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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. Humidity indicates the likelihood for precipitation, dew, or fog to be present.

Humidity depends on the temperature and pressure of the system of interest. The same amount of water vapor results in higher relative humidity in cool air than warm air. A related parameter is the dew point. 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, but only 8 g of water per cubic metre of air at 8 °C.

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 meter) or as mass of water vapor per mass of dry air (usually in grams per kilogram). Relative humidity, often expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature. Specific humidity is the ratio 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, or alternatively using a similar 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. There is a very small difference described under "Enhancement factor" below, which can be neglected in many calculations unless great accuracy is required.

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 per cubic metre when the air is saturated at 30 °C.

Absolute humidity is the mass of the water vapor $$ (m_{\text{H}_2\text{O}}) $$, divided by the volume of the air and water vapor mixture $$ (V_\text{net} )$$, which can be expressed as:

$$ AH = \frac{m_{\text{H}_2\text{O}}}{V_\text{net}}. $$If the volume is not set, the absolute humidity varies with changes in air temperature or pressure. Because of this, it is inappropriate for computations in chemical engineering, such as drying, where temperature variations might be significant. 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 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
Relative humidity is the ratio of how much water vapour is in the air to how much water vapour the air could potentially contain at a given temperature. It varies with the temperature of the air: colder air can contain less vapour, and water will tend to condense out of the air more at lower temperatures. 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 droplets are prone to total evaporation due to the heat.

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.

In a scientific notion, the relative humidity ($$RH$$ or $$\varphi$$) of an air-water mixture is defined as the ratio of the partial pressure of water vapor ($$p$$) in air to the saturation vapor pressure ($$p_s$$) of water at the same temperature, usually expressed as a percentage:

$$ \varphi = 100\% \cdot p/p_s$$

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 would feel like 83.6 ±.

Relative humidity is also a key metric used to evaluate when it is appropriate to install flooring over a concrete slab.

Specific humidity
Specific humidity (or moisture content) is the ratio of the mass of water vapor to the total mass of the air parcel. Specific humidity is approximately equal to the mixing ratio, which is defined as the ratio of the mass of water vapor in an air parcel to the mass of dry air for the same parcel. As temperature decreases, the amount of water vapor needed to reach saturation also decreases. As the temperature of a parcel of air becomes lower it will eventually reach the point of saturation without adding or losing water mass.

Related concepts
The term relative humidity is reserved for systems of water vapor in air. The term relative saturation is used to describe the analogous property for systems consisting of a condensable phase other than water in a non-condensable phase other than air.

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

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

$$ e^*_w = \left(1.0007 + 3.46 \times 10^{-6} P\right) \times 6.1121\, e^{17.502 T/(240.97 + T)},$$

where $$ T $$ is the dry-bulb temperature expressed in degrees Celsius (°C), $$ P $$ is the absolute pressure expressed in millibars, and $$ e^*_w $$ is the equilibrium vapor pressure expressed in millibars. Buck has reported that the maximal relative error is less than 0.20% between −20, and when this particular form of the generalized formula is used to estimate the equilibrium vapor pressure of water.

There are various devices used to measure and regulate humidity. Calibration standards for the most accurate measurement include the gravimetric hygrometer, chilled mirror hygrometer, and electrolytic hygrometer. The gravimetric method, while the most accurate, is very cumbersome. For fast and very accurate measurement the chilled mirror method is effective. For process on-line measurements, the most commonly used sensors nowadays are based on capacitance measurements to measure relative humidity, frequently with internal conversions to display absolute humidity as well. These are cheap, simple, generally accurate and relatively robust. All humidity sensors face problems in measuring dust-laden gas, such as exhaust streams from 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. 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. Therefore, when some number N of water molecules (vapor) is introduced into a volume of dry air, the number of air molecules in that volume must decrease by the same number N for the pressure to remain constant without using a change in temperature. The numbers are exactly equal if we consider the gases as ideal. 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.

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 $$ (f_w) $$ is defined as the ratio of the saturated vapor pressure of water in moist air $$ (e'_w) $$ to the saturated vapor pressure of pure water:

$$ f_W = \frac{e'_w}{e^*_w}. $$

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.

Effects
Climate control refers to the control of temperature and relative humidity in buildings, vehicles and other enclosed spaces for the purpose of providing for human comfort, health and safety, and of meeting environmental requirements of machines, sensitive materials (for example, historic) and technical processes.

Climate
While humidity itself is a climate variable, it also affects other climate variables. Environmental humidity is affected by winds and by rainfall.

The most humid cities on Earth are generally located closer to the equator, near coastal regions. Cities in parts of Asia and Oceania are among the most humid. Bangkok, 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 in the monsoon season.

High temperatures combine with the high dew point to create heat index in excess of 65 C. 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.

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. Water vapor is thus a "slave" to the non-condensible greenhouse gases.

Animal and plant life
Humidity is one of the fundamental abiotic factors that defines any habitat (the tundra, wetlands, and the desert are a few examples), and is a determinant of which animals and plants can thrive in a given environment.

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

Domesticated plants and animals (e.g. lizards) require regular upkeep of humidity percents when grown in-home and container conditions, for optimal thriving environment. Steps towards plant humidity include: Humidity trays (pebble trays), Humidity dome, Humidifier, manual daily misting, greenhouses (or indoor propagation box), or growing your plants in a naturally humid room (like the bathroom).

Human comfort
Although humidity is an important factor for thermal comfort, humans are more sensitive to variations in temperature than they are to changes in relative humidity. Humidity has a small effect on thermal comfort outdoors when air temperatures are low, a slightly more pronounced effect at moderate air temperatures, and a much stronger influence at higher air temperatures.

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

Humans can be comfortable within a wide range of humidities depending on the temperature—from 30 to 70%—but ideally not above the Absolute (60 °F Dew Point), between 40   % and 60    %. In general, higher temperatures will require lower humidities to achieve thermal comfort compared to lower temperatures, with all other factors held constant. For example, with clothing level = 1, metabolic rate = 1.1, and air speed 0.1 m/s, a change in air temperature and mean radiant temperature from 20 °C to 24 °C would lower the maximum acceptable relative humidity from 100% to 65% to maintain thermal comfort conditions. The CBE Thermal Comfort Tool can be used to demonstrate the effect of relative humidity for specific thermal comfort conditions and it can be used to demonstrate compliance with ASHRAE Standard 55–2017.

