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Humidity is the concentration of water vapor in the air. The concentration can be expressed as absolute humidity, specific humidity, or relative humidity. A device used to measure humidity is called a hygrometer. A humidistat is used to regulate the humidity of a building with a de-humidifier. These can be analogous to a thermometer and thermostat for climate control. Due to the 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).

Absolute humidity

Absolute humidity is a method of expressing the amount of water vapor by using the ratio of the mass of the water vapor to the volume of the air. Absolute humidity is expressed as a ratio of kilograms of water vapor, m_w, per cubic meter of air, v_a .

As such, its value changes as the air pressure changes.

Specific humidity

Specific humidity or the mixing ratio is a method of expressing the amount of aqueous vapor in air by using a ratio of water vapor to dry air. Specific humidity is expressed as a ratio of kilograms of water vapor, m_w, per kilogram of air, m_a .

That ratio can be given as:

Partial pressure of water vapor and air can also be used to express the ratio.

Relative humidity

Main article: Relative humidity

Relative humidity is the ratio of the current vapor pressure of water in any gas (especially air), known as the absolute vapor pressure (AVP), to the equilibrium vapor pressure or saturation vapor pressure (SVP), at which the gas is called saturated at the current temperature, expressed as a percentage.

Formulaically expressed as:

Equivalently, it is the ratio of the current mass of water per volume of gas and the mass per volume of a saturated gas.

The numerators of these ratios are the two ways of expressing absolute humidity. The following graph compares dew point (maximum humidity in red) to 50% relative humidity (green line halfway between zero and the dew point across the range of temperatures).

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, created by Charles T. R. Wilson for nuclear physics experiments, which uses an extremely brief state of supersaturation to accomplish its function.

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 heat index table.

Effects on human body

Under conditions of high humidity, the evaporation of sweat from the skin is decreased and the body's efforts to maintain an acceptable body temperature may be significantly impaired. Also, if the atmosphere is as warm as or warmer than the skin during times of high humidity, blood brought to the body surface cannot lose its heat, resulting in a condition called hyperpyrexia. With so much blood going to the external surface of the body, relatively 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. This resulting condition is called heat stroke or hyperthermia.

Recommendations for Comfort

Humans and animals control their body temperature by sweating. The US Environmental Protection Agency cites the ASHRAE Standard 55-1992 Thermal Environmental Conditions for Human Occupancy, which recommends keeping relative humidity between 30% and 60%, with below 50% preferred to control dust mites. At high humidity sweating is less effective so we feel hotter. Dry air feels colder at room temperature leading to discomfort, lower productivity and demands for more heating. When relative humidity is ideal, temperatures in buildings can be lowered without causing discomfort to the people in them.

Popular culture

First spoken by Warren Hymer in the 1939 movie Mr. Moto on Danger Island, the expression It's not the heat, it's the humidity refers to unpleasantly muggy weather; that is, the fact that humid air can be significantly less comfortable than drier air at the same temperature.

See also

• Weather forecasting
• Concentration
• Steam
• Vapor barrier

References

• United States Environmental Protection Agency. "IAQ in large buildings". Retrieved Jan. 9, 2006.

External links

• Glossary definition of specific humidity
• National Science Digital Library - Absolute Humidity
• Glossary definition of psychrometric tables
• Humidity Sensor producer

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