Relative humidity

Relative humidity (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. Relative humidity depends on temperature and the 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 that of dewpoint.

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


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[1] at a given temperature:[2][3]

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 dewpoint.


Climate control

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.

Relative humidity and thermal comfort

Along with air temperature, mean radiant temperature, air speed, metabolic rate, and clothing level, relative humidity plays a role in human thermal comfort.  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.[4]  However, the recommended range of indoor relative humidity in air conditioned buildings is generally 30-60%.[5][6]

In general, higher temperatures will require lower relative 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 degrees C to 24 degrees 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.[7]

When using the adaptive model to predict thermal comfort indoors, relative humidity is not taken into account.[4]

Although relative humidity is an important factor for thermal comfort, humans are more sensitive to variations in temperature than they are to changes in relative humidity.[8] Relative 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.[9]

Human discomfort caused by low relative humidity

In cold climates, the outdoor temperature causes lower capacity for water vapor to flow about. Thus although it may be snowing and the relative humidity outdoors is high, once that air comes into a building and heats up, its new relative humidity is very low, making the air very dry, which can cause discomfort. Dry cracked skin can result from dry air.

Low humidity causes tissue lining nasal passages to dry, crack and become more susceptible to penetration of Rhinovirus cold viruses.[10] Low humidity is a common cause of nosebleeds. The use of a humidifier in homes, especially bedrooms, can help with these symptoms.[11]

Indoor relative humidities should be kept above 30% to reduce the likelihood of the occupant's nasal passages drying out.[12][13]

Humans can be comfortable within a wide range of humidities depending on the temperature—from 30% to 70%[14]—but ideally between 50%[15] and 60%.[16] Very low humidity can create discomfort, respiratory problems, and aggravate allergies in some individuals. In the winter, it is advisable to maintain relative humidity at 30% or above.[17] Extremely low (below 20%) relative humidities may also cause eye irritation.[12][18]


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 percent, 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 10%, 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.


A hygrometer is a device used for measuring the humidity of air.

The humidity of an air–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.

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

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 °C and +50 °C when this particular form of the generalized formula is used to estimate the equilibrium vapor pressure of water.

Water vapor is independent of air

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.[1][20] There is a very small difference described under "Enhancement factor" below, which can be neglected in many calculations unless high accuracy is required.

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

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.[21]

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.[22]

Other important facts

Relative Humidity

A gas in this context is referred to as saturated when the vapor pressure of water in the air is at the equilibrium vapor pressure for water vapor at the temperature of the gas and water vapor mixture; liquid water (and ice, at the appropriate temperature) will fail to lose mass through evaporation when exposed to saturated air. It may also correspond to the possibility of dew or fog forming, within a space that lacks temperature differences among its portions, for instance in response to decreasing temperature. Fog consists of very minute droplets of liquid, primarily held aloft by isostatic motion (in other words, the droplets fall through the air at terminal velocity, but as they are very small, this terminal velocity is very small too, so it doesn't look to us like they are falling, and they seem to be held aloft).

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.

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 or 10 °C increase in temperature. Thus, the relative humidity will drop by a factor of 2 for each 20 °F or 10 °C increase in temperature, assuming conservation of absolute moisture. For example, in the range of normal temperatures, air at 68 °F or 20 °C and 50% relative humidity will become saturated if cooled to 50 °F or 10 °C, its dew point, and 41 °F or 5 °C air at 80% relative humidity warmed to 68 °F or 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.[23]

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 precipitation, 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