Some people experience difficulty breathing in humid environments. Some cases may possibly be related to respiratory conditions such as asthma, while others may be the product of anxiety. Affected people will often hyperventilate in response, causing sensations of numbness, faintness, and loss of concentration, among others.

Very low humidity can create discomfort, respiratory problems, and aggravate allergies in some individuals. Low humidity causes tissue lining nasal passages to dry, crack and become more susceptible to penetration of rhinovirus cold viruses. Extremely low (below 20   %) relative humidities may also cause eye irritation. The use of a humidifier in homes, especially bedrooms, can help with these symptoms. Indoor relative humidities should be kept above 30% to reduce the likelihood of the occupant's nasal passages drying out, especially in winter.

Air conditioning reduces discomfort by reducing not just temperature but humidity as well. Heating cold outdoor air can decrease relative humidity levels indoors to below 30%. According to ASHRAE Standard 55-2017: Thermal Environmental Conditions for Human Occupancy, indoor thermal comfort can be achieved through the PMV method with relative humidities ranging from 0% to 100%, depending on the levels of the other factors contributing to thermal comfort. However, the recommended range of indoor relative humidity in air conditioned buildings is generally 30–60%.

Human health
Higher humidity reduces the infectivity of aerosolized influenza virus. A study concluded, "Maintaining indoor relative humidity >40% will significantly reduce the infectivity of aerosolized virus."

Excess moisture in buildings expose occupants to fungal spores, cell fragments, or mycotoxins. Infants in homes with mold have a much greater risk of developing asthma and allergic rhinitis. More than half of adult workers in moldy/humid buildings develop nasal or sinus symptoms due to mold exposure.

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.

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.

Building construction
Common construction methods often produce building enclosures with a poor thermal boundary, requiring an insulation and air barrier system designed to retain indoor environmental conditions while resisting external environmental conditions. The energy-efficient, heavily sealed architecture introduced in the 20th century also sealed off the movement of moisture, and this has resulted in a secondary problem of condensation 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   %, 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

Electronic devices are often rated to operate only under certain humidity conditions (e.g., 10% to 90%). The optimal humidity for electronic devices is 30% to 65%. At the top end of the range, moisture may increase the conductivity of permeable 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).

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.

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.

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 compared to no more than about 0.5% by mass at 0 C. 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-change increase in temperature. Thus, the relative humidity will drop by a factor of 2 for each 20 F-change increase in temperature, assuming conservation of absolute moisture. For example, in the range of normal temperatures, air at 68 F and 50% relative humidity will become saturated if cooled to 50 F, its dew point, and 41 F air at 80% relative humidity warmed to 68 F 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 though no lower humidity limit is established.

Water vapor is a lighter gas than other gaseous components of air at the same temperature, so humid air will tend to rise by natural convection. 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.

General sources

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

title: Humidity


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Dialectic

Dialectic (διαλεκτική, dialektikḗ; Dialektik), also known as the dialectical method, refers originally to dialogue between people holding different points of view about a subject but wishing to arrive at the truth through reasoned argumentation. Dialectic resembles debate, but the concept excludes subjective elements such as emotional appeal and rhetoric. It has its origins in ancient philosophy and continued to be developed in the Middle Ages.

Hegelianism refigured "dialectic" to no longer refer to a literal dialogue. Instead, the term takes on the specialized meaning of development by way of overcoming internal contradictions. Dialectical materialism, a theory advanced by Karl Marx and Friedrich Engels, adapted the Hegelian dialectic into a materialist theory of history. The legacy of Hegelian and Marxian dialectics has been criticized by philosophers such as Karl Popper and Mario Bunge, who considered it unscientific.

Dialectic implies a developmental process and so does not naturally fit within classical logic. Nevertheless, some twentieth-century logicians have attempted to formalize it.

History
There are a variety of meanings of dialectic or dialectics within Western philosophy.

Classical philosophy
In classical philosophy, dialectic (διαλεκτική) is a form of reasoning based upon dialogue of arguments and counter-arguments, advocating propositions (theses) and counter-propositions (antitheses). The outcome of such a dialectic might be the refutation of a relevant proposition, or of a synthesis, or a combination of the opposing assertions, or a qualitative improvement of the dialogue.

The term "dialectic" owes much of its prestige to its role in the philosophies of Socrates and Plato, in the Greek Classical period (5th to 4th centuries BC). Aristotle said that it was the pre-Socratic philosopher Zeno of Elea who invented dialectic, of which the dialogues of Plato are examples of the Socratic dialectical method.

Socratic method
The Socratic dialogues are a particular form of dialectic known as the method of elenchus (literally, "refutation, scrutiny" ) whereby a series of questions clarifies a more precise statement of a vague belief, logical consequences of that statement are explored, and a contradiction is discovered. The method is largely destructive, in that false belief is exposed and only constructive in that this exposure may lead to further search for truth. The detection of error does not amount to a proof of the antithesis. For example, a contradiction in the consequences of a definition of piety does not provide a correct definition. The principal aim of Socratic activity may be to improve the soul of the interlocutors, by freeing them from unrecognized errors, or indeed, by teaching them the spirit of inquiry.

In common cases, Socrates uses enthymemes as the foundation of his argument.

For example, in the Euthyphro, Socrates asks Euthyphro to provide a definition of piety. Euthyphro replies that the pious is that which is loved by the gods. But, Socrates also has Euthyphro agreeing that the gods are quarrelsome and their quarrels, like human quarrels, concern objects of love or hatred. Therefore, Socrates reasons, at least one thing exists that certain gods love but other gods hate. Again, Euthyphro agrees. Socrates concludes that if Euthyphro's definition of piety is acceptable, then there must exist at least one thing that is both pious and impious (as it is both loved and hated by the gods)—which Euthyphro admits is absurd. Thus, Euthyphro is brought to a realization by this dialectical method that his definition of piety is not sufficiently meaningful.

In another example, in Plato's Gorgias, dialectic occurs between Socrates, the Sophist Gorgias, and two men, Polus and Callicles. Because Socrates' ultimate goal was to reach true knowledge, he was even willing to change his own views in order to arrive at the truth. The fundamental goal of dialectic, in this instance, was to establish a precise definition of the subject (in this case, rhetoric) and with the use of argumentation and questioning, make the subject even more precise. In the Gorgias, Socrates reaches the truth by asking a series of questions and in return, receiving short, clear answers.

