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Wikipedia

Humidity

Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye.[2] Humidity indicates the likelihood for precipitation, dew, or fog to be present.

Global distribution of relative humidity at the surface averaged over the years 1981–2010 from the CHELSA-BIOCLIM+ data set[1]

Humidity depends on the temperature and pressure of the system of interest. The same amount of water vapor results in higher relative humidity in cool air than warm air. A related parameter is the dew point. The amount of water vapor needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing water mass. The amount of water vapor contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 g of water per cubic metre of air at 30 °C (86 °F), but only 8 g of water per cubic metre of air at 8 °C (46 °F).

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)[3] or as mass of water vapor per mass of dry air (usually in grams per kilogram).[4] Relative humidity, often expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature. Specific humidity is the ratio of water vapor mass to total moist air parcel mass.

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

The notion of air "holding" water vapor or being "saturated" by it is often mentioned in connection with the concept of relative humidity. This, however, is misleading—the amount of water vapor that enters (or can enter) a given space at a given temperature is almost independent of the amount of air (nitrogen, oxygen, etc.) that is present. Indeed, a vacuum has approximately the same equilibrium capacity to hold water vapor as the same volume filled with air; both are given by the equilibrium vapor pressure of water at the given temperature.[5][6] There is a very small difference described under "Enhancement factor" below, which can be neglected in many calculations unless great accuracy is required.

Definitions

 
Paranal Observatory on Cerro Paranal in the Atacama Desert is one of the driest places on Earth.[7]

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 30 g (1.1 oz) per cubic metre when the air is saturated at 30 °C (86 °F).[8][9]

Absolute humidity is the mass of the water vapor  , divided by the volume of the air and water vapor mixture  , which can be expressed as:

 

The absolute humidity changes as air temperature or pressure changes, if the volume is not fixed. This makes it unsuitable for chemical engineering calculations, e.g. in drying, 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 BS 1339 [10] 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.

Relative humidity

The relative humidity   or   of an air-water mixture is defined as the ratio of the partial pressure of water vapor   in air to the saturation vapor pressure   of water at the same temperature, usually expressed as a percentage:[11][12][5]

 

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 changing the temperature of air can change the relative humidity, even when the absolute humidity remains constant.

Chilling air increases the relative humidity, and can cause the water vapour to condense (if the relative humidity rises over 100%, the dew point). Likewise, warming air decreases the relative humidity. 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.

Relative humidity only considers the invisible water vapour. Mists, clouds, fogs and aerosols of water do not count towards the measure of relative humidity of the air, although their presence is an indication that a body of air may be close to the dew point.

Relative humidity is normally expressed as a percentage; a higher percentage means that the air–water mixture is more humid. At 100% relative humidity, the air is saturated and is at its dew point. In the absence of a foreign body on which droplets or crystals can nucleate, the relative humidity can exceed 100%, in which case the air is said to be supersaturated. Introduction of some particles or a surface to a body of air above 100% relative humidity will allow condensation or ice to form on those nuclei, thereby removing some of the vapour and lowering the humidity.

Relative humidity is an important metric used in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, a rise in relative humidity increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin. For example, according to the heat index, a relative humidity of 75% at air temperature of 80.0 °F (26.7 °C) would feel like 83.6 °F ±1.3 °F (28.7 °C ±0.7 °C).[13][14]

Relationship between absolute-, relative-humidity, and temperature

In the Earth's atmosphere at sea level:

