
to grow the nearby ice. Relative humidity can exceed 100%, in which case the air is
supersaturated Supersaturation occurs with a chemical solution when the concentration of a solute exceeds the concentration specified by the value equilibrium solubility. Most commonly the term is applied to a solution of a solid in a liquid. A supersaturated sol ...
. Cloud formation requires
supersaturated Supersaturation occurs with a chemical solution when the concentration of a solute exceeds the concentration specified by the value equilibrium solubility. Most commonly the term is applied to a solution of a solid in a liquid. A supersaturated sol ...
air.
Cloud condensation nuclei Cloud condensation nuclei or CCNs (also known as cloud seeds) are small particles typically 0.2 µm, or 1/100 the size of a cloud In meteorology Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry a ...
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 A cloud chamber, also known as a Wilson cloud chamber, is a particle detector used for visualizing the passage of ionizing radiation. A cloud chamber consists of a sealed environment containing a supersaturation, supersaturated vapour of wate ...
, 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 Warmer air can hold more moisture than colder air. The dew point is the temperature to which air must be cooled to become saturated with water vapor. It is assumed that air pressure and water content is constant. When cooled further, the air ...
and its corresponding
absolute humidity Humidity is the concentration of water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, ...
, the relative humidity will change inversely, albeit nonlinearly, with the
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

. This is because the partial pressure of water increases with temperature—the operative principle behind everything from
hair dryer A hair dryer, hairdryer or blow dryer is an electromechanical In engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, ro ...

s to
dehumidifier A dehumidifier is an air conditioning Air conditioning (also A/C, air conditioner) is the process of removing heat and controlling the humidity Humidity is the concentration of water vapour (99.9839 °C) , - , Boiling point , ...

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 compared to no more than about 0.5% by mass at . This explains the low levels (in the absence of measures to add moisture) of humidity in heated structures during winter, resulting in dry
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have differ ...

,
itch ITCH is a HECT domain In molecular biology, the HECT domain is a protein domain A protein domain is a region of the protein's Peptide, polypeptide chain that is self-stabilizing and that folds independently from the rest. Each domain forms a c ...

y
eye Eyes are organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma Parenchyma () is t ...

s, and persistence of 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 Humidity is the concentration of water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, ...
doubles for every increase in temperature. Thus, the relative humidity will drop by a factor of 2 for each increase in temperature, assuming conservation of absolute moisture. For example, in the range of normal temperatures, air at and 50% relative humidity will become saturated if cooled to , its
dew point Warmer air can hold more moisture than colder air. The dew point is the temperature to which air must be cooled to become saturated with water vapor. It is assumed that air pressure and water content is constant. When cooled further, the air ...
, and air at 80% relative humidity warmed to will have a relative humidity of only 29% and feel dry. By comparison, thermal comfort standard
ASHRAE 55 ''ANSI/ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy'' is an American National Standard The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of Standard ...
requires systems designed to control humidity to maintain a dew point of though no lower humidity limit is established. 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 Convection is single or multiphase fluid flow that occurs Spontaneous process, spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When t ...

. This is a mechanism behind
thunderstorms A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning Lightning is a naturally occurring electrostatic discharge Electrostatic discharge (ESD) is the sudden fl ...

and other
weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a p ...

phenomena. Relative humidity is often mentioned in
weather forecasts Weather forecasting is the application of science and technology forecasting, to predict the conditions of the Earth's atmosphere, atmosphere for a given location and time. People have attempted to predict the weather informally for millennia an ...
and reports, as it is an indicator of the likelihood of dew, or fog. In hot summer
weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a p ...

, it also increases the
apparent temperature Apparent temperature is the temperature equivalent perceived by humans, caused by the combined effects of air temperature Temperature is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy, present in ...
to
human Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A speci ...

s (and other
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

s) by hindering the
evaporation Evaporation is a type of that occurs on the of a as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other bas ...

of perspiration from the skin as the relative humidity rises. This effect is calculated as the heat index or
humidex The humidex (short for humidity index) is an index number used by Canadian Canadians (french: Canadiens) are people identified with the country of Canada. This connection may be residential, legal, historical or cultural. For most Canadians, m ...
. A device used to measure humidity is called a
hygrometer A hair tension dial hygrometer with a nonlinear scale. A hygrometer is an instrument used to measure the amount of water vapor (99.9839 °C) , - , Boiling point The boiling point of a substance is the temperature at which the vapor ...

; one used to regulate it is called a humidistat, or sometimes hygrostat. (These are
analogous Analogy (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximate ...
to a
thermometer A thermometer is a device that measures temperature or a temperature gradient A temperature gradient is a physical quantity A physical quantity is a physical property of a material or system that can be Quantification (science), quantified by ...

and
thermostat A thermostat is a regulating device component which senses the temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen ...

for temperature, respectively.)

*
Concentration In chemistry Chemistry is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in t ...

*
Dew point depression The dew point depression (T-Td) is the difference between the temperature and dew point temperature at a certain height in the Earth's atmosphere, atmosphere. For a constant temperature, the smaller the difference, the more moisture there is, and ...
*
Heat index The heat index (HI) is an index that combines air File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmospher ...
* Humidity buffering * Humidity indicator card * Humidity indicator *
Psychrometrics Psychrometrics, psychrometry, and hygrometry are names for the field of engineering concerned with the physical and thermodynamic Thermodynamics is a branch of physics Physics is the natural science that studies matter, its Elementa ...
* Saturation vapor density *
Water activity Water activity (''aw'') is the partial Partial may refer to: Mathematics *Partial derivative In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure ...

# References

## General sources

* * * * * * * * * * * United States Environmental Protection Agency
"IAQ in Large Buildings"
Retrieved Jan. 9, 2006.