Plato
In Platonism and Neoplatonism, dialectic assumed an ontological and metaphysical role in that it became the process whereby the intellect passes from sensibles to intelligibles, rising from idea to idea until it finally grasps the supreme idea, the first principle which is the origin of all. The philosopher is consequently a "dialectician". In this sense, dialectic is a process of inquiry that does away with hypotheses up to the first principle. It slowly embraces multiplicity in unity. The philosopher Simon Blackburn wrote that the dialectic in this sense is used to understand "the total process of enlightenment, whereby the philosopher is educated so as to achieve knowledge of the supreme good, the Form of the Good".

Medieval philosophy
Logic, which could be considered to include dialectic, was one of the three liberal arts taught in medieval universities as part of the trivium; the other elements were rhetoric and grammar.

Based mainly on Aristotle, the first medieval philosopher to work on dialectics was Boethius (480–524). After him, many scholastic philosophers also made use of dialectics in their works, such as Abelard, William of Sherwood, Garlandus Compotista, Walter Burley, Roger Swyneshed, William of Ockham, and Thomas Aquinas.

This dialectic (a quaestio disputata) was formed as follows:
 * 1) The question to be determined ("It is asked whether...");
 * 2) A provisory answer to the question ("And it seems that...");
 * 3) The principal arguments in favor of the provisory answer;
 * 4) An argument against the provisory answer, traditionally a single argument from authority ("On the contrary...");
 * 5) The determination of the question after weighing the evidence ("I answer that...");
 * 6) The replies to each of the initial objections. ("To the first, to the second etc., I answer that...")

Modern philosophy
The concept of dialectics was given new life at the start of the 19th century by Georg Wilhelm Friedrich Hegel, whose dialectical model of nature and of history made dialectics a fundamental aspect of reality, instead of regarding the contradictions into which dialectics leads as evidence of the limits of pure reason, as Immanuel Kant had argued. Hegel was influenced by Johann Gottlieb Fichte's conception of synthesis, although Hegel didn't adopt Fichte's "thesis–antithesis–synthesis" language except to describe Kant's philosophy: rather, Hegel argued that such language was "a lifeless schema" imposed on various contents, whereas he saw his own dialectic as flowing out of "the inner life and self-movement" of the content itself.

In the mid-19th century, Hegelian dialectic was appropriated by Karl Marx and Friedrich Engels and retooled in what they considered to be a nonidealistic manner. It would also become a crucial part of later representations of Marxism as a philosophy of dialectical materialism. These representations often contrasted dramatically and led to vigorous debate among different Marxist groups.

Hegelian dialectic
The Hegelian dialectic describes changes in the forms of thought through their own internal contradictions into concrete forms that overcome previous oppositions.

This dialectic is sometimes presented in a threefold manner, as first stated by Heinrich Moritz Chalybäus, as comprising three dialectical stages of development: a thesis, giving rise to its reaction; an antithesis, which contradicts or negates the thesis; and the tension between the two being resolved by means of a synthesis.

By contrast, the terms abstract, negative, and concrete suggest a flaw or an incompleteness in any initial thesis. For Hegel, the concrete must always pass through the phase of the negative, that is, mediation. This is the essence of what is popularly called Hegelian dialectics.

To describe the activity of overcoming the negative, Hegel often used the term Aufhebung, variously translated into English as "sublation" or "overcoming", to conceive of the working of the dialectic. Roughly, the term indicates preserving the true portion of an idea, thing, society, and so forth, while moving beyond its limitations. What is sublated, on the one hand, is overcome, but, on the other hand, is preserved and maintained.

As in the Socratic dialectic, Hegel claimed to proceed by making implicit contradictions explicit: each stage of the process is the product of contradictions inherent or implicit in the preceding stage. On his view, the purpose of dialectics is "to study things in their own being and movement and thus to demonstrate the finitude of the partial categories of understanding".

For Hegel, even history can be reconstructed as a unified dialectic, the major stages of which chart a progression from self-alienation as servitude to self-unification and realization as the rational constitutional state of free and equal citizens.

Marxist dialectic
Marxist dialectic is a form of Hegelian dialectic which applies to the study of historical materialism. Marxist dialectic is thus a method by which one can examine social and economic behaviors. It is the foundation of the philosophy of dialectical materialism, which forms the basis of historical materialism.

In the Marxist tradition, "dialectic" refers to regular and mutual relationships, interactions, and processes in nature, society, and human thought.

A dialectical relationship is a relationship in which two phenomena or ideas mutually impact each other, leading to development and negation. Development refers to the change and motion of phenomena and ideas from less advanced to more advanced or from less complete to more complete. Dialectical negation refers to a stage of development in which a contradiction between two previous subjects gives rise to a new subject. In the Marxist view, dialectical negation is never an endpoint, but instead creates new conditions for further development and negation.

Karl Marx and Friedrich Engels, writing several decades after Hegel's death, proposed that Hegel's dialectic is too abstract. Against this, Marx presented his own dialectic method, which he claimed to be "direct opposite" of Hegel's method.

Marxist dialectics is exemplified in Das Kapital. As Marx explained dialectical materialism,

"it includes in its comprehension an affirmative recognition of the existing state of things, at the same time, also, the recognition of the negation of that state, of its inevitable breaking up; because it regards every historically developed social form as in fluid movement, and therefore takes into account its transient nature not less than its momentary existence; because it lets nothing impose upon it, and is in its essence critical and revolutionary."

Class struggle is the primary contradiction to be resolved by Marxist dialectics because of its central role in the social and political lives of a society. Nonetheless, Marx and Marxists developed the concept of class struggle to comprehend the dialectical contradictions between mental and manual labor and between town and country. Hence, philosophic contradiction is central to the development of dialectics: the progress from quantity to quality, the acceleration of gradual social change; the negation of the initial development of the status quo; the negation of that negation; and the high-level recurrence of features of the original status quo.

Friedrich Engels further proposed that nature itself is dialectical, and that this is "a very simple process, which is taking place everywhere and every day".

In Marxism, the dialectical method of historical study is intertwined with historical materialism, the school of thought exemplified by the works of Marx, Engels, and Vladimir Lenin.

For Lenin, the primary feature of Marx's "dialectical materialism" (Lenin's term) is its application of materialist philosophy to history and social sciences. Lenin's main contribution to the philosophy of dialectical materialism is his theory of reflection, which presents human consciousness as a dynamic reflection of the objective material world that fully shapes its contents and structure.

Later, Stalin's works on the subject established a rigid and formalistic division of Marxist–Leninist theory into dialectical materialism and historical materialism. While the first was supposed to be the key method and theory of the philosophy of nature, the second was the Soviet version of the philosophy of history.