Absolute humidity in g/m3 (oz/cu. yd)[15][16]
Temperature Relative humidity
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
50 °C (122 °F) 0 (0) 8.3 (0.22) 16.6 (0.45) 24.9 (0.67) 33.2 (0.90) 41.5 (1.12) 49.8 (1.34) 58.1 (1.57) 66.4 (1.79) 74.7 (2.01) 83.0 (2.24)
45 °C (113 °F) 0 (0) 6.5 (0.18) 13.1 (0.35) 19.6 (0.53) 26.2 (0.71) 32.7 (0.88) 39.3 (1.06) 45.8 (1.24) 52.4 (1.41) 58.9 (1.59) 65.4 (1.76)
40 °C (104 °F) 0 (0) 5.1 (0.14) 10.2 (0.28) 15.3 (0.41) 20.5 (0.55) 25.6 (0.69) 30.7 (0.83) 35.8 (0.97) 40.9 (1.10) 46.0 (1.24) 51.1 (1.38)
35 °C (95 °F) 0 (0) 4.0 (0.11) 7.9 (0.21) 11.9 (0.32) 15.8 (0.43) 19.8 (0.53) 23.8 (0.64) 27.7 (0.75) 31.7 (0.85) 35.6 (0.96) 39.6 (1.07)
30 °C (86 °F) 0 (0) 3.0 (0.081) 6.1 (0.16) 9.1 (0.25) 12.1 (0.33) 15.2 (0.41) 18.2 (0.49) 21.3 (0.57) 24.3 (0.66) 27.3 (0.74) 30.4 (0.82)
25 °C (77 °F) 0 (0) 2.3 (0.062) 4.6 (0.12) 6.9 (0.19) 9.2 (0.25) 11.5 (0.31) 13.8 (0.37) 16.1 (0.43) 18.4 (0.50) 20.7 (0.56) 23.0 (0.62)
20 °C (68 °F) 0 (0) 1.7 (0.046) 3.5 (0.094) 5.2 (0.14) 6.9 (0.19) 8.7 (0.23) 10.4 (0.28) 12.1 (0.33) 13.8 (0.37) 15.6 (0.42) 17.3 (0.47)
15 °C (59 °F) 0 (0) 1.3 (0.035) 2.6 (0.070) 3.9 (0.11) 5.1 (0.14) 6.4 (0.17) 7.7 (0.21) 9.0 (0.24) 10.3 (0.28) 11.5 (0.31) 12.8 (0.35)
10 °C (50 °F) 0 (0) 0.9 (0.024) 1.9 (0.051) 2.8 (0.076) 3.8 (0.10) 4.7 (0.13) 5.6 (0.15) 6.6 (0.18) 7.5 (0.20) 8.5 (0.23) 9.4 (0.25)
5 °C (41 °F) 0 (0) 0.7 (0.019) 1.4 (0.038) 2.0 (0.054) 2.7 (0.073) 3.4 (0.092) 4.1 (0.11) 4.8 (0.13) 5.4 (0.15) 6.1 (0.16) 6.8 (0.18)
0 °C (32 °F) 0 (0) 0.5 (0.013) 1.0 (0.027) 1.5 (0.040) 1.9 (0.051) 2.4 (0.065) 2.9 (0.078) 3.4 (0.092) 3.9 (0.11) 4.4 (0.12) 4.8 (0.13)
−5 °C (23 °F) 0 (0) 0.3 (0.0081) 0.7 (0.019) 1.0 (0.027) 1.4 (0.038) 1.7 (0.046) 2.1 (0.057) 2.4 (0.065) 2.7 (0.073) 3.1 (0.084) 3.4 (0.092)
−10 °C (14 °F) 0 (0) 0.2 (0.0054) 0.5 (0.013) 0.7 (0.019) 0.9 (0.024) 1.2 (0.032) 1.4 (0.038) 1.6 (0.043) 1.9 (0.051) 2.1 (0.057) 2.3 (0.062)
−15 °C (5 °F) 0 (0) 0.2 (0.0054) 0.3 (0.0081) 0.5 (0.013) 0.6 (0.016) 0.8 (0.022) 1.0 (0.027) 1.1 (0.030) 1.3 (0.035) 1.5 (0.040) 1.6 (0.043)
−20 °C (−4 °F) 0 (0) 0.1 (0.0027) 0.2 (0.0054) 0.3 (0.0081) 0.4 (0.011) 0.4 (0.011) 0.5 (0.013) 0.6 (0.016) 0.7 (0.019) 0.8 (0.022) 0.9 (0.024)
−25 °C (−13 °F) 0 (0) 0.1 (0.0027) 0.1 (0.0027) 0.2 (0.0054) 0.2 (0.0054) 0.3 (0.0081) 0.3 (0.0081) 0.4 (0.011) 0.4 (0.011) 0.5 (0.013) 0.6 (0.016)

Specific humidity

Specific humidity (or moisture content) is the ratio of the mass of water vapor to the total mass of the air parcel.[17] Specific humidity is approximately equal to the mixing ratio, 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.[18]

Measurement

 
A hygrothermograph for humidity and temperature recording
 
Hygrometer for domestic use, wet/dry psychrometer type
 
Thermo hygrometer displaying temperature and relative humidity

A device used to measure humidity of air is called a psychrometer or hygrometer. A humidistat is a humidity-triggered switch, often used to control a dehumidifier.

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 (Tw) of the mixture are known. These quantities are readily estimated by using a sling psychrometer.

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 is among the least complex of these, having only three parameters (A, B, and C). Other formulas, such as the Goff–Gratch equation and the Magnus–Tetens approximation, are more complicated but yield better accuracy.[citation needed]

The Arden Buck equation is commonly encountered in the literature regarding this topic:[19]

 

where   is the dry-bulb temperature expressed in degrees Celsius (°C),   is the absolute pressure expressed in millibars, and   is the equilibrium vapor pressure expressed in millibars. Buck has reported that the maximal relative error is less than 0.20% between −20, and +50 °C (−4, and 122 °F) 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.[20] For process on-line measurements, the most commonly used sensors nowadays are based on capacitance measurements to measure relative humidity,[21] 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 clothes dryers.

Humidity is also measured on a global scale using remotely placed satellites. These satellites are able to detect the concentration of water in the troposphere at altitudes between 4 and 12 km (2.5 and 7.5 mi). Satellites that can measure water vapor have sensors that are sensitive to infrared radiation. 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.

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. 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 (M ≈ 28) or a molecule of oxygen (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. 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 discovered this phenomenon and wrote about it in his book Opticks.[22]

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   is defined as the ratio of the saturated vapor pressure of water in moist air   to the saturated vapor pressure of pure water:

 

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.[19]

Effects

 
Hygrostat set to 50% relative humidity
 
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

 
Average humidity around Australia year-round at 9 am
  80–90%
  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, 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 points in the country, frequently exceeding 30 °C (86 °F) in the monsoon season.[23]

High temperatures combine with the high dew point to create heat index in excess of 65 °C (149 °F). 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 35 °C (95 °F).

Global climate

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 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.[24][25][26] Water vapor is thus a "slave" to the non-condensible greenhouse gases.[27][28][29]

Animal and plant life

 
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 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.[30]

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.

