FACTOID # 11: Oklahoma has the highest rate of women in State or Federal correctional facilities.

 Home Encyclopedia Statistics States A-Z Flags Maps FAQ About

 WHAT'S NEW

SEARCH ALL

Search encyclopedia, statistics and forums:

(* = Graphable)

Encyclopedia > Humidity

The term humidity is usually taken in daily language to refer to relative humidity. Relative humidity is defined as the amount of water vapor in a sample of air compared to the maximum amount of water vapor the air can hold at any specific temperature in a form of 1 to 100%. Humidity may also be expressed as Absolute humidity and specific humidity. Relative humidity is an important metric used in forecasting weather. Humidity indicates the likelihood of precipitation, dew, or fog. High humidity makes people feel hotter outside in the summer because it reduces the effectiveness of sweating to cool the body by preventing the evaporation of perspiration from the skin. This effect is calculated in a heat index table. Warm water vapor has more thermal energy than cool water vapor and therefore more of it evaporates into warm air than into cold air. Image File history File links Broom_icon. ... A hygrometer used to measure the humidity of air. ... Water vapor or water vapour (see spelling differences), also aqueous vapor, is the gas phase of water. ... Look up air in Wiktionary, the free dictionary. ... The largest and the smallest element of a set are called extreme values, or extreme records. ... For other uses, see Temperature (disambiguation). ... Modern weather predictions aid in timely evacuations and potentially save lives and property damage Human beings have attempted to predict the weather since time immemorial. ... Dew on a spider web Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening. ... For other uses, see Fog (disambiguation). ... Perspiration (also called sweating or sometimes transpiration) is the production and evaporation of a fluid, consisting primarily of water as well as a smaller amount of sodium chloride (the main constituent of table salt), that is excreted by the sweat glands in the skin of mammals. ... Vaporization redirects here. ... The heat index (HI) or humidex is an index that combines air temperature and relative humidity to determine an apparent temperature â€” how hot it actually feels. ... In thermal physics, thermal energy is the energy portion of a system that increases with its temperature. ...

Absolute humidity is the quantity of water in a particular volume of air. The most common units are grams per cubic meter, although any mass unit and any volume unit could be used. Pounds per cubic foot is common in the U.S., and occasionally even other units mixing the English and metric systems are used.

If all the water in one cubic meter of air were condensed into a container, the container could be weighed to determine absolute humidity. The amount of vapor in that cube of air is the absolute humidity of that cubic meter of air. More technically: the mass of water vapor mw, per cubic meter of air, Va . The cubic metre (symbol m³) is the SI derived unit of volume. ...

$AH = {m_w over V_a}$

Absolute humidity ranges from 0 gram per cubic metre in dry air to 30 grams per cubic metre (0.03 ounce per cubic foot) when the vapour is saturated at 30 °C.[1] (See also Absolute Humidity table)

The absolute humidity changes as air pressure changes. This is very inconvenient for chemical engineering calculations, e.g. for dryers, where temperature can vary considerably. As a result, absolute humidity is generally defined in chemical engineering as mass of water vapor per unit mass of dry air, also known as the mass mixing ratio (see below), which is much more rigorous for heat and mass balance calculations. Mass of water per unit volume as in the equation above would then be defined as volumetric humidity. Because of the potential confusion, British Standard BS 1339 (revised 2002) suggests avoiding the term "absolute humidity." Units should always be carefully checked. Most humidity charts are given in g/kg or kg/kg, but any mass units may be used. This article is about pressure in the physical sciences. ... Chemical engineers design, construct and operate plants Chemical engineering is the branch of engineering that deals with the application of physical science (e. ... An electric clothes dryer A clothes dryer or tumble dryer is a household appliance that is used to remove the residual moisture from a load of clothing and other textiles, generally shortly after they are cleaned in a washing machine. ... For other uses, see Temperature (disambiguation). ... British Standards is the new name of the British Standards Institute and is part of BSI Group which also includes a testing organisation. ...