Dialectical naturalism
Dialectical naturalism is a term coined by American philosopher Murray Bookchin to describe the philosophical underpinnings of the political program of social ecology. Dialectical naturalism explores the complex interrelationship between social problems, and the direct consequences they have on the ecological impact of human society. Bookchin offered dialectical naturalism as a contrast to what he saw as the "empyrean, basically antinaturalistic dialectical idealism" of Hegel, and "the wooden, often scientistic dialectical materialism of orthodox Marxists".

Theological dialectics
Neo-orthodoxy, in Europe also known as theology of crisis and dialectical theology, is an approach to theology in Protestantism that was developed in the aftermath of the First World War (1914–1918). It is characterized as a reaction against doctrines of 19th-century liberal theology and a more positive reevaluation of the teachings of the Reformation, much of which had been in decline (especially in western Europe) since the late 18th century. It is primarily associated with two Swiss professors and pastors, Karl Barth (1886–1968) and Emil Brunner (1899–1966), even though Barth himself expressed his unease in the use of the term.

In dialectical theology the difference and opposition between God and human beings is stressed in such a way that all human attempts at overcoming this opposition through moral, religious or philosophical idealism must be characterized as 'sin'. In the death of Christ humanity is negated and overcome, but this judgment also points forwards to the resurrection in which humanity is reestablished in Christ. For Barth this meant that only through God's 'no' to everything human can his 'yes' be perceived. Applied to traditional themes of Protestant theology, such as double predestination, this means that election and reprobation cannot be viewed as a quantitative limitation of God's action. Rather it must be seen as its "qualitative definition". As Christ bore the rejection as well as the election of God for all humanity, every person is subject to both aspects of God's double predestination.

Dialectic prominently figured in Bernard Lonergan's philosophy, in his books Insight and Method in Theology. Michael Shute wrote about Lonergan's use of dialectic in The Origins of Lonergan's Notion of the Dialectic of History. For Lonergan, dialectic is both individual and operative in community. Simply described, it is a dynamic process that results in something new:

"For the sake of greater precision, let us say that a dialectic is a concrete unfolding of linked but opposed principles of change. Thus there will be a dialectic if (1) there is an aggregate of events of a determinate character, (2) the events may be traced to either or both of two principles, (3) the principles are opposed yet bound together, and (4) they are modified by the changes that successively result from them."

Dialectic is one of the eight functional specialties Lonergan envisaged for theology to bring this discipline into the modern world. Lonergan believed that the lack of an agreed method among scholars had inhibited substantive agreement from being reached and progress from being made compared to the natural sciences. Karl Rahner, S.J., however, criticized Lonergan's theological method in a short article entitled "Some Critical Thoughts on 'Functional Specialties in Theology'" where he stated: "Lonergan's theological methodology seems to me to be so generic that it really fits every science, and hence is not the methodology of theology as such, but only a very general methodology of science."

Criticisms
Friedrich Nietzsche viewed dialectic as a method that imposes artificial boundaries and suppresses the richness and diversity of reality. He rejected the notion that truth can be fully grasped through dialectical reasoning and offered a critique of dialectic, challenging its traditional framework and emphasizing the limitations of its approach to understanding reality. He expressed skepticism towards its methodology and implications in his work Twilight of the Idols: "I mistrust all systematizers and I avoid them. The will to a system is a lack of integrity". In the same book, Nietzsche criticized Socrates' dialectics because he believed it prioritized reason over instinct, resulting in the suppression of individual passions and the imposition of an artificial morality.

Karl Popper attacked the dialectic repeatedly. In 1937, he wrote and delivered a paper entitled "What Is Dialectic?" in which he criticized the dialectics of Hegel, Marx, and Engels for their willingness "to put up with contradictions". He argued that accepting contradiction as a valid form of logic would lead to the principle of explosion and thus trivialism. Popper concluded the essay with these words: "The whole development of dialectic should be a warning against the dangers inherent in philosophical system-building. It should remind us that philosophy should not be made a basis for any sort of scientific system and that philosophers should be much more modest in their claims. One task which they can fulfill quite usefully is the study of the critical methods of science". Seventy years later, Nicholas Rescher responded that "Popper's critique touches only a hyperbolic version of dialectic", and he quipped: "Ironically, there is something decidedly dialectical about Popper's critique of dialectics."

The philosopher of science and physicist Mario Bunge repeatedly criticized Hegelian and Marxian dialectics, calling them "fuzzy and remote from science" and a "disastrous legacy". He concluded: "The so-called laws of dialectics, such as formulated by Engels (1940, 1954) and Lenin (1947, 1981), are false insofar as they are intelligible." Poe Yu-ze Wan, reviewing Bunge's criticisms of dialectics, found Bunge's arguments to be important and sensible, but he thought that dialectics could still serve some heuristic purposes for scientists.

Even some Marxists are critical of the term "dialectics". For instance, Michael Heinrich wrote, "More often than not, the grandiose rhetoric about dialectics is reducible to the simple fact that everything is dependent upon everything else and is in a state of interaction and that it's all rather complicated—which is true in most cases, but doesn't really say anything."

Mathematics
Mathematician William Lawvere interpreted dialectics in the setting of categorical logic in terms of adjunctions between idempotent monads. This perspective may be useful in the context of theoretical computer science where the duality between syntax and semantics can be interpreted as a dialectic in this sense. For example, the Curry-Howard equivalence is such an adjunction or more generally the duality between closed monoidal categories and their internal logic.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Dialectic


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Sierra Entertainment

Sierra Entertainment, Inc. (formerly On-Line Systems and Sierra On-Line, Inc.) was an American video game developer and publisher founded in 1979 by Ken and Roberta Williams. The company is known for pioneering the graphic adventure game genre, including the first such game, Mystery House. It is known for its graphical adventure game series King's Quest, Space Quest, Police Quest, Gabriel Knight, Leisure Suit Larry, and Quest for Glory, and as the original publisher of Valve's Half-Life series.

After seventeen years as an independent company, Sierra was acquired by CUC International in February 1996 to become part of CUC Software. However, CUC International was caught in an accounting scandal in 1998, and many of the original founders of Sierra including the Williamses left the company. Sierra remained as part of CUC Software as it was sold and renamed several times over the next few years. Sierra was formally disestablished as a company and reformed as a division of this group in August 2004. The former CUC Software group was acquired by Vivendi and branded as Vivendi Games in 2006. The Sierra division continued to operate through Vivendi Games's merger with Activision to form Activision Blizzard on July 10, 2008, but was shut down later that year. The Sierra brand was revived by Activision in 2014 to re-release former Sierra games and some independently developed games.