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.[31] 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.[32]

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%[33]—but ideally not above the Absolute (60°F Dew Point),[34] between 40%[35] and 60%.[36] 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 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.[37]

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. Sufferers will often hyperventilate in response, causing sensations of numbness, faintness, and loss of concentration, among others.[38]

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.[39] Extremely low (below 20%) relative humidities may also cause eye irritation.[40][41] The use of a humidifier in homes, especially bedrooms, can help with these symptoms.[42] Indoor relative humidities should be kept above 30% to reduce the likelihood of the occupant's nasal passages drying out, especially in winter.[40][43][44]

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%.[45] 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.[46] However, the recommended range of indoor relative humidity in air conditioned buildings is generally 30–60%.[47][48]

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."[49]

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 g water/m3 than at 30 g water/m3.[50]

Increased humidity can also lead to changes in total body water that usually leads to moderate weight gain, especially if one is acclimated to working or exercising in hot and humid weather.[51]

Building construction

 
Effects of high humidity level in a building structure (primary efflorescence)

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.[52] 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, dehumidifiers 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 shorting of electrical components. In vehicles and pressure vessels such as pressurized airliners, submersibles and spacecraft, these considerations may be critical to safety, and complex environmental control systems including equipment to maintain pressure are needed.

Aviation

Airliners operate with low internal relative humidity, often under 20%,[53] 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. Naturally aspirated internal combustion engines have a further danger of ice forming inside the carburetor. Aviation weather reports (METARs) 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

 
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%). At the top end of the range, moisture may increase the conductivity of permeable 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).[54] 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 devices, resulting in irreversible damage. Data centers 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.[55]

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.[56]

Other important facts

 

At 100% relative humidity, air is saturated and at its dew point: the water vapor pressure would permit neither evaporation of nearby liquid water nor condensation to grow the nearby water; neither sublimation of nearby ice nor deposition to grow the nearby ice.

Relative humidity can exceed 100%, in which case the air is supersaturated. Cloud formation requires supersaturated air. Cloud condensation nuclei lower the level of supersaturation required to form fogs and clouds - in the absence of nuclei around which droplets or ice can form, a higher level of supersaturation is required for these droplets or ice crystals to form spontaneously. In the Wilson cloud chamber, which is used in nuclear physics experiments, a state of supersaturation is created within the chamber, and moving subatomic particles act as condensation nuclei so trails of fog show the paths of those particles.

For a given dew point and its corresponding absolute humidity, the relative humidity will change inversely, albeit nonlinearly, with the temperature. This is because the vapor pressure of water increases with temperature—the operative principle behind everything from hair dryers to dehumidifiers.

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 30 °C (86 °F) compared to no more than about 0.5% by mass at 0 °C (32 °F). This explains the low levels (in the absence of measures to add moisture) of humidity in heated structures during winter, resulting in dry skin, itchy eyes, and persistence of static electric charges. Even with saturation (100% relative humidity) outdoors, heating of infiltrated outside air that comes indoors raises its moisture capacity, which lowers relative humidity and increases evaporation rates from moist surfaces indoors (including human bodies and household plants.)

Similarly, during summer in humid climates a great deal of liquid water condenses from air cooled in air conditioners. Warmer air is cooled below its dew point, and the excess water vapor condenses. This phenomenon is the same as that which causes water droplets to form on the outside of a cup containing an ice-cold drink.

A useful rule of thumb is that the maximum absolute humidity doubles for every 20 °F (11 °C) increase in temperature. Thus, the relative humidity will drop by a factor of 2 for each 20 °F (11 °C) increase in temperature, assuming conservation of absolute moisture. For example, in the range of normal temperatures, air at 68 °F (20 °C) and 50% relative humidity will become saturated if cooled to 50 °F (10 °C), its dew point, and 41 °F (5 °C) air at 80% relative humidity warmed to 68 °F (20 °C) will have a relative humidity of only 29% and feel dry. By comparison, thermal comfort standard ASHRAE 55 requires systems designed to control humidity to maintain a dew point of 16.8 °C (62.2 °F) though no lower humidity limit is established.[46]

Water vapor is a lighter gas than other gaseous components of air at the same temperature, so humid air will tend to rise by natural convection. This is a mechanism behind thunderstorms and other weather phenomena. Relative humidity is often mentioned in weather forecasts and reports, as it is an indicator of the likelihood of dew, or fog. In hot summer weather, it also increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin as the relative humidity rises. This effect is calculated as the heat index or humidex.

A device used to measure humidity is called a hygrometer; one used to regulate it is called a humidistat, or sometimes hygrostat. (These are analogous to a thermometer and thermostat for temperature, respectively.)