## Mixing ratio or humidity ratio

Mixing or humidity ratio is expressed as a ratio of kilograms of water vapor, mw, per kilogram of dry air, md, at a given pressure. The colloquial term Moisture Content is also used instead of Mixing/Humidity Ratio. Humidity Ratio is a standard axis on psychrometric charts, and is a useful parameter in psychrometrics calculations because it does not change with temperature except when the air cools below dewpoint For the parapsychology phenomenon of distance knowledge, see psychometry. ...

That ratio can be given as:

$MRi = {m_w over m_d}$

Partial pressure of water vapor and air can also be used to express the ratio. In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ...

## Relative humidity

Main article: Relative humidity

Relative humidity is defined as the ratio of the partial pressure of water vapor in a gaseous mixture of air and water vapor to the saturated vapor pressure of water at a given temperature. Relative humidity is expressed as a percentage and is calculated in the following manner: A hygrometer used to measure the humidity of air. ... Gas phase particles (atoms, molecules, or ions) move around freely Gas is one of the four major states of matter, consisting of freely moving atoms or molecules without a definite shape and without a definite volume. ...

$RH = {p_{(H_2O)} over p^*_{(H_2O)}} times 100%$

where

${p_{(H_2O)}}$ is the partial pressure of water vapor in the gas mixture;
${p^*_{(H_2O)}}$ is the saturation vapor pressure of water at the temperature of the gas mixture; and
$RH_{,_,}$ is the relative humidity of the gas mixture being considered.

In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. ... The saturation vapor pressure is the vapor pressure of water when air is saturated with water (having the maximum amount of water vapor that air can hold for a given temperature and pressure). ...

## Specific humidity

Specific humidity is the ratio of water vapor to dry air in a particular volume. Specific humidity ratio is expressed as a ratio of kilograms of water vapor, mw, per kilogram of mixture, mt .

That ratio can be showed as:

$SH = {m_w over m_t} = {m_w over m_a+m_w}$

Specific humidity is related to mixing ratio (and vice versa) by:

$SH = {MR over 1+MR}$
$MR = {SH over 1-SH}$

## Measuring and regulating humidity

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

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 kilometers. 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 future weather forecasts. For other uses, see Satellite (disambiguation). ... Atmosphere diagram showing the mesosphere and other layers. ... For other uses, see Infrared (disambiguation). ... This article or section does not cite its references or sources. ...

## Why humidity can be less than 100% when it's raining

Humidity is a measure of the amount of water vapor dissolved in the air, not including any liquid water or ice falling through the air. For clouds to form, and rain to start, the air does have to reach 100% relative humidity, but only where the clouds are forming or where the rain is falling from. This normally happens when the air rises and cools. Typically, rain falls into air with less than saturated humidity. Some water from the rain may evaporate into the air as it falls, increasing the humidity, but not necessarilly enough to raise the humidity to 100%. It is even possible for rain falling through warm, humid air to be cold enough to lower the air temperature to the dew point, thus condensing water vapor out of the air. Although that would indeed raise the relative humidity to 100%, the water lost from the air (as dew) would also lower the absolute humidity. For other uses, see Cloud (disambiguation). ... This article is about precipitation. ... The dew point (or dewpoint) is the temperature which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. ... A hygrometer used to measure the humidity of air. ... Absolute humidity is the mass of water vapor in a given volume of air or gas, expressed by weight and usually measured in grams per cubic meter, though grains per cubic foot has also been used in the United States. ...