  1. ^ a b "Water Vapor Myths: A Brief Tutorial". Archived from the original on 2016-01-25.
  2. ^ Perry, R. H. and Green, D. W, Perry's Chemical Engineers' Handbook (7th Edition), McGraw-Hill, ISBN 0-07-049841-5 , Eqn 12-7
  3. ^ Lide, David (2005). CRC Handbook of Chemistry and Physics (85 ed.). CRC Press. pp. 15–25. ISBN 0-8493-0485-7. Archived from the original on 2008-05-23.
  4. ^ a b ASHRAE Standard 55 (2017). "Thermal Environmental Conditions for Human Occupancy".
  5. ^ 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.
  6. ^ ASHRAE Standard 160 (2016). "Criteria for Moisture-Control Design Analysis in Buildings"
  7. ^ 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.
  8. ^ Fanger, P. O. (1970). Thermal comfort: analysis and applications in environmental engineering. Danish Technical Press.
  9. ^ 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)". International Journal of Biometeorology. 56 (3): 481–494. doi:10.1007/s00484-011-0454-1. ISSN 0020-7128. PMID 21626294.
  10. ^ "Archived copy". Archived from the original on 2016-02-04. Retrieved 2016-01-24.CS1 maint: Archived copy as title (link) University of Rochester Medical Center | What causes the common cold? | Health Encyclopedia
  11. ^ "Archived copy". Archived from the original on 2015-11-10. Retrieved 2015-11-01.CS1 maint: Archived copy as title (link) Nosebleeds - Prevention | WebMD Medical Reference
  12. ^ a b 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.
  13. ^ "Archived copy" (PDF). Archived (PDF) from the original on 2015-09-22. Retrieved 2016-01-24.CS1 maint: Archived copy as title (link) INDOOR AIR QUALITY | NH DHHS, Division of Public Health Services | NH Department of Environmental Services
  14. ^ Gilmore, C. P. (September 1972). "More Comfort for Your Heating Dollar". Popular Science: 99.
  15. ^ "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.
  16. ^ "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.
  17. ^ "Archived copy" (PDF). Archived (PDF) from the original on 2015-01-20. Retrieved 2015-11-01.CS1 maint: Archived copy as title (link) School Indoor Air Quality | Best Management Practices Manual | November 2003 | Office | Office of Environmental Health and Safety | Indoor Air Quality Program DOH 333-044 November 2003 | Washington State Department of Health
  18. ^ "Indoor air quality testing". Archived from the original on 2017-09-21.
  19. ^ Buck, Arden (December 1981). "New Equations for Computing Vapor Pressure and Enhancement Factor" (PDF). National Center for Atmospheric Research. Archived (PDF) from the original on March 4, 2016. Retrieved July 21, 2017.
  20. ^ "Bad Clouds FAQ". Archived from the original on 2006-06-17.
  21. ^ Buck, A. 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.
  22. ^ [1] Archived May 8, 2006, at the Wayback Machine
  23. ^ "Thermal Environmental Conditions for Human Occupancy". ASHRAE Standard 55. 2013.

External links

1984 Dallas Cowboys season

The 1984 Dallas Cowboys season was the team's 25th in the National Football League. The Cowboys finished the season with a record of nine wins and seven losses, and missed the playoffs for the first time in 10 years. A division record of 3–5 caused them to finish fourth in the NFC East, despite equaling the overall records of the New York Giants and St. Louis Cardinals. A loss to the winless Buffalo Bills in week 12 cost the team a critical win. Nonetheless, the Cowboys had a 9-5 record and would have made the playoffs had they won one of their two remaining games, and would have won the division had they won both games. The team gave up a 15-point lead against the Washington Redskins in week 15, and then lost to the Miami Dolphins by one touchdown (surrendered with less than a minute to play) in the final week of the season. The season was overshadowed by a quarterback controversy between Danny White and Gary Hogeboom, with Hogeboom getting the majority of the starts.


Apartadó (Spanish pronunciation: [apaɾtaˈðo]) is a town and municipality in the Antioquia Department, Colombia.

Apartadó means river of plantains in the local Indian language. The town is located near the Atlantic Ocean in the Gulf of Urabá, the economy is based in bananas, plantain, corn, cassava, cocoa, wood and livestock.

The mean temperature is 30 degrees Celsius and the relative humidity is above 80% all year round.

Apartadó is divided in 48 neighborhoods, and here is the best high school of the region of Urabá_Antioquia.

Today, the government is stimulating industrialization because it is near the Caribbean Sea and to the center of the country.

Inhabitants: 150,000 in 2009.

Climate of Gibraltar

The climate of Gibraltar is Mediterranean/Subtropical with mild winters and warm summers. Gibraltar has two main prevailing winds, an easterly one known as the Levante coming from the Sahara in Africa which brings humid weather and warmer sea currents and the other as Poniente which is westerly and brings fresher air and colder sea. Its terrain consists of the 430-metre (1,411 ft) high Rock of Gibraltar and the narrow coastal lowland surrounding it. Rain occurs mainly in winter; the summers are generally dry.