Currently, the Sierra brand is under Microsoft's ownership through its gaming division, following the acquisition of Activision Blizzard.

Founding as On-Line Systems (1979–1982)

Sierra Entertainment was founded in 1979 as On-Line Systems in Simi Valley, California, by the husband-and-wife duo Ken and Roberta Williams. Ken, a programmer for IBM, had planned to use the company to create business software for the TRS-80 and Apple II. Ken had brought a teletype terminal home one day in 1979 and, while looking through the host system's software catalog, discovered the text adventure Colossal Cave Adventure. He encouraged Roberta to join him in playing it, and she was enthralled by the game. After Ken had brought an Apple II to their home, she played through other text adventures such as those by Scott Adams and Softape to study them. Dissatisfied with the text-only format, she realized that the graphics display capability of the Apple II could enhance the adventure gaming experience. With Ken's help in some of the programming, Roberta designed Mystery House, inspired by the novel And Then There Were None and the board game Clue, using text commands and printout combined with rudimentary graphics depicting the current setting.

Mystery House was released through mail-order in May 1980. It was an instant hit with about 15,000 copies sold, earning US$167,000 1980. It is the first computer adventure game to have graphics, although made with crude, static, monochrome, line drawings. The two shifted the focus to developing more graphical adventure games. Mystery House became the first in the Hi-Res Adventure series. The Hi-Res Adventure series continued with Mission Asteroid, which was released as Hi-Res Adventure #0 though being the second release. The next release, Wizard and the Princess, also known as Adventure in Serenia, is considered a prelude to the later King's Quest series in both story and concept. Through 1981 and 1982, more games were released in the series including Cranston Manor, Ulysses and the Golden Fleece, Time Zone, and The Dark Crystal. A simplified version of The Dark Crystal, intended for a younger audience, was written by Al Lowe and released as Gelfling Adventure.

Rebranding to Sierra On-Line (1982–1988)

On-Line Systems was renamed Sierra On-Line in 1982, and moved to Oakhurst, California. The "Sierra" name was taken from the Sierra Nevada mountain range that Oakhurst was near, and its new logo incorporated the imagery of Half Dome mountain. By early 1984, InfoWorld estimated that Sierra was the world's 12th-largest microcomputer software company, with US$12.5 million in 1983 sales.

The company weathered the video game crash of 1983 with only a 20% increase in sales, after analysts in 1982 had predicted a doubling in 1983 of the entire software market. The company had spent much of 1983 developing for a Commodore machine and the TI-99/4A which were both obsolete by the end of the year. Ken Williams was reportedly described as "bewildered by the pace at which computers come into and fall out of favor", and Williams said, "I've learned my lesson. I'm not moving until I understand the market better."

Many of Sierra's best known series began in the 1980s. In 1983, Sierra On-Line was contacted by IBM to create a game for the new PCjr. IBM offered to fund the entire development and marketing of the game, paying royalties. Ken and Roberta Williams accepted and started on the project. Roberta Williams created a story featuring classic fairy-tale elements. Her game concept includes animated color graphics, a pseudo 3D-perspective where the main character is visible on the screen, a more competent text parser that understands advanced commands from the player, and music playing in the background through the PCjr sound hardware. For the game, a complete development system called Adventure Game Interpreter (AGI) was developed. In mid-1984, King's Quest: Quest for the Crown was released to much acclaim, beginning the King's Quest series.

While finishing The Black Cauldron, programmers Mark Crowe and Scott Murphy began to plan for an adventure game of their own. After a simple demonstration to Ken Williams, he allowed them to start working on the full game, which was named Space Quest: The Sarien Encounter. The game was released in October 1986 as an instant success, spawning many sequels in the Space Quest series in the following years.

Al Lowe, who had been working at Sierra On-Line for many years, was asked by Ken Williams to write a modern version of Chuck Benton's Softporn Adventure from 1981, the only pure text adventure that the company had ever released. Leisure Suit Larry in the Land of the Lounge Lizards was a great hit and won the Software Publishers Association's Best Adventure Game award of 1987. It can be deduced that the game first became famous as an early example of software piracy, as Sierra sold many more hintbooks than actual copies of the game. A series of Leisure Suit Larry games followed.

Ken Williams befriended a retired highway patrol officer named Jim Walls and asked him to produce an adventure series based on a police theme. Walls proceeded to create Police Quest: In Pursuit of the Death Angel, which was released in 1987. Several sequels followed, and the series was touted for adherence to police protocol (relevant parts of which were explained in the games' manuals), and presenting some real-life situations encountered by Walls during his career as an officer.

Quest for Glory is a series of hybrid adventure/role-playing video games designed by Corey and Lori Ann Cole. The first game in the series, Quest for Glory: So You Want to Be a Hero, was released in 1989. The series combines humor, puzzle elements, themes and characters borrowed from various legends, puns, and memorable characters, creating a five-part series of the Sierra stable. Although the series was originally titled Hero's Quest, Sierra failed to trademark the name. Milton Bradley successfully trademarked an electronic version of their unrelated joint Games Workshop board game, HeroQuest, which forced Sierra to change the series' title to Quest for Glory. This decision caused all future games in the series, newer releases of Hero's Quest I, to switch to the new name.

In 1987, Sierra On-Line started to publish its own gaming magazine, about its upcoming games and interviews with the developers. The magazine was initially named The Sierra Newsletter, The Sierra News Magazine, and The Sierra/Dynamix Newsmagazine. However, since Sierra Club already published a magazine called Sierra Magazine, the name of the magazine published by Sierra On-Line was changed to InterAction in 1991. It was discontinued in 1999.

Sierra's Adventure Game Interpreter engine, introduced with King's Quest, was replaced in 1988 with Sierra's Creative Interpreter in King's Quest IV. The game was released under both engines, so those who had newer computers could use the new engine and better rendering technology.

Going public (1989–1995)

Sierra became a public company in 1989, trading on the NASDAQ under the stock ticker "SIER". Additional public investment allowed the company to engage in further acquisitions over the next several years.

In 1990, Sierra released King's Quest V, the first Sierra On-Line game ever to have more than 500,000 copies sold and the highest selling game for five years. It won several awards, such as the Best Adventure Game of the Year from both the Software Publishers Association and Computer Gaming World magazine.