References

Citations

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General sources

  • Arundel, A. V.; Sterling, E. M.; Biggin, J. H.; Sterling, T. D. (1986). "Indirect health effects of relative humidity in indoor environments". Environ. Health Perspect. 65: 351–61. doi:10.1289/ehp.8665351. PMC 1474709. PMID 3709462.
  • Bröde, Peter; Fiala, Dusan; Błażejczyk, Krzysztof; Holmér, Ingvar; Jendritzky, Gerd; Kampmann, Bernhard; Tinz, Birger; Havenith, George (2011-05-31). "Deriving the operational procedure for the Universal Thermal Climate Index (UTCI)" (PDF). International Journal of Biometeorology. 56 (3): 481–494. doi:10.1007/s00484-011-0454-1. ISSN 0020-7128. PMID 21626294. S2CID 37771005.
  • Buck, Arden L. (1981). "New Equations for Computing Vapor Pressure and Enhancement Factor". Journal of Applied Meteorology. 20 (12): 1527–1532. Bibcode:1981JApMe..20.1527B. doi:10.1175/1520-0450(1981)020<1527:NEFCVP>2.0.CO;2. ISSN 0021-8952.
  • Fanger, P. O. (1970). Thermal Comfort: Analysis and Applications in Environmental Engineering. Danish Technical Press. ISBN 978-87-571-0341-0.
  • Gilmore, C. P. (September 1972). "More Comfort for Your Heating Dollar". Popular Science. p. 99.
  • Schiavon, Stefano; Hoyt, Tyler; Piccioli, Alberto (2013-12-27). "Web application for thermal comfort visualization and calculation according to ASHRAE Standard 55". Building Simulation. 7 (4): 321–334. doi:10.1007/s12273-013-0162-3. ISSN 1996-3599. S2CID 56274353.
  • Wolkoff, Peder; Kjaergaard, Søren K. (August 2007). "The dichotomy of relative humidity on indoor air quality". Environment International. 33 (6): 850–857. doi:10.1016/j.envint.2007.04.004. ISSN 0160-4120. PMID 17499853.
  • United States Environmental Protection Agency, "IAQ in Large Buildings". Retrieved Jan. 9, 2006.