Average morning relative humidity: 82%, evening relative humidity: 64%. Sunshine hours are up to 2,778 per year, from 150 in November (~5 hours of sunshine per day) to 341 in July (~11 hours of sunshine per day).

Collections maintenance

Collection maintenance is a form of collections care that consists of the day-to-day hands on care of collections and cultural heritage. The primary goal of collections maintenance is to prevent further decay of cultural heritage by ensuring proper storage and upkeep including performing regular housekeeping of the spaces and objects and monitoring and controlling storage environments. Collections maintenance is closely linked to collections care and collections management. The professionals most influenced by collections maintenance include collection managers, registrars, and archivists.

Convective condensation level

The convective condensation level (CCL) represents the height (or pressure) where an air parcel becomes saturated when heated from below and lifted adiabatically due to buoyancy.

In the atmosphere, assuming a constant water vapor mixing ratio, the dew point temperature (the temperature where the

relative humidity is 100%) decreases with increasing height because the pressure of the atmosphere decreases with height. The CCL is determined by plotting the dew point (100%RH) verses altitude and locating the intersection with the actual measured temperature sounding. It marks where the cloud base begins when air is heated from below to the convective temperature, without mechanical lift. Once the CCL is determined, the surface temperature necessary to raise a mass of air to that height can be found by using the Dry Adiabatic Lapse Rate (DALR) to determine the potential temperature. In the early morning, this temperature is typically larger than the surface temperature, in the mid-afternoon, it may be the same.

Compare this to the Lifting Condensation Level (LCL) where the air is lifted and cooled without first increasing the surface temperature. The LCL is less than or equal to the CCL depending on the temperature profile.

Both condensation levels indicate the altitude (or pressure) where relative humidity reaches 100%. However, since the actual condensation level depends on the availability of condensation nuclei, clouds typically do not form until the relative humidity is somewhat above 100%.

Critical relative humidity

The critical relative humidity (CRH) of a salt is defined as the relative humidity of the surrounding atmosphere (at a certain temperature) at which the material begins to absorb moisture from the atmosphere and below which it will not absorb atmospheric moisture.

When the humidity of the atmosphere is equal to (or is greater than) the critical relative humidity of a sample of salt, the sample will take up water until all of the salt is dissolved to yield a saturated solution. All water-soluble salts and mixtures have characteristic critical humidities; it is a unique material property.

The critical relative humidity of most salts decreases with increasing temperature. For instance, the critical relative humidity of ammonium nitrate decreases 22% with a temperature from 0°C to 40°C (32°F to 104°F).

The critical relative humidity of several fertilizer salts is given in table 1:

Table 1: Critical relative humidities of pure salts at 30°C.

Mixtures of salts usually have lower critical humidities than either of the constituents. Fertilizers that contain Urea as an ingredient usually exhibit a much lower Critical Relative Humidity than Fertilizers without Urea.Table 2 shows CRH data for two-component mixtures:

Table 2: Critical relative humidities of mixtures of salts at 30°C (values are percent relative humidity).

As shown, the effect of salt mixing is most dramatic in the case of ammonium nitrate with urea. This mixture has an extremely low critical relative humidity and can therefore only be used in liquid fertilisers (so called UAN-solutions).

Dead man zone

The dead man zone is the area directly around a bushfire that is likely to burn within five minutes given the current wind conditions or an anticipated change in wind direction. The distance this zone extends from the firefront is highly dependent on terrain, windspeed, fuel type and composition, relative humidity and ambient temperature, and can range from under 100 m to well over 1 km.

Dew point

The dew point is the temperature to which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense to form liquid water (dew). When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface. When the temperature is below the freezing point of water, the dew point is called the frost point, as frost is formed rather than dew. The measurement of the dew point is related to humidity. A higher dew point means there will be more moisture in the air.