The Sierra Network was launched on May 6, 1991, as the first game-only online environment. Development of the network began in 1989, as Ken Williams was inspired by the launch of the Prodigy service in 1988 to create something similar for Sierra's games. As a free service other than access use charges, the network provided a "land-based" precursor to MMORPGs and internet chat rooms, each land theme for the type of content provided multi-player gaming and category based bulletin boards and chat rooms throughout the continental United States. By July 1993, having reached about 40,000 subscribers, AT&T announced a plan to invest US$5 million into the network and add more games, gaining partial control as part of its expansion into the growing online services. AT&T later took sole possession of the network on November 15, 1994, so the name was changed to the ImagiNation Network. The network failed to find a mass audience, and was sold to America On-Line in 1996.

In 1991, Sierra released the first game in the Dr. Brain series, Castle of Dr. Brain, a hybrid puzzle adventure education game, which has several sequels. In 1993, Gabriel Knight: Sins of the Fathers was released, beginning the Gabriel Knight series. The game and subsequent sequels were critically acclaimed in the mainstream press at the time.

Sierra and Broderbund started merger discussions in March 1991, but the idea was discontinued later that month.

Sierra needed a new building due to growth, and moved its headquarters and much of its key staff to Bellevue, Washington. Sierra's original location in Oakhurst was later renamed Yosemite Entertainment, and continued under that name until closing in early 1999.

The company was now made up of five separate and largely autonomous development divisions: Sierra Publishing, Sierra Northwest, Dynamix, Bright Star Technology, and Coktel Vision, with each group working separately on product development but sharing manufacturing, distribution, and sales resources.

1995 was a successful year for the company. Sierra was the market-share leader in PC games for the year. With $83.4 million in sales from software-publishing, earnings improved by 19 percent, and a net income of $11.9 million. In June, Sierra and Pioneer Electric Corp. signed an agreement to create a joint venture that would develop, publish, manufacture, and market entertainment software for the Japanese software market. This joint venture created a new company called Sierra Venture. With Sierra and Pioneer investing over $12 million, the new company immediately manufactured and shipped over twenty of Sierra's most popular products to Japan and created new titles for the Japanese market. 1995 also saw Sierra acquiring a number of development companies, both small home developers and larger companies.

Phantasmagoria was by far the largest project ever undertaken by Sierra. The anticipation for the game was high at release in late 1995. Although nearly one million copies were sold when the game was first released in August 1995, Sierra's bestselling adventure game created, the game received mixed reviews from industry critics.

Sale to CUC International (1996–1998)
In February 1996, early e-commerce pioneer CUC International, seeking to expand into interactive entertainment, offered to buy Sierra at a price of about US$1.5 billion. Walter Forbes, the CEO of CUC International, and a member of Sierra's own Board of Directors since 1991, surprised Ken Williams with the deal after a board meeting. At this time, Sierra had modest revenues of about US$158 million in the current fiscal year, so the sum surprised Ken. Forbes had posited the idea to Ken that this would be the start of a large company eventually to bring in LucasArts Entertainment, Broderbund, and Davidson & Associates (which at the time owned Blizzard Entertainment) under one entity and be a major publisher in the video game industry, as a great boon to the Williams and to Sierra's shareholders. Roberta had expressed her concerns about the offer to Ken and to executive officers, but he remained interested in the potential that Forbes offered. Ken accepted the offer, believing it was in the best interest for Sierra's future and stockholders, and CUC announced by the end of February 1996 the beginning of closing the acquisition of both Sierra and Davidson for US$1.06 billion and US$1.14 billion in CUC stock, respectively. The deal to obtain LucasArts and Broderbund failed.

Sierra's acquisition closed on July 24, 1996. The terms included naming Ken Williams a vice-chairman of CUC International, a Member of the Office of the President of CUC, and that he would remain responsible for Sierra's R&D and remain Sierra's CEO. He also requested creation of a "software board" consisting of him, Michael Brochu (Sierra's President and COO), Bob Davidson (founder and CEO of Davidson & Associates), and Forbes. It functioned as a governing body of what would become CUC Software, regulating major decisions and product lines.

In September 1996, CUC announced consolidation of some functions of its game companies into a single company called CUC Software Inc., headquartered in Torrance, California. Bob Davidson, founder and CEO of Davidson & Associates became the CEO for the publishing body. CUC Software consolidated manufacturing, distribution, and sales resources of all of divisions including what was to become Sierra, Davidson, Blizzard, Knowledge Adventure, and Gryphon Software.

CUC Software utilized its various labels' market specialties. For example, in October 1996, Sierra published Stay Tooned!, an adventure game developed by Funnybone Interactive (a subsidiary of Davidson & Associates) as Sierra was more known as an adventure game publisher than Davidson.

In November 1996, Ken Williams met with the founders of Valve and negotiated Sierra's exclusive rights to publish Half-Life, which Ken Williams debuted at E3 in May 1997. In December 1996, Sierra released The Realm Online, an online fantasy role-playing game.

After the sale, Ken Williams remained within the software division so that he could provide strategic guidance to Sierra, although he began to grow disillusioned as he soon found that his new titles at CUC meant very little and the software board met only once. He began disputes with Davidson over Davidson's conservative management style and his disdain for Sierra's more risque product lines such as Phantasmagoria and Leisure Suit Larry.

In January 1997, Davidson stepped down as CEO of CUC Software, and CUC Executive Chris MacLeod was named as his replacement. After this, Ken Williams shifted his focus work on CUC's online product distributor, NetMarket while remaining as CEO of Sierra in name only. In November, Ken Williams departed from CUC International, while Roberta Williams remained with Sierra until the release of King's Quest: Mask of Eternity in December 1998. Brochu, who had been hired in 1995 by Ken Williams, to handle the daily business affairs of Sierra, replaced Ken Williams and remained as President of Sierra until October 1997, when he too departed the company.

In April 1997, to further expand upon their role in the edutainment business, Sierra purchased Books That Work and CUC International purchased Berkeley Systems and transferred management of the studio to Sierra as an internal developer. In December 1997, in order to secure the rights to Return to Krondor, Sierra purchased PyroTechnix, who were developing the game.

On November 5, 1997, after the departure of Brochu in October, Sierra was split into three business units, all of which reported directly to MacLeod.

In 1998, Sierra divided into 5 sub-brands and corporate divisions:
 * Sierra Attractions (for casual games such as poker) - composed of Berkeley Systems
 * Sierra Home (for home/lifestyle software) - composed of Sierra's gardening, home design, and cooking software divisions
 * Sierra Sports (For sports games) - composed of Dynamix's Sports Titles, Synergistic Software, and Papyrus
 * Sierra Studios (general publishing division) - Composed of Sierra Northwest/Bellevue, Pyrotechnix, and Impressions Software
 * Sierra FX (adventure games and online multiplayer games) - Based at Sierra's old headquarters in Oakhurst, which was publicly referred to as Yosemite Entertainment.

