relative humidity

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

Relative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water vapor.

Definition

The relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a prescribed temperature. Relative humidity is normally expressed as a percentage and is defined in the following manner^[1]:

where:

is the relative humidity of the mixture being considered;

is the partial pressure of water vapor in the mixture; and

is the saturated vapor pressure of water at the temperature of the mixture.

The international symbols U^[2][3] and U_w,^[4] expressed in per cent, are gaining recognition.

Measurement

A hygrometer is a device that is used to measure humidity.

The relative humidity of an air-water vapor mixture can be estimated if both the measurement temperature (T_m) and the dew point temperature (T_d) of the mixture are known. This approach to measuring humidity requires an equation expressing the saturation vapor pressure as a function of temperature. There are many such equations^[5], ranging from the approximate Antoine equation, to the more detailed Goff-Gratch equation. A reasonable compromise proposed by many is the Magnus-Teton formulation:

where the saturated vapor pressure of water at any temperature T is estimated by:

Where T is expressed in degrees Celsius, various researchers have published fitted values for the α,β and λ parameters, some of which are listed by Vömel^[5].

A slightly more complex variant of this equation was proposed by Buck.

Whichever equation is chosen for , the Relative humidity is calculated as:

In practice both T_m and T_d are readily estimated by using a sling psychrometer and the relative humidity of the atmosphere can be calculated.

A common misconception

Often the notion of air holding water vapor is presented to describe the concept of relative humidity. However, air simply acts as a transporter of water vapour and is not a holder of it. Therefore, relative humidity is wholly understood in terms of the physical properties of water alone and thus is unrelated to this concept.^[6][7] In fact, water vapor can be present in an airless volume and therefore the relative humidity of this volume can be readily determined.

The misconception that air holds water is likely the result of the use of the word saturation which is often misused in descriptions of relative humidity. In the present context the word saturation refers to the state of water vapor,^[8] not the solubility of one material in another ( ideal gases mix in any proportion).

The thermophysical properties of water-air mixtures encountered at atmospheric conditions are reasonably approximated by assuming they behave as a mixture of ideal gases. For many practical purposes the assumption that both components (air and water) behave independently of each other is reasonable. Therefore the physical properties of an air-water mixture can be estimated by considering the physical properties of each component separately.

Significance of relative humidity

Climate control

Climate control refers to the control of temperature and relative humidity for human comfort, health and safety, and for the technical requirements of machines and processes, in buildings, vehicles and other enclosed spaces.

Comfort

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^{[citation needed]} we feel warmer when the relative humidity is high than when it is low.

For example, if the air temperature is 24 °C (75 °F) and the relative humidity is zero percent, then the air temperature feels like 21 °C (69 °F).^[9] If the relative humidity is 100 percent at the same air temperature, then it feels like 27 °C (80 °F).^[9] In other words, if the air is 24 °C and contains saturated water vapor, then the human body cools itself at the same rate as it would if it were 27 °C and dry.^[9] The heat index and the humidex are indices that reflect the combined effect of temperature and humidity on the cooling effect of the atmosphere on the human body.

Buildings

When controlling the climate in buildings using HVAC systems the key is to control the relative humidity in 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 hanging outdoors 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 high 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.

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 same basic principles as in buildings, above, apply. 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 sealed vehicles and pressure vessels such as pressurised airliners, submersibles and spacecraft these considerations may be critical to safety, and complex environmental control systems including equipment to maintain pressure are needed. For example, airliner fuselages are susceptible to corrosion from humidity, and avionics are susceptible to condensation, and as the failure of either is potentially catastrophic, 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 dry air, often 5% relative humidity or below, which is found at airliner cruising altitudes. 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 dry air is essential to safe flight.

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 isobariacally heated (heating with no change in system pressure) then the relative humidity of the system decreases because the saturated 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 increasing system pressure. This is shown in State C.

Therefore a change in relative humidity can be explained by a change in system temperature, a change in the absolute pressure of the system, or change in both of these system properties.

Pressure enhancement factor (open system)

Saturation water vapor pressure of air-water mixtures shows a very slight dependence on system pressure. The empirical equations of Buck^[10] show that the difference in water vapor pressure between normal atmospheric pressure and no air at all is about 0.4% at room temperature. This is equivalent to the vapor pressure change from a temperature decrease of less that 0.1°C.

Consider two locations:

1. sea level, and
2. 5500m up a mountain, where air pressure is half the value at sea level.

Assuming a local temperature of 20°C and a local dew point of 10°C, the relative humidities calculate to be 52.504% and 52.498%, a difference that would rarely be measureable. On the other hand, the mixing ratio (mass of water per unit mass of air) is halved at the high altitude, because the vapor pressure of water is almost unchanged (same mass per unit volume of gas), while the air density is halved. The volumetric (absolute) humidity is essentially unchanged since it is the ratio of mass of water to volume of gas.

If we were somehow to take a roomful of our air from sea level and release it into a room at 5500m, displacing all the original air without mixing, and leaking out enough to match the outside pressure, then we would reach opposite conclusions. This would be effectively a closed system and the relative humidity and volumetric humidity would be half what they were at sea level, because the dew point of the air in the room would now be close to 0°C. The mixing ratio would be unchanged from the sea level value.

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.^[11]

Other important facts

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 being 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. An arguable exception is the Wilson cloud chamber which uses, in nuclear physics experiments, an extremely brief state of "supersaturation" to accomplish its function.

For a given dewpoint 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, indicated by 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.)

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 dewpoint and the excess water vapor condenses. This phenomenon is the same as that which causes water droplets to form on the outside of a cup containing an ice-cold drink.

A useful rule of thumb is that the maximum absolute humidity doubles for every 20 °F (11.1 °C) increase in temperature. Thus, the relative humidity will drop by a factor of 2 for each 20 °F (11.1 °C) increase in temperature, assuming conservation of absolute moisture. For example, in the range of normal temperatures, air at 70 °F (21.1 °C) and 50% relative humidity will become saturated if cooled to 50°F (10 °C), its dewpoint and 40 °F (4.4 °C) air at 80% relative humidity warmed to 70 °F (21.1 °C) will have a relative humidity of only 29% and feel dry. By comparison, a relative humidity between 40% and 60% is considered healthy and comfortable in comfort controlled environments (ASHRAE Standard 55 - see thermal comfort).

Water vapor is a lighter gas than 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

• Humidity
  • Absolute Humidity
  • Specific Humidity
• Concentration
• Heat index
• Dew point
  • Dew point depression
• Humidity indicator
• Hygrometer
  • Psychrometrics
• Saturation vapor density

References

• Himmelblau, David M. (1985454545). Basic Principles And Calculations In Chemical Engineering. Prentice Hall. ISBN 0-13-066572-X. 
• Perry, R.H. and Green, D.W (1997). Perry's Chemical Engineers' Handbook (7th Edition). McGraw-Hill. ISBN 0-07-049841-5. 

External links

• Glossary definition of psychrometric tables - National Snow and Ice Data Center
• Bad Clouds FAQ, PSU.edu