Diablo wind

Diablo wind is a name that has been occasionally used for the hot, dry wind from the northeast that typically occurs in the San Francisco Bay Area of Northern California, during the spring and fall. The same wind pattern also affects other parts of California's coastal ranges. The term first appeared shortly after the 1991 Oakland firestorm, perhaps to distinguish it from the comparable, and more familiar, hot dry wind in Southern California known as the Santa Ana winds. In fact, in decades previous to the 1991 fire, the term "Santa Ana" was occasionally used as well for the Bay Area dry northeasterly wind, such as the one that was associated with the 1923 Berkeley Fire.The name "Diablo wind" refers to the fact that the wind blows into the inner Bay Area from the direction of Mount Diablo in adjacent Contra Costa County, and mindful of the fiery, romantic connotation inherent in the term that translates to "devil wind". The Diablo wind is created by the combination of strong inland high pressure at the surface, strongly sinking air aloft, and lower pressure off the California coast. The air descending from aloft as well as from the Coast Ranges compresses as it sinks to sea level where it warms as much as 20 °F (11 °C), and loses relative humidity.Because of the elevation of the coastal ranges in north-central California, the thermodynamic structure that occurs with the Diablo wind pattern favors the development of strong ridge-top and lee-side downslope winds associated with a phenomenon called the "hydraulic jump". While hydraulic jumps can occur with Santa Ana winds, the same thermodynamic structure that occurs with them typically favors "gap" flow more frequently. Thus, Santa Anas are strongest in canyons, whereas a Diablo wind is first noted and blows strongest atop and on the western slopes of the various mountain peaks and ridges around the Bay Area, although channeling by canyons is also significant.

In both cases, as the air sinks, it heats up by compression and its relative humidity drops. This warming is in addition to, and usually greater than, any contact heating that occurs as the air stream crosses the Central Valley and the Diablo Valley. This is the reverse of the normal summertime weather pattern in which an area of low pressure (called the California Thermal Low) rather than high pressure lies east of the Bay Area, drawing in cooler, more humid air from the ocean. The dry offshore wind, already strong because of the offshore pressure gradient, can become quite strong with gusts reaching speeds of 40 miles per hour (64 km/h) or higher, particularly along and in the lee of the ridges of the Coast Range. This effect is especially dangerous with respect to wildfires as it can enhance the updraft generated by the heat in such fires.

While the Diablo wind pattern occurs in both the spring and fall, it is most dangerous in the fall, when vegetation is at its driest.

Heat index

The heat index (HI) or humiture is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade. The result is also known as the "felt air temperature", "apparent temperature", "real feel" or "feels like". For example, when the temperature is 32 °C (90 °F) with 70% relative humidity, the heat index is 41 °C (106 °F). This heat index temperature has an implied (unstated) humidity of 20%. This is the value of relative humidity for which the heat index number equals the actual air temperature.

The human body normally cools itself by perspiration, or sweating. Heat is removed from the body by evaporation of that sweat. However, high relative humidity reduces the evaporation rate. This results in a lower rate of heat removal from the body, hence the sensation of being overheated. This effect is subjective, with different individuals perceiving heat differently for various reasons (such as differences in body shape, metabolic differences, differences in hydration, pregnancy, menopause, effects of drugs and/or drug withdrawal); its measurement has been based on subjective descriptions of how hot subjects feel for a given temperature and humidity. This results in a heat index that relates one combination of temperature and humidity to another.

Because the humidity index is based on temperatures in the shade, while people often move across sunny areas, then the heat index can give a much lower temperature than actual conditions of typical outdoor activities. Also, for people exercising or active, at the time, then the heat index could give a temperature lower than the felt conditions. For example, with a temperature in the shade of only 28 °C (82 °F) at 60% relative humidity, then the heat index would seem 29 °C (84 °F), but movement across sunny areas of 39 °C (102 °F), would give a heat index of over 58 °C (136 °F), as more indicative of the oppressive and sweltering heat. Plus when actively working, or not wearing a hat in sunny areas, then the feels-like conditions would seem even hotter. Hence, the heat index could seem unrealistically low, unless resting inactive (idle) in heavily shaded areas.


A humidistat is an electronic device analogous to a thermostat but which responds to relative humidity, not temperature. Humidistats are used in a number of devices including dehumidifiers, humidifiers, and microwave ovens. In humidifiers and dehumidifiers the humidistat is used where constant relative humidity conditions need to be maintained such as a refrigerator, greenhouse, or climate-controlled warehouse. When adjusting the controls in these applications the humidistat would be what is being set. In microwaves they are used in conjunction with "smart cooking" one-button features such as those for microwave popcorn. Humidistats employ hygrometers but are not the same. A humidistat has the functionality of a switch and is not just a measuring instrument like a hygrometer is.