On November 24, 1997, Sierra published Diablo: Hellfire, the official expansion pack for the widely popular game Diablo developed by Synergistic Software, a division of Sierra.

On November 19, 1998, Sierra published Half-Life for the PC, developed by Valve, which became a huge success.

During these events, CUC merged with Henry Silverman's HFS Incorporated in December 1997 and became the Cendant Corporation. The merger did not immediately affect operations of Sierra. However, Silverman, who served as CEO of Cendant, had become more involved with the bookkeeping of the merged companies and noticed irregularities from CUC's past bookkeeping, leading to the discovery of massive accounting fraud at CUC in March 1998. Forbes was later convicted on three charges related to fraud by the Security and Exchange Commission in 2007. With the news, Cendant announced intention to sell the computer entertainment division, and on November 20, 1998, announced the sale of the consumer software division to Paris-based Havas S.A. Sierra became a part of Havas Interactive, the interactive entertainment division of the company.

Fallout from CUC's acquisition (1999–2003)
On February 22, 1999, Sierra announced a major reorganization of the company, resulting in the shutdown of several of their development studios, cutbacks on others and the relocation of key projects, and employees from those studios, to Bellevue. About 250 people in total lost their jobs. Development groups within Sierra such as PyroTechnix were shut down. Others such as Books That Work were relocated to Bellevue. Also shut down was Yosemite Entertainment, the division occupying the original headquarters of Sierra On-Line. The company sold the rights of Headgate Studios back to the original owner. With the exception of the warehouse and distribution department, the entire studio was shut down. Game designers Al Lowe and Scott Murphy were laid off. Lowe had just started work on Leisure Suit Larry 8. Murphy was involved in a Space Quest 7 project at the time. Layoffs continued on March 1, when Sierra terminated 30 employees at the previously unaffected Dynamix, or 15 percent of their workforce.

Despite the layoffs, Sierra continued to publish games for smaller development houses. In September 1999, they released Homeworld, a real-time space-combat strategy game developed by Relic Entertainment. The game design was revolutionary for the genre, and the game received great critical acclaim and many awards.

UK-based game developer and publisher Codemasters, in an effort to establish themselves in the United States, announced the launch of a new development studio in Oakhurst, using the abandoned Sierra facilities and hiring much of the Yosemite Entertainment's laid-off staff in mid-September 1999. In early October, the company announced plans to take over management and maintenance of the online RPG The Realm and acquiring the complete yet previously canceled Navy SEALs. The company also reported they obtained the rights to continue using the name Yosemite Entertainment for the development house.

Meanwhile, Sierra announced another reorganization, this time into three business units: Core Games, Casual Entertainment, and Home Productivity. This reorganization resulted in even more layoffs, eliminating 105 additional jobs and a number of games in production. After 1999, Sierra almost entirely ceased to be a developer of games and, as time went on, instead became a publisher of games by independent developers.

At the end of June 2000, a strategic business alliance between Vivendi, Seagram, and Canal+ was announced, and Vivendi Universal, a leading global media and communications company, was formed after the merger with Seagram (the parent company of Universal Studios). Havas S.A. was renamed Vivendi Universal Publishing and became the publishing division of the new group, divided into five groups: games, education, literature, health, and information. The merger was followed by many more layoffs of Sierra employees.

In August 2001, Sierra announced a major reorganization, which included the closure of Dynamix as well as the layoffs of 148 employees located at the company headquarters in Bellevue.

On February 19, 2002, Sierra On-Line officially announced the name change to Sierra Entertainment, Inc.

In 2002, Sierra, working with High Voltage Software, announced the development of a new chapter in the Leisure Suit Larry franchise, titled Leisure Suit Larry: Magna Cum Laude. Released to mostly mixed to negative reviews; Larry's creator, Lowe, was not involved with the project.

The newly renamed Sierra Entertainment continued to develop mostly unsuccessful interactive entertainment products. However, hit Homeworld 2 once again cemented Sierra's reputation as a respectable publisher.

In 2003, Sierra Entertainment released the second video game adaptation of The Hobbit, as well as NASCAR Racing 2003 Season.

Acquisition and absorption under Vivendi Games (2004–2008)
In early 2004, cost-cutting measures were taken at Sierra's parent company Vivendi Universal Games due to financial troubles and because of Sierra's lack of profitability as a working developer. Sierra's last owned studios Impressions Games and the Papyrus Design Group were both shut down in early 2004, laying off 50 people; 180 Sierra-related positions were eliminated at Vivendi's Los Angeles offices; and by June 2004, Vivendi had completely shut down Sierra's Bellevue location, laid off more than 100 employees, dispersed Sierra's work to other VU Games divisions, and re-located the remains of Sierra's assets to Vivendi's corporate headquarters in Fresno, California. In total, 350 people were laid off. Some assets were retired in the process, including Print Artist, and some like the Hoyle franchise were sold to other publishers or developers. Sierra was simply a publishing label and brand name for Vivendi assets, being used in tandem with its own name for publishing. As a company, Sierra was disestablished on August 24, 2004. The business continued to operate as a division of Vivendi Games.

In late 2005, the Sierra brand was re-launched from Los Angeles. A new subsidiary called Sierra Online (no-relation to Sierra's former name Sierra On-Line) was also founded within this time, which focused on downloadable and online-only games.

Throughout 2005 and 2006, Vivendi acquired several game development studios including Massive Entertainment, High Moon Studios, Radical Entertainment, Secret Lair Studios / Studio Ch'in (based in Seattle and Shanghai) and Swordfish Studios and integrated them into Sierra, alongside the creative licenses from other Vivendi divisions and from companies partnered with Vivendi and the copyrights of several notable intellectual properties, such as Crash Bandicoot, Spyro the Dragon, 50 Cent: Bulletproof and Scarface. Vivendi also ceased publishing under their own name by this point after their name change, with all major releases being under the Sierra brand name.

Caesar IV was published September 26, 2006, in North America, in partnership with Tilted Mill Entertainment.

In mid 2007, Sierra Online began launching Xbox Live Arcade games for the Xbox 360. One of the first releases is the conversion of the successful "German-style" board game Carcassonne, which had been in development at Secret Lair Studios.

