Humidity: Difference between revisions
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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 [[concentration|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). |
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==Absolute humidity== |
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'''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, <math> m_w </math>, per cubic meter of air, <math> v_a </math> . |
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:<math> AH = {m_w \over v_a} </math> |
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As such, its value changes as the air pressure changes. |
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==Specific humidity== |
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'''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, <math> m_w </math>, per kilogram of air, <math> m_a </math> . |
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That ratio can be given as: |
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:<math> SH = {m_w \over m_a} </math> |
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[[Partial pressure]] of water vapor and air can also be used to express the ratio. |
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[[Image:relative_humidity.jpg|right]] |
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==Relative humidity== |
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''Main article:'' [[Relative humidity]] |
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'''[[Relative humidity]]''' is the ratio of the current [[vapor pressure]] of [[water vapor|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 [[saturation (chemistry)|saturated]] at the current temperature, expressed as a percentage. |
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Formulaically expressed as: |
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:<math> RH = {AVP \over SVP} </math> |
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Equivalently, it is the ratio of the current mass of water per volume of gas and the mass per volume of a saturated gas. |
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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). |
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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.) |
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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. |
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Relative humidity is often mentioned in [[weather forecasting|weather forecasts]] and reports, as it is an indicator of the likelihood of [[precipitation (meteorology)|precipitation]], dew, or fog. In hot [[summer]] [[weather]], it also increases the apparent temperature to [[human]]s (and other [[animal]]s) by preventing the [[evaporation]] of [[perspiration]] from the skin. This effect is calculated in a [[heat index]] table. |
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==Effects on human body== |
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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 [[muscle]]s, the [[brain]], and other internal [[organ (anatomy)|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]]. |
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==Recommendations for Comfort== |
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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 |
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effective so we feel hotter. Dry air feels colder at room temperature leading to |
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discomfort, lower productivity and demands for more heating. When relative |
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humidity is ideal, temperatures in buildings can be lowered without causing |
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discomfort to the people in them. |
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==Popular culture== |
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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. |
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==See also== |
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*[[Weather forecasting]] |
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*[[Concentration]] |
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*[[Steam]] |
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*[[Vapor barrier]] |
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==References== |
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* United States Environmental Protection Agency. [http://www.epa.gov/iaq/largebldgs/i-beam_html/ch2-hvac.htm#F2.6 "IAQ in large buildings"]. Retrieved Jan. 9, 2006. |
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{{Met_vars}} |
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==External links== |
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*[http://nsidc.org/arcticmet/glossary/specific_humidity.html Glossary definition of specific humidity] |
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*[http://www.nsdl.arm.gov/Library/glossary.shtml#absolute_humidity National Science Digital Library - Absolute Humidity] |
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*[http://nsidc.org/arcticmet/glossary/psychrometric_tables.html Glossary definition of psychrometric tables] |
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*[http://www.ist-ag.ch/english/products/humid/humidity.html Humidity Sensor producer] |
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[[Category:Physical quantity|Humidity]] |
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[[Category:Psychrometrics]] |
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[[Category:Weather]] |
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{{Link FA|de}} |
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{{Link FA|zh}} |
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[[ca:Humitat]] |
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[[da:Luftfugtighed]] |
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[[de:Luftfeuchtigkeit]] |
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[[es:Humedad ambiental]] |
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[[fi:Ilmankosteus]] |
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[[fr:Hygrométrie]] |
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[[he:לחות מוחלטת]] |
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[[he:לחות]] |
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[[ja:湿度]] |
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[[nl:Luchtvochtigheid]] |
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[[pl:Wilgotność bezwzględna]] |
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[[pl:wilgotność powietrza]] |
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[[pt:Humidade]] |
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[[sl:Vlažnost]] |
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[[zh:湿度]] |
[[zh:湿度]] |
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