Further reading

External links

  • Current map of global relative humidity

humidity, serbian, film, film, concentration, water, vapor, present, water, vapor, gaseous, state, water, generally, invisible, human, indicates, likelihood, precipitation, present, global, distribution, relative, humidity, surface, averaged, over, years, 1981. For the Serbian film see Humidity film Humidity is the concentration of water vapor present in the air Water vapor the gaseous state of water is generally invisible to the human eye 2 Humidity indicates the likelihood for precipitation dew or fog to be present Global distribution of relative humidity at the surface averaged over the years 1981 2010 from the CHELSA BIOCLIM data set 1 Humidity depends on the temperature and pressure of the system of interest The same amount of water vapor results in higher relative humidity in cool air than warm air A related parameter is the dew point The amount of water vapor needed to achieve saturation increases as the temperature increases As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing water mass The amount of water vapor contained within a parcel of air can vary significantly For example a parcel of air near saturation may contain 28 g of water per cubic metre of air at 30 C 86 F but only 8 g of water per cubic metre of air at 8 C 46 F 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 3 or as mass of water vapor per mass of dry air usually in grams per kilogram 4 Relative humidity often expressed as a percentage indicates a present state of absolute humidity relative to a maximum humidity given the same temperature Specific humidity is the ratio of water vapor mass to total moist air parcel mass Humidity plays an important role for surface life For animal life dependent on perspiration sweating to regulate internal body temperature high humidity impairs heat exchange efficiency by reducing the rate of moisture evaporation from skin surfaces This effect can be calculated using a heat index table also known as a humidex The notion of air holding water vapor or being saturated by it is often mentioned in connection with the concept of relative humidity This however is misleading the amount of water vapor that enters or can enter a given space at a given temperature is almost independent of the amount of air nitrogen oxygen etc that is present Indeed a vacuum has approximately the same equilibrium capacity to hold water vapor as the same volume filled with air both are given by the equilibrium vapor pressure of water at the given temperature 5 6 There is a very small difference described under Enhancement factor below which can be neglected in many calculations unless great accuracy is required Contents 1 Definitions 1 1 Absolute humidity 1 2 Relative humidity 1 3 Relationship between absolute relative humidity and temperature 1 4 Specific humidity 1 5 Related concepts 2 Measurement 3 Air density and volume 4 Pressure dependence 4 1 Enhancement factor 5 Effects 5 1 Climate 5 2 Global climate 5 3 Animal and plant life 5 4 Human comfort 5 5 Human health 5 6 Building construction 5 7 Vehicles 5 8 Aviation 5 9 Electronics 5 10 Industry 5 11 Baking 6 Other important facts 7 References 7 1 Citations 7 2 General sources 8 Further reading 9 External linksDefinitions Edit Paranal Observatory on Cerro Paranal in the Atacama Desert is one of the driest places on Earth 7 Absolute humidity Edit 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 30 g 1 1 oz per cubic metre when the air is saturated at 30 C 86 F 8 9 Absolute humidity is the mass of the water vapor m H 2 O displaystyle m H 2 O divided by the volume of the air and water vapor mixture V n e t displaystyle V net which can be expressed as A H m H 2 O V n e t displaystyle AH frac m H 2 O V net The absolute humidity changes as air temperature or pressure changes if the volume is not fixed This makes it unsuitable for chemical engineering calculations e g in drying 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 BS 1339 10 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 Relative humidity Edit The relative humidity R H displaystyle RH or ϕ displaystyle phi of an air water mixture is defined as the ratio of the partial pressure of water vapor p displaystyle p in air to the saturation vapor pressure p s displaystyle p s of water at the same temperature usually expressed as a percentage 11 12 5 ϕ 100 p p s displaystyle phi 100 p p s 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 changing the temperature of air can change the relative humidity even when the absolute humidity remains constant Chilling air increases the relative humidity and can cause the water vapour to condense if the relative humidity rises over 100 the dew point Likewise warming air decreases the relative humidity 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 Relative humidity only considers the invisible water vapour Mists clouds fogs and aerosols of water do not count towards the measure of relative humidity of the air although their presence is an indication that a body of air may be close to the dew point Relative humidity is normally expressed as a percentage a higher percentage means that the air water mixture is more humid At 100 relative humidity the air is saturated and is at its dew point In the absence of a foreign body on which droplets or crystals can nucleate the relative humidity can exceed 100 in which case the air is said to be supersaturated Introduction of some particles or a surface to a body of air above 100 relative humidity will allow condensation or ice to form on those nuclei thereby removing some of the vapour and lowering the humidity Relative humidity is an important metric used in weather forecasts and reports as it is an indicator of the likelihood of precipitation dew or fog In hot summer weather a rise in relative humidity increases the apparent temperature to humans and other animals by hindering the evaporation of perspiration from the skin For example according to the heat index a relative humidity of 75 at air temperature of 80 0 F 26 7 C would feel like 83 6 F 1 3 F 28 7 C 0 7 C 13 14 Relationship between absolute relative humidity and temperature Edit In the Earth s atmosphere at sea level Absolute humidity in g m3 oz cu yd 15 16 Temperature Relative humidity0 10 20 30 40 50 60 70 80 90 100 50 C 122 F 0 0 8 3 0 22 16 6 0 45 24 9 0 67 33 2 0 90 41 5 1 12 49 8 1 34 58 1 1 57 66 4 1 79 74 7 2 01 83 0 2 24 45 C 113 F 0 0 6 5 0 18 13 1 0 35 19 6 0 53 26 2 0 71 32 7 0 88 39 3 1 06 45 8 1 24 52 4 1 41 58 9 1 59 65 4 1 76 40 C 104 F 0 0 5 1 0 14 10 2 0 28 15 3 0 41 20 5 0 55 25 6 0 69 30 7 0 83 35 8 0 97 40 9 1 10 46 0 1 24 51 1 1 38 35 C 95 F 0 0 4 0 0 11 7 9 0 21 11 9 0 32 15 8 0 43 19 8 0 53 23 8 0 64 27 7 0 75 31 