For heating, ventilation, and air conditioning (HVAC) of buildings, humidistats or humidity sensors are used to sense the air relative humidity in the controlled space and turn on and off the HVAC equipment.


Humidity is the amount of water vapour present in air. Water vapour, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. The amount of water vapour 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 vapour contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 grams of water per cubic metre of air at 30 °C, but only 8 grams of water per cubic metre of air at 8 °C.

Three primary measurements of humidity are widely employed: absolute, relative and specific. Absolute humidity describes the water content of air and is expressed in either grams per cubic metre or grams per kilogram. Relative humidity, 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.


A hygrometer () is an instrument used to measure the amount of humidity and water vapor in the atmosphere, in soil, or in confined spaces. Humidity measurement instruments usually rely on measurements of some other quantity such as temperature, pressure, mass, a mechanical or electrical change in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can lead to a measurement of humidity. Modern electronic devices use temperature of condensation (called the dew point), or changes in electrical capacitance or resistance to measure humidity differences. The first crude hygrometer was invented by the Italian Renaissance polymath Leonardo da Vinci in 1480 and a more modern version was created by Swiss polymath Johann Heinrich Lambert in 1755.

The maximum amount of water vapor that can be held in a given volume of air (saturation) varies greatly by temperature; cold air can hold less mass of water per unit volume than hot air. Temperature can change humidity. Most instruments respond to (or are calibrated to read) relative humidity (RH), which is the amount of water relative to the maximum at a particular temperature expressed as per cent.

ISO 3977

ISO 3977 is an international standard related to the design and procurement of gas turbine system applications. ISO 3977 is based primarily on the ASME 133 series on gas turbines, as well as the API 616 and API 11PGT standards. The standard environmental design point of any gas turbine system is 15 °C, 60% relative humidity, and sea level elevation. The standard is divided into eight parts and covers procurement, design requirements, installation, and reliability.

Modified atmosphere/modified humidity packaging

Modified atmosphere/modified humidity (MA/MH) packaging is a technology used to preserve the quality of fresh produce so that it can be sold to markets far away from where it is grown, extend the marketing period, and to help suppliers reduce food waste within the cold chain. Commercial examples of MA/MH include sea freight of Galia and cantaloupe melons from Central and South America to Europe (a 21-day journey) and North America (a 7-day journey); transport of white asparagus from fields in Peru to markets in Western Europe (a 20-day journey by land and sea); and trucking of cherries from orchards in Turkey to supermarkets in the UK (a 7-day journey).


Psychrometrics, psychrometry, and hygrometry are names for the field of engineering concerned with the physical and thermodynamic properties of gas-vapor mixtures. The term comes from the Greek psuchron (ψυχρόν) meaning "cold" and metron (μέτρον) meaning "means of measurement".


Pyroglyphidae is a family of non-parasitic mites. It includes the house dust mite that live in human dwellings, many species that live in the burrows and nests of other animals, and some pests of dried products stored in humid conditions.

Storage of cultural heritage objects

The storage of cultural heritage objects typically falls to the responsibility of cultural heritage institutions, or individuals. The proper storage of these objects can help to ensure a longer lifespan for the object with minimal damage or degradation. With so many different types of artifacts, materials, and combinations of materials, keepers of these artifacts often have considerable knowledge of the best practices in storing these objects to preserve their original state.

Wet-bulb temperature

The wet-bulb temperature (WBT) is the temperature read by a thermometer covered in water-soaked cloth (wet-bulb thermometer) over which air is passed. At 100% relative humidity, the wet-bulb temperature is equal to the air temperature (dry-bulb temperature) and it is lower at lower humidity. It is defined as the temperature of a parcel of air cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat supplied by the parcel. A wet-bulb thermometer indicates a temperature close to the true (thermodynamic) wet-bulb temperature. The wet-bulb temperature is the lowest temperature that can be reached under current ambient conditions by the evaporation of water only.

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