Humidity and hygrometry
Cloud forest mount kinabalu.jpg
Specific concepts
General concepts
Measures and Instruments


Humidity depends on the temperature and pressure of the system of interest. The same amount of water vapor results in higher 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 (0.99 oz) of water per cubic metre of air at 30 °C (86 °F), but only 8 g (0.28 oz) of water per cubic metre of air at 8 °C (46 °F).





Paranal Observatory on Cerro Paranal in the Atacama Desert is one of the driest places on Earth.[6]

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).[7][8]




Relative humidity

The relative humidity or of an air-water mixture is defined as the ratio of the partial pressure of water vapor in the mixture to the equilibrium vapor pressure of water over a flat surface of pure water[10] at a given temperature:[11][12][4]

In other words, relative humidity is the ratio of how much water is in the air and how much water the air could potentially contain. 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. 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 a some 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.

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.


Specific humidity


Related concepts



A hygrometer for humidity control and recording.
Hygrometer for domestic use, wet/dry psychrometer type.
Clock with hygrometer, showing a very low humidity in winter.

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:[17]

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.[18] For process on-line measurements, the most commonly used sensors nowadays are based on capacitance measurements to measure relative humidity,[19] 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 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




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).

Changes in Relative Humidity.png

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.[17]


Acoustic guitar room at Guitar Center humidified to 50% relative humidity in winter.
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.


Climates of the Earth basing on humidity values.
 Humid climate
 Semi-arid climate
 Arid climate


The most humid cities on earth are generally located closer to the equator, near coastal regions. Cities in parts of Asia are among the most humid. Bangkok, Ho Chi Minh City, Kuala Lumpur, Hong Kong, Manila, Jakarta, 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 Cochin 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.[21]


Global climate



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.[22][23][24] Water vapor is thus a "slave" to the non-condensible greenhouse gases.[25][26][27]

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.



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.[29] 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.[30]


Humans can be comfortable within a wide range of humidities depending on the temperature — from 30–70%[31] — but ideally not above the Absolute (60°F Dew Point).[32], ... between 40%[33] and 60%.[34] 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.[35]


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.[37] Extremely low (below 20%) relative humidities may also cause eye irritation.[38][39] The use of a humidifier in homes, especially bedrooms, can help with these symptoms.[40] Indoor relative humidities should be kept above 30% to reduce the likelihood of the occupant's nasal passages drying out, especially in winter.[38][41][42]

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%.[43] 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.[44] However, the recommended range of indoor relative humidity in air conditioned buildings is generally 30–60%.[45][46]

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."[47]

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.[48]

Building construction

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


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.


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.


Airliners operate with low internal relative humidity, often under 20%,[50] 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. 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.


Desiccant bag (silica gel), commonly included in packages containing electronic products to control humidity.







Other important facts

Relative Humidity.png


The statement that relative humidity (RH%) can never be above 100%, while a fairly good guide, is not absolutely accurate, without a more sophisticated definition of humidity than the one given here. Cloud formation, in which aerosol particles are activated to form cloud condensation nuclei, requires the supersaturation of an air parcel to a relative humidity of slightly above 100%. One smaller-scale example is found in the Wilson cloud chamber in nuclear physics experiments, in which a state of supersaturation is induced to accomplish its function.

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 partial pressure of water increases with temperature—the operative principle behind everything from hair dryers to dehumidifiers.


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.[44]

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.)

See also



  4. ^ a b "Water Vapor Myths: A Brief Tutorial". www.atmos.umd.edu. Archived from the original on 2016-01-25.
  5. ^ Fraser, Alistair B. "Bad Clouds FAQ". www.ems.psu.edu. Archived from the original on 2006-06-17.
  10. ^ "Water Vapor Myths: A Brief Tutorial".
  14. ^ 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. doi:10.1175/1520-0450(1979)018<0861:TAOSPI>2.0.CO;2. ISSN 0021-8952.
  15. ^ 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.
  16. ^ "Vapor-Liquid/Solid System, 201 Class Page". University of Arizona. Archived from the original on May 8, 2006.
  17. ^ a b Buck 1981, pp. 1527–1532.
  19. ^ "BS1339" Part 3
  21. ^ "Weather History for Sukkur, Pakistan – Weather Underground".
  22. ^ "Blackbody Radiation".
  23. ^ "Lecture notes".
  29. ^ Fanger 1970, p. 48.
  30. ^ Bröde et al. 2011, pp. 481–494.
  31. ^ Gilmore 1972, p. 99.
  32. ^ [1] ASHRAE Std 62.1-2019
  33. ^ "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.
  34. ^ "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.
  35. ^ Schiavon, Hoyt & Piccioli 2013, pp. 321-334.
  36. ^ "Heat and humidity - the lung association". www.lung.ca. Retrieved 14 March 2018.
  37. ^ "What causes the common cold?". University of Rochester Medical Center. Retrieved 2016-01-24.
  38. ^ a b Arundel et al. 1986, pp. 351–361.
  39. ^ "Indoor air quality testing". Archived from the original on 2017-09-21.
  40. ^ "Nosebleeds". WebMD Medical Reference. Retrieved 2015-11-01.
  41. ^ "Indoor Air Quality" (PDF). NH DHHS, Division of Public Health Services. Archived (PDF) from the original on 2015-09-22. Retrieved 2016-01-24.
  42. ^ "School Indoor Air Quality: Best Management Practices Manual" (PDF). Washington State Department of Health. November 2003. Retrieved 2015-11-01.
  43. ^ "Optimum Humidity Levels for Home". AirBetter.org. 3 August 2014.
  44. ^ a b ASHRAE Standard 55 (2017). "Thermal Environmental Conditions for Human Occupancy".
  45. ^ Wolkoff & Kjaergaard 2007, pp. 850–857.
  46. ^ ASHRAE Standard 160 (2016). "Criteria for Moisture-Control Design Analysis in Buildings"
  48. ^ 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.
  49. ^ "Free publications".
  50. ^ "Airplane Humidity". Aviator Atlas. 5 April 2020. Retrieved 11 September 2020.
  51. ^ "Fogging Glasses".
  53. ^ "Why is humidity important in cooking?".

General sources

External links

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