7 0 85 35 6 0 96 39 6 1 07 30 C 86 F 0 0 3 0 0 081 6 1 0 16 9 1 0 25 12 1 0 33 15 2 0 41 18 2 0 49 21 3 0 57 24 3 0 66 27 3 0 74 30 4 0 82 25 C 77 F 0 0 2 3 0 062 4 6 0 12 6 9 0 19 9 2 0 25 11 5 0 31 13 8 0 37 16 1 0 43 18 4 0 50 20 7 0 56 23 0 0 62 20 C 68 F 0 0 1 7 0 046 3 5 0 094 5 2 0 14 6 9 0 19 8 7 0 23 10 4 0 28 12 1 0 33 13 8 0 37 15 6 0 42 17 3 0 47 15 C 59 F 0 0 1 3 0 035 2 6 0 070 3 9 0 11 5 1 0 14 6 4 0 17 7 7 0 21 9 0 0 24 10 3 0 28 11 5 0 31 12 8 0 35 10 C 50 F 0 0 0 9 0 024 1 9 0 051 2 8 0 076 3 8 0 10 4 7 0 13 5 6 0 15 6 6 0 18 7 5 0 20 8 5 0 23 9 4 0 25 5 C 41 F 0 0 0 7 0 019 1 4 0 038 2 0 0 054 2 7 0 073 3 4 0 092 4 1 0 11 4 8 0 13 5 4 0 15 6 1 0 16 6 8 0 18 0 C 32 F 0 0 0 5 0 013 1 0 0 027 1 5 0 040 1 9 0 051 2 4 0 065 2 9 0 078 3 4 0 092 3 9 0 11 4 4 0 12 4 8 0 13 5 C 23 F 0 0 0 3 0 0081 0 7 0 019 1 0 0 027 1 4 0 038 1 7 0 046 2 1 0 057 2 4 0 065 2 7 0 073 3 1 0 084 3 4 0 092 10 C 14 F 0 0 0 2 0 0054 0 5 0 013 0 7 0 019 0 9 0 024 1 2 0 032 1 4 0 038 1 6 0 043 1 9 0 051 2 1 0 057 2 3 0 062 15 C 5 F 0 0 0 2 0 0054 0 3 0 0081 0 5 0 013 0 6 0 016 0 8 0 022 1 0 0 027 1 1 0 030 1 3 0 035 1 5 0 040 1 6 0 043 20 C 4 F 0 0 0 1 0 0027 0 2 0 0054 0 3 0 0081 0 4 0 011 0 4 0 011 0 5 0 013 0 6 0 016 0 7 0 019 0 8 0 022 0 9 0 024 25 C 13 F 0 0 0 1 0 0027 0 1 0 0027 0 2 0 0054 0 2 0 0054 0 3 0 0081 0 3 0 0081 0 4 0 011 0 4 0 011 0 5 0 013 0 6 0 016 Specific humidity Edit Specific humidity or moisture content is the ratio of the mass of water vapor to the total mass of the air parcel 17 Specific humidity is approximately equal to the mixing ratio 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 Edit 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 18 Measurement Edit A hygrothermograph for humidity and temperature recording Hygrometer for domestic use wet dry psychrometer type Thermo hygrometer displaying temperature and relative humidity A device used to measure humidity of air is called a psychrometer or hygrometer A humidistat is a humidity triggered switch often used to control a dehumidifier 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 Tw of the mixture are known These quantities are readily estimated by using a sling psychrometer 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 is among the least complex of these having only three parameters A B and C Other formulas such as the Goff Gratch equation and the Magnus Tetens approximation are more complicated but yield better accuracy citation needed The Arden Buck equation is commonly encountered in the literature regarding this topic 19 e w 1 0007 3 46 10 6 P 6 1121 e 17 502 T 240 97 T displaystyle e w left 1 0007 3 46 times 10 6 P right times 6 1121 e 17 502T 240 97 T where T displaystyle T is the dry bulb temperature expressed in degrees Celsius C P displaystyle P is the absolute pressure expressed in millibars and e w displaystyle e w is the equilibrium vapor pressure expressed in millibars Buck has reported that the maximal relative error is less than 0 20 between 20 and 50 C 4 and 122 F 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 20 For process on line measurements the most commonly used sensors nowadays are based on capacitance measurements to measure relative humidity 21 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 clothes dryers Humidity is also measured on a global scale using remotely placed satellites These satellites are able to detect the concentration of water in the troposphere at altitudes between 4 and 12 km 2 5 and 7 5 mi Satellites that can measure water vapor have sensors that are sensitive to infrared radiation 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 Air density and volume EditMain articles Volume thermodynamics Density of air and Ideal gas law 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 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 M 28 or a molecule of oxygen 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 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 discovered this phenomenon and wrote about it in his book Opticks 22 Pressure dependence EditThe 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 Edit The enhancement factor f w displaystyle f w is defined as the ratio of the saturated vapor pressure of water in moist air e w displaystyle e w to the saturated vapor pressure of pure water f W e w e w displaystyle f W frac e w e w 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 19 Effects Edit Hygrostat set to 50 relative humidity 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 Edit See also Precipitation meteorology and Humid subtropical climate Average humidity around Australia year round at 9 am 80 90 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 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 points in the country frequently exceeding 30 C 86 F in the monsoon season 23 High temperatures combine with the high dew point to create heat index in excess of 65 C 149 F 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 35 C 95 F Global climate Edit 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 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 24 25 26 Water vapor is thus a slave to the non condensible greenhouse gases 27 28 29 Animal and plant life Edit 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 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 30 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 Human comfort Edit 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 31 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 32 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 33 but ideally not above the Absolute 60 F Dew Point 34 between 40 35 and 60 36 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 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 37 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 Sufferers will often hyperventilate in response causing sensations of numbness faintness and loss of concentration among others 38 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 39 Extremely low below 20 relative humidities may also cause eye irritation 40 41 The use of a humidifier in homes especially bedrooms can help with these symptoms 42 Indoor relative humidities should be kept above 30 to reduce the likelihood of the occupant s nasal passages drying out especially in winter 40 43 44 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 45 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 46 However the recommended range of indoor relative humidity in air conditioned buildings is generally 30 60 47 48 Human health Edit Higher humidity reduces the infectivity of aerosolized influenza virus A study concluded Maintaining indoor relative humidity gt 40 will significantly reduce the infectivity of aerosolized virus 49 