Relative Humidity

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Relative humidity is a term used to describe the quantity of water vapor that exists in a gaseous mixture of air and water (see: common misconceptions below).

Contents

1 Definition
2 Common misconceptions
3 Related concepts
4 Other important facts
5 See also
6 External links

Definition

Relative humidity is expressed as a percentage and is calculated in the following manner:

[ RH = \over p^*_} \times 100% ]

Where symbols above have the following meaning:

p_(H2O) is the partial pressure of water vapor in the gas mixture;

p^*_(H2O) is the saturation vapor pressure of water at the temperature and pressure of the gas mixture; and

RH is the relative humidity of the mixture under consideration.

Common misconceptions

Relative humidity is the ratio of the amount of water vapor in air to the maximum amount of water vapor that could be in the air if the vapor were at its saturation conditions. Often the concept of air holding water vapor is used in the description of relative humidity. Relative humidity is wholly understood in terms of the physical properties of water alone and therefore is unrelated to this concept.[link]

This misconception is likely a result of the word saturation which is commonly used in the definition of relative humidity. The term "saturation" in this context refers to the saturation state of water vapor, rather than saturation as related to solubility.

The relative humidity of a gas mixture is a function of not only the temperature, but also the pressure of the gas mixture. This dependence exists because the saturated vapor pressure of water depends on these intensive properties. It is meaningless to state the relative humidity of a system without stating both the temperature and pressure of interest. A gaseous mixture of air and water at 90% relative humidity and 25 °C is very different from a similar mixture at 90% relative humidity and 10 °C.

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 or a non-condensable phase other than air.

Other important facts

A gas in this context is referred to as saturated when the vapor pressure of water is at the equilibrium vapor pressure for water vapor; liquid water (and ice, at the appropriate temperature) will fail to lose mass through evaporation when exposed to saturated air. It also corresponds to the possibilility 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 droplets of liquid. (Even though these droplets may be so small as to fall imperceptibly slowly through the mixed gas we call air, this behavior is too different from that of water vapor to reflect it in the same scale. This explains the restriction of relative-humidity discussions to 100% and below.)

The statement that relative humidity 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 with the temperature. This is because the partial pressure of water increases with temperature – the principle behind everything from hair dryers to dehumidifiers.

Due to this changing partial pressure of water vapor in air as temperature changes, the water content of air at sea level can get as high as 3% at 30 °C (86 °F), and no more than about 0.5% 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, reflected by dry skin, itchy eyes, and persistence of static electric charges. Even with saturation (100% humidity) outdoors, heating of whatever outside air comes indoors raises its moisture capacity, reflected in decreased relative humidity and increased evaporation rates from moist surfaces.

Similarly, during summer in humid climates a great deal of water condenses from air cooled in air conditioners. Warmer air is cooled below its dewpoint and that water 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.

Water vapor is a lighter gas than dry air, so humid air will tend to rise through drier air at the same temperature. This phenomemon is a mechanism behind thunderstorms, since as the humid rising air also becomes colder as it rises due to adiabatic cooling, and as the air cools past its dew point, water vapor condenses into small droplets (which may form clouds). They will further condense into larger water droplets if certain conditions are met, for example, the presence of small particles (often referred to as "seeds"). The droplets will descend as rain if they become too large for the updraft to lift.

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 preventing the evaporation of perspiration from the skin. This effect is calculated in a table, resulting in 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.)

External links

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