Humidity is the amount of water vapor in the air. The term "humidity" is usually taken in daily language to refer to relative humidity. Relative humidity is a measurement of the amount of Water vapor that exists in a gaseous mixture of air and water 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 0 to 100%. General properties of water vapor Evaporation/sublimation Whenever a water molecule leaves a surface it is said to have evaporated Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Humidity may also be expressed as absolute humidity and specific humidity. Relative humidity is an important metric used in forecasting weather. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location Humidity indicates the likelihood of precipitation, dew, or fog. In Meteorology, precipitation (also known as one class of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric Formation Water will condense into droplets depending on the temperature Fog is a cloud that is in contact with the ground Stratus clouds are usually the only clouds that touch the ground 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. Evaporation is the process by which Molecules in a Liquid state (e This effect is calculated in a heat index table. The heat index ( HI) is an index that combines Air Temperature and Relative humidity in an attempt to determine the human-perceived equivalent Warm water vapor has more thermal energy than cool water vapor and therefore more of it evaporates into warm air than into cold air. Thermal energy is the sum of the sensible energy and latent energy.

## Absolute humidity

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. CM3 redirects here If you were looking for the 3rd game in the Cooking Mama series abbreviated as CM3 see here. 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 .

$AH = {m_w \over V_a}$

Absolute humidity ranges from 0 gram per cubic meter in dry air to 30 grams per cubic meter (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. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface This is very inconvenient for chemical engineering calculations, e. Chemical engineering is the branch of Engineering that deals with the application of Physical science (e g. for dryers, where temperature can vary considerably. A clothes dryer or tumble dryer is a household appliance that is used to remove the moisture from a load of Clothing and other Textiles, generally Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature 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". British Standards are produced by BSI British Standards, a division of BSI Group that is incorporated under a Royal Charter and is formally designated Units should always be carefully checked. Most humidity charts are given in g/kg or kg/kg, but any mass units may be used.

### 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

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. Psychrometrics or psychrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures 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 a measurement of the amount of Water vapor that exists in a gaseous mixture of air and water This page is about the physical properties of gas as a state of matter Relative humidity is expressed as a percentage and is calculated in the following manner:

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

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 Vapor pressure (also known as equilibrium vapor pressure or saturation vapor pressure) is the Pressure of a Vapor in equilibrium

Relative humidity is often mentioned in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location In Meteorology, precipitation (also known as one class of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric 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. Summer is one of the four Temperate Seasons Summer marks the warmest time of year with the longest days The weather is a set of all the phenomena occurring in a given Atmosphere at a given Time. The heat index ( HI) is an index that combines Air Temperature and Relative humidity in an attempt to determine the human-perceived equivalent Human beings, humans or man (Origin 1590–1600 L homō man OL hemō the earthly one (see Humus Evaporation is the process by which Molecules in a Liquid state