In late 2007, Sierra released games like the first-person shooter game TimeShift (which was originally going to be published by Atari but was delayed various times during development) and the real-time tactical video game World in Conflict.

Closure and sale of properties (2008–2009)
In 2008, Sierra Entertainment's parent company Vivendi Games merged with video game publisher Activision to form the Activision Blizzard holding company. Vivendi Games was absorbed into Activision after the merger and the ownership of Sierra properties went to Activision. Later that year, Sierra was closed down for possible future sale. Leading up to the merger, Activision's management was confident that Sierra would cease operations post-merger.

On July 29, 2008, Activision announced a long-term strategy for its library of Sierra games and announced that they would only publish five titles: Crash: Mind Over Mutant, The Legend of Spyro: Dawn of the Dragon, Prototype, and Ice Age: Dawn of the Dinosaurs, and an unannounced new property. The rest of the properties, including the Sierra Online division, were deemed by the company to be "non-strategic" with its then-current business practices. A selection of earlier sales and reversions followed:
 * July 30, 2008 - The video game rights to the Robert Ludlum/Bourne franchise reverted back to Robert Ludlum's estate through their Ludlum Entertainment subsidiary. Ludlum subsequently licensed these rights to Electronic Arts in February 2009.
 * August 6, 2008 - Activision announces that they have put Massive Entertainment up for sale, citifying their disregard of owning a European strategy developer, and that they were likely to sell World in Conflict and it's dropped expansion pack Soviet Assault separately. Swordfish Studios was also put up for sale.
 * August 13, 2008 - The video game licensing rights to Scarface, including a planned sequel to Scarface: The World is Yours, reverted back to Universal Pictures.
 * September 8, 2008 - Warner Bros. Interactive Entertainment acquired the F.E.A.R. trademark as well as the original title, consisting of the base game and its outsourced expansion packs Extraction Point and Perseus Mandate. The purchase of the trademark allowed Monolith Productions, who developed the original game, to rename its upcoming sequel Project Origin as F.E.A.R. 2: Project Origin.
 * October 8, 2008 - Activision announces the closure of the online servers of several older Sierra games on November 1. The Sierra Online division was also closed within that period despite the fact that two more games would be published under that name in 2009.
 * October 13, 2008 - THQ picked up the rights to 50 Cent: Blood on the Sand.
 * October 30, 2008 - Infogrames, the parent company of Atari, purchases publishing rights to Ghostbusters: The Video Game and The Chronicles of Riddick: Assault on Dark Athena, and secures licensing rights from Sony and Universal respectively for future titles using both licenses.
 * November 11, 2008 - Ubisoft purchases Massive Entertainment, in addition to purchasing the rights to World in Conflict.
 * November 12, 2008 - Monumental Games purchases Swordfish Studios' Manchester development studio.
 * November 15, 2008 - Codemasters purchases Swordfish Studios' remaining studio in Birmingham.
 * December 12, 2008 - Electronic Arts purchases publishing rights to Brütal Legend.
 * February 2009 - Codemasters purchases the Leisure Suit Larry IP and the publishing rights to Leisure Suit Larry: Box Office Bust.
 * April 27, 2009 - Bethesda Softworks purchases the publishing rights to WET.
 * July 14, 2009 - Rebellion Developments acquires several Sierra titles - Evil Genius, Ground Control, Empire Earth, Lords of Magic and Lords of the Realm.
 * October 1, 2009 - Jeff Tunnell acquires The Incredible Machine through his company PushButton Labs.

Brand name revival under Activision (2014–2016)
On August 7, 2014, the website for Sierra, which previously redirected to Activision's website, was updated to showcase a new logo, teasing: "More to be revealed at Gamescom 2014." Activision confirmed that the Sierra label would re-release some of its older games, re-imagine its older franchises, and collaborate with indie studios to create new "innovative, edgy and graphically unique" projects. Sierra plans to focus on publishing downloadable games through PlayStation Network, Steam, and Xbox Live. Ken Williams said, "We're very proud of what we created all those years ago with Sierra On-Line, and today's news about carrying Sierra forward as an indie-specific brand is very encouraging. We look forward to seeing Sierra's independent spirit live on." That day, King's Quest and Geometry Wars 3: Dimensions were announced as the first two games published under the revived Sierra brand. On December 5, 2014, the company was awarded with the "Industry Icon" award during the 2014 The Game Awards, and it introduced the first footage of the reboot of King's Quest.

In August 2018, TJX Companies acquired the domain sierra.com from Activision, subsequently shortening the name of its Sierra Trading Post subsidiary to Sierra.

Development

 * Books That Work was acquired in April 1997, folded into Sierra in February 1999.
 * Bright Star Technology in Bellevue, Washington was founded in 1980 and acquired in 1992.
 * Berkeley Systems was purchased by CUC International in April 1997 and integrated into Sierra as an internal studio.
 * Coktel Vision in Paris was founded in 1984 and acquired in October 1993.
 * Arion Software was acquired in 1995 and absorbed into Sierra On-Line.
 * The Pixellite Group was founded in 1983, acquired in May 1995, and absorbed into Sierra On-Line.
 * Sublogic, based in Champaign, Illinois, was acquired in 1995 and absorbed into Sierra On-Line.
 * Dynamix in Eugene, Oregon was founded in 1984, acquired in August 1990, and shut down in August 2001.
 * Green Thumb Software in Boulder, Colorado was acquired and absorbed in July 1995.
 * Headgate Studios in Bountiful, Utah was founded in 1992, acquired in April 1996, and sold to the original owner in 1999.
 * Impressions Games in Cambridge, Massachusetts was founded in 1989, acquired in 1995, and closed in May 2004.
 * Synergistic Software was founded in 1978, acquired in 1996, and folded into Sierra in February 1999.
 * Papyrus Design Group in Watertown, Massachusetts was founded in 1987, acquired in 1995, and closed in May 2004.
 * PyroTechnix was founded as Computer Presentation, acquired December 1997, and folded into Sierra in February 1999.
 * Yosemite Entertainment in Oakhurst, California was formed in 1998, and folded into Sierra in February 1999.

Publishing

 * Sierra Attractions; 1998–2001
 * Sierra FX; 1998
 * Sierra Home; 1996–2004
 * Sierra Sports; February 1998 –2000
 * Sierra Studios; 1998–2001

Games
Sierra both developed its own games and published several games from its divisions and from third-party developers. As a developer, Sierra launched the video game series King's Quest, Space Quest, Police Quest, Gabriel Knight, Leisure Suit Larry, and Quest for Glory.

title: Sierra Entertainment