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 g water m3 than at 30 g water m3 50 Increased humidity can also lead to changes in total body water that usually leads to moderate weight gain especially if one is acclimated to working or exercising in hot and humid weather 51 Building construction Edit Effects of high humidity level in a building structure primary efflorescence 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 52 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 dehumidifiers and associated control systems Vehicles Edit 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 shorting of electrical components In vehicles and pressure vessels such as pressurized airliners submersibles and spacecraft these considerations may be critical to safety and complex environmental control systems including equipment to maintain pressure are needed Aviation Edit Airliners operate with low internal relative humidity often under 20 53 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 Naturally aspirated internal combustion engines have a further danger of ice forming inside the carburetor Aviation weather reports METARs 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 Edit 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 At the top end of the range moisture may increase the conductivity of permeable 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 54 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 devices resulting in irreversible damage Data centers often monitor relative humidity levels for these reasons Industry Edit 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 55 Baking Edit 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 56 Other important facts Edit At 100 relative humidity air is saturated and at its dew point the water vapor pressure would permit neither evaporation of nearby liquid water nor condensation to grow the nearby water neither sublimation of nearby ice nor deposition to grow the nearby ice Relative humidity can exceed 100 in which case the air is supersaturated Cloud formation requires supersaturated air Cloud condensation nuclei lower the level of supersaturation required to form fogs and clouds in the absence of nuclei around which droplets or ice can form a higher level of supersaturation is required for these droplets or ice crystals to form spontaneously In the Wilson cloud chamber which is used in nuclear physics experiments a state of supersaturation is created within the chamber and moving subatomic particles act as condensation nuclei so trails of fog show the paths of those particles For a given dew point and its corresponding absolute humidity the relative humidity will change inversely albeit nonlinearly with the temperature This is because the vapor pressure of water increases with temperature the operative principle behind everything from hair dryers to dehumidifiers 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 30 C 86 F compared to no more than about 0 5 by mass at 0 C 32 F This explains the low levels in the absence of measures to add moisture of humidity in heated structures during winter resulting in dry skin itchy eyes and persistence of static electric charges Even with saturation 100 relative humidity outdoors heating of infiltrated outside air that comes indoors raises its moisture capacity which lowers relative humidity and increases evaporation rates from moist surfaces indoors including human bodies and household plants Similarly during summer in humid climates a great deal of liquid water condenses from air cooled in air conditioners Warmer air is cooled below its dew point and the excess water vapor condenses This phenomenon is the same as that which causes water droplets to form on the outside of a cup containing an ice cold drink A useful rule of thumb is that the maximum absolute humidity doubles for every 20 F 11 C increase in temperature Thus the relative humidity will drop by a factor of 2 for each 20 F 11 C increase in temperature assuming conservation of absolute moisture For example in the range of normal temperatures air at 68 F 20 C and 50 relative humidity will become saturated if cooled to 50 F 10 C its dew point and 41 F 5 C air at 80 relative humidity warmed to 68 F 20 C will have a relative humidity of only 29 and feel dry By comparison thermal comfort standard ASHRAE 55 requires systems designed to control humidity to maintain a dew point of 16 8 C 62 2 F though no lower humidity limit is established 46 Water vapor is a lighter gas than other gaseous components of air at the same temperature so humid air will tend to rise by natural convection This is a mechanism behind thunderstorms and other weather phenomena Relative humidity is often mentioned in weather forecasts and reports as it is an indicator of the likelihood of dew or fog In hot summer weather it also increases the apparent temperature to humans and other animals by hindering the evaporation of perspiration from the skin as the relative humidity rises This effect is calculated as the heat index or humidex A device used to measure humidity is called a hygrometer one used to regulate it is called a humidistat or sometimes hygrostat These are analogous to a thermometer and thermostat for temperature respectively References EditCitations Edit Brun P Zimmermann N E Hari C Pellissier L Karger D N preprint Global climate related predictors at kilometre resolution for the past and future Earth Syst Sci Data Discuss https doi org 10 5194 essd 2022 212 What is Water Vapor Retrieved 2012 08 28 Wyer Samuel S 1906 Fundamental Physical Laws and Definitions A Treatise on Producer Gas and Gas Producers McGraw Hill Book Company p 23 Perry R H and Green D W 2007 Perry s Chemical Engineers Handbook 8th Edition Section 12 Psychrometry Evaporative Cooling and Solids Drying McGraw Hill ISBN 978 0 07 151135 3 a b Babin Steven M 1998 Relative Humidity amp Saturation Vapor Pressure A Brief Tutorial Johns Hopkins University Applied Physics Laboratory Archived from the original on 1998 07 13 Retrieved 2022 11 28 Alternate title Water Vapor Myths A Brief Tutorial Fraser Alistair B Bad Clouds FAQ www ems psu edu Archived from the original on 2006 06 17 Antarctic Air Visits Paranal ESO Picture of the Week Retrieved 4 February 2014 Climate Humidity indexes Encyclopaedia Britannica Retrieved 15 February 2018 Climate humidity table Transport Information Service of the German Insurance Association Retrieved 15 February 2018 British Standard BS 1339 revised Humidity and Dewpoint Parts 1 3 2002 2007 Perry R H and Green D W Perry s Chemical Engineers Handbook 8th Edition McGraw Hill ISBN 0 07 142294 3 pg 12 4 Lide David 2005 CRC Handbook of Chemistry and Physics 85 ed CRC Press pp 15 25 ISBN 0 8493 0485 7 Lans P Rothfusz The Heat Index Equation or More Than You Ever Wanted to Know About Heat Index Scientific Services Division NWS Southern Region Headquarters 1 July 1990 The Heat Index Equation or More Than You Ever Wanted to Know About Heat Index PDF Archived from the original PDF on 2011 12 01 Retrieved 2022 11 06 Steadman R G 1979 The Assessment of Sultriness Part I A Temperature Humidity Index Based on Human Physiology and Clothing Science Journal of Applied Meteorology 18 7 861 873 Bibcode 1979JApMe 18 861S doi 10 1175 1520 0450 1979 018 lt 0861 TAOSPI gt 2 0 CO 2 ISSN 0021 8952 Climate humidity table Transport Informations Service www tis gdv de Retrieved 2021 06 17 Absolute Humidity Table PDF mercury pr erau edu Retrieved 2021 06 17 Seidel Dian What is atmospheric humidity and how is it measured broken link National Oceanic and Atmospheric Administration National Oceanic and Atmospheric Administration Archived from the original on 18 October 2017 Retrieved 3 March 2017 Vapor Liquid Solid System 201 Class Page University of Arizona Archived from the original on May 8 2006 a b Buck 1981 pp 1527 1532 Pieter R Wiederhold 1997 Water Vapor Measurement Methods and Instrumentation Marcel Dekker New York NY ISBN 9780824793197 BS1339 Part 3 Isaac Newton 1704 Opticks Dover ISBN 978 0 486 60205 9 Weather History for Sukkur Pakistan Weather Underground Blackbody Radiation Lecture notes Archived from the original on 2017 10 23 Retrieved 2015 01 11 Radiative Balance Earth s Temperature and Greenhouse Gases lecture notes Alley R 2014 GEOSC 10 Optional Enrichment Article 1 Archived from the original on 2018 09 08 Retrieved 2015 01 11 Businger S Lecture 28 Future Global Warming Modeling Climate Change PDF Archived from the original PDF on 2015 01 30 Schwieterman E Comparing the Greenhouse Effect on Earth Mars Venus and Titan Present Day and through Time PDF C Michael Hogan 2010 Abiotic factor Encyclopedia of Earth eds Emily Monosson and C Cleveland National Council for Science and the Environment Archived June 8 2013 at the Wayback Machine Washington DC Fanger 1970 p 48 Brode et al 2011 pp 481 494 Gilmore 1972 p 99 1 ASHRAE Std 62 1 2019 Winter Indoor Comfort and Relative Humidity Information please database Pearson 2007 archived from the original on 2013 04 27 retrieved 2013 05 01 by increasing the relative humidity to above 50 within the above temperature range 80 or more of all average dressed persons would feel comfortable Recommended relative humidity level The engineering toolbox archived from the original on 2013 05 11 retrieved 2013 05 01 Relative humidity above 60 feels uncomfortable wet Human comfort requires the relative humidity to be in the range 25 60 RH Schiavon Hoyt amp Piccioli 2013 pp 321 334 Heat and humidity the lung association www lung ca 26 August 2014 Retrieved 14 March 2018 What causes the common cold University of Rochester Medical Center Retrieved 2016 01 24 a b Arundel et al 1986 pp 351 361 Indoor air quality testing Archived from the original on 2017 09 21 Nosebleeds WebMD Medical Reference Retrieved 2015 11 01 Indoor Air Quality PDF NH DHHS Division of Public Health Services Archived PDF from the original on 2015 09 22 Retrieved 2016 01 24 School Indoor Air Quality Best Management Practices Manual PDF Washington State Department of Health November 2003 Retrieved 2015 11 01 Optimum Humidity Levels for Home AirBetter org 3 August 2014 a b ASHRAE Standard 55 2017 Thermal Environmental Conditions for Human Occupancy Wolkoff amp Kjaergaard 2007 pp 850 857 ASHRAE Standard 160 2016 Criteria for Moisture Control Design Analysis in Buildings Noti John D Blachere Francoise M McMillen Cynthia M Lindsley William G Kashon Michael L Slaughter Denzil R Beezhold Donald H 2013 High Humidity Leads to Loss of Infectious Influenza Virus from Simulated Coughs PLOS ONE 8 2 e57485 Bibcode 2013PLoSO 857485N doi 10 1371 journal pone 0057485 PMC 3583861 PMID 23460865 Pieterse A Hanekom SD 2018 Criteria for enhancing mucus transport a systematic scoping review Multidisciplinary Respiratory Medicine 13 22 doi 10 1186 s40248 018 0127 6 PMC 6034335 PMID 29988934 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 Scientific American Retrieved 2021 06 09 Free publications Retrieved 2013 12 23 Airplane Humidity Aviator Atlas 5 April 2020 Retrieved 11 September 2020 Fogging Glasses Archived from the original on 2015 02 26 Retrieved 2012 08 08 Everything You Need to Know About Combustion Chemistry amp Analysis Industrial Controls Why is humidity important in cooking General sources Edit Arundel A V Sterling E M Biggin J H Sterling T D 1986 Indirect health effects of relative humidity in indoor environments Environ Health Perspect 65 351 61 doi 10 1289 ehp 8665351 PMC 1474709 PMID 3709462 Brode Peter Fiala Dusan Blazejczyk Krzysztof Holmer Ingvar Jendritzky Gerd Kampmann Bernhard Tinz Birger Havenith George 2011 05 31 Deriving the operational procedure for the Universal Thermal Climate Index UTCI PDF International Journal of Biometeorology 56 3 481 494 doi 10 1007 s00484 011 0454 1 ISSN 0020 7128 PMID 21626294 S2CID 37771005 Buck Arden L 1981 New Equations for Computing Vapor Pressure and Enhancement Factor Journal of Applied Meteorology 20 12 1527 1532 Bibcode 1981JApMe 20 1527B doi 10 1175 1520 0450 1981 020 lt 1527 NEFCVP gt 2 0 CO 2 ISSN 0021 8952 Fanger P O 1970 Thermal Comfort Analysis and Applications in Environmental Engineering Danish Technical Press ISBN 978 87 571 0341 0 Gilmore C P September 1972 More Comfort for Your Heating Dollar Popular Science p 99 Schiavon Stefano Hoyt Tyler Piccioli Alberto 2013 12 27 Web application for thermal comfort visualization and calculation according to ASHRAE Standard 55 Building Simulation 7 4 321 334 doi 10 1007 s12273 013 0162 3 ISSN 1996 3599 S2CID 56274353 Wolkoff Peder Kjaergaard Soren K August 2007 The dichotomy of relative humidity on indoor air quality Environment International 33 6 850 857 doi 10 1016 j envint 2007 04 004 ISSN 0160 4120 PMID 17499853 United States Environmental Protection Agency IAQ in Large Buildings Retrieved Jan 9 2006 Further reading EditHimmelblau David M 1989 Basic Principles And Calculations In Chemical Engineering Prentice Hall ISBN 0 13 066572 X Lide David 2005 CRC Handbook of Chemistry and Physics 85 ed CRC Press ISBN 9780849304859 Perry R H Green D W 1997 Perry s Chemical Engineers Handbook 7th ed McGraw Hill ISBN 0 07 049841 5 External links Edit Look up humidity in Wiktionary the free dictionary Wikisource has the text of the 1905 New International Encyclopedia article Humidity Current map of global relative humidity Retrieved from https en wikipedia org w index php title Humidity amp oldid 1133226762 Relative humidity, wikipedia, wiki, book, books, library,

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