## Humidity and air density

Humid air is less dense than dry air because a molecule of water weighs less than molecules of nitrogen and oxygen. Isaac Newton discovered this phenomenon and wrote about it in his book Opticks.[1] Avogadro's ideal gas law states that a fixed volume of gas at a given temperature and pressure always contains the same number of molecules regardless of what type of gas it is. Consider a cubic meter of dry air. About 78% of the molecules are nitrogen (N2), with a molecular weight of 28. Another 21% of the molecules are oxygen (O2), with a molecular weight of 32. The final 1% is a mixture of other gases. Combining these weights in the correct proportions gives an average molecular weight for air of about 29. If molecules of water vapor (H2O), of molecular weight 18, replace the diatomic nitrogen or oxygen molecules in this fixed volume then the weight of the air decreases, and hence the density decreases. Thus, humid air has a lower density than dry air at a specified temperature and pressure. General Name, symbol, number nitrogen, N, 7 Chemical series nonmetals Group, period, block 15, 2, p Appearance colorless gas Standard atomic weight 14. ... This article is about the chemical element and its most stable form, or dioxygen. ... Sir Isaac Newton FRS (4 January 1643 â€“ 31 March 1727) [ OS: 25 December 1642 â€“ 20 March 1727][1] was an English physicist, mathematician, astronomer, natural philosopher, and alchemist. ... Isotherms of an ideal gas The ideal gas law is the equation of state of a hypothetical ideal gas, first stated by BenoÃ®t Paul Ã‰mile Clapeyron in 1834. ...

## Dew point and frost point

Associated with relative humidity is dew point (If the dew point is below freezing, it is referred to as the frost point). Dew point is the temperature at which water vapor saturates from an air mass into liquid or solid usually forming rain, snow, frost, or dew. Dew point normally occurs when a mass of air has a relative humidity of 100%. This happens in the atmosphere as a result of cooling through a number of different processes. The dew point (or dewpoint) is the temperature which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. ... For other uses, see Temperature (disambiguation). ... This article is about precipitation. ... For other uses, see Snow (disambiguation). ... Frost on black pipes Frost is a solid deposition of water vapor from saturated air. ... Dew on a spider web Dew is water in the form of droplets that appears on thin, exposed objects in the morning or evening. ... Air redirects here. ...

Image File history File links No higher resolution available. ...

## Most humid places on earth

The most humid cities on earth are generally located closer to the equator, near coastal regions. Cities in South and Southeast Asia seem to be among the most humid. Kolkata, India; Kerala, India; and Bangkok, Thailand experience extreme humidity during their rainy seasons combined with warmth giving the feel of a lukewarm Sauna.[2] Darwin, Australia experiences an extremely humid wet season from December to April. Kuala Lumpur and Singapore have very high humidity all year round because of their proximity to water bodies and the Equator and overcast weather; despite sunshine, perfectly clear days are rare in these locations and it is often misty. 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 30 degrees Celsius. , â€œCalcuttaâ€ redirects here. ... , Kerala ( ; Malayalam: à´•àµ‡à´°à´³à´‚; ) is a state on the Malabar Coast of southwestern India. ... Location within in Thailand Coordinates: , Country Settled Ayutthaya Period Founded as capital 21 April 1782 Government  - Type Special administrative area  - Governor Apirak Kosayothin Area  - City 1,568. ... Nickname: Motto: Maju dan makmur (English: Progress and Prosper) Location in Malaysia Coordinates: , Country State Establishment 1857 Granted city status 1974 Government  - Mayor (Datuk Bandar) Datuk Abdul Hakim Borhan From 14 December 2006 Area  - Total 243. ... World map showing the equator in red In tourist areas, the equator is often marked on the sides of roads The equator marked as it crosses IlhÃ©u das Rolas, in SÃ£o TomÃ© and PrÃ­ncipe. ...

In the United States the most humid cities, strictly in terms of relative humidity, are Forks and Olympia, Washington.[3] This fact may come as a surprise to many, as the climate in this region rarely exhibits the discomfort usually associated with high humidity. Dew points are typically much lower on the West Coast than on the East. Because high dew points play a more significant role than relative humidity in the discomfort created during humid days, the air in these western cities usually does not feel "humid." A hygrometer used to measure the humidity of air. ... Forks is a city in Clallam County, Washington, United States. ... Coordinates: , Country State County Thurston Incorporated January 28, 1859 Government  - Mayor Mark Foutch Area  - Total 18. ...

The highest dew points are found in coastal Florida and Texas. When comparing Key West and Houston, two of the most humid cities from those states, coastal Florida seems to have the higher dew points on average. But, as noted by Jack Williams of USA Today,[4] Houston lacks the coastal breeze present in Key West. This article is about the U.S. State of Florida. ... For other uses, see Texas (disambiguation). ...

## Effects on human body

The human body sheds heat by a combination of evaporation of perspiration, heat convection in the surrounding air, and thermal radiation. 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 shed heat by conduction to the air, and a condition called hyperpyrexia results. 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. ... Radiant heat redirects here. ... For other uses, see Blood (disambiguation). ... In medicine, hyperpyrexia is an excessive and unusual elevation of body temperature above 107. ... For other uses of Muscles, see Muscles (disambiguation). ... Human brain In animals, the brain (enkephale) (Greek for in the skull), is the control center of the central nervous system, responsible for behavior. ... This article is about the biological unit. ... Physical strength is the ability of a person or animal to exert force on physical objects using muscles. ... The word fatigue is used in everyday living to describe a range of afflictions, varying from a general state of lethargy to a specific work induced burning sensation within muscle. ... Hyperthermia in its advanced state referred to as heat stroke or sunstroke, is an acute condition which occurs when the body produces or absorbs more heat than it can dissipate. ...

## Effects on electronics

Many electronic devices have humidity specifications, for example, 5 to 95%. 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 (eg garage, car, shed) to a warm humid place (house), 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. It is advisable to allow electronic equipment to acclimatise for several hours, after being brought in from the cold, before powering on. The inverse is also true. For other uses, see Condensation (disambiguation). ... For alternate meanings see Short circuit (disambiguation) A short circuit (sometimes known as simply a short) is a fault whereby electricity moves through a circuit in an unintended path, usually due to a connection forming where none was expected. ... Evaporation is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state. ...

Excessively high humidity causes corrosion in electronics. Low humidity causes static electricity and spontaneous shutdown of servers in data centres. Electronics, and more specific TTL technology, cannot handle voltages that exceed the supply voltage by a small margin before it will blow and cause it to malfunction. Therefore humidity is an important measure in the control of data centre facilities.

## Recommendations for comfort

Humans control their body temperature by sweating and shivering. The United States 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; thus the desire to remove humidity from air with air conditioning in the summer. In the winter, heating cold outdoor air can decrease indoor relative humidity levels to below 30%, leading to discomfort such as dry skin and excessive thirst. Perspiration (also called sweating or sometimes transpiration) is the production and evaporation of a fluid, consisting primarily of water as well as a smaller amount of sodium chloride (the main constituent of table salt), that is excreted by the sweat glands in the skin of mammals. ... Shivering is a human bodily function in response to cold. ... EPA redirects here. ... The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) is an international voluntary organization for people involved in heating, ventilation, air conditioning, or refrigeration (HVAC&R). ... Binomial name Dermatophagoides pteronyssinus Trouessart, 1897 The house dust mite (Dermatophagoides pteronyssinus in Europe and Dermatophagoides farinae in North America), sometimes abbreviated by allergists to HDM, is a cosmopolitan guest in human habitation. ... Note: in the broadest sense, air conditioning can refer to any form of heating, ventilation, and air-conditioning. ...

## References

1. ^ Newton, Isaac (1704). Opticks. Dover.
2. ^ http://www.bbc.co.uk/weather/world/city_guides/results.shtml?tt=TT002890
3. ^ http://www.komotv.com/news/archive/4092941.html
• United States Environmental Protection Agency, "IAQ in Large Buildings". Retrieved Jan. 9, 2006.

Results from FactBites:

 Humidity - Wikipedia, the free encyclopedia (1369 words) Humidity is the concentration of water vapor in the air. 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. 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.
 Relative humidity - definition of Relative humidity in Encyclopedia (654 words) Relative humidity is the ratio of the current vapor pressure of water in any gas (especially air) to the vapor pressure at which the gas would become saturated at the current temperature, normally expressed as a percentage. This phenomemon is the 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 condenses as rain. Relative humidity is often mentioned in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog.
More results at FactBites »

Share your thoughts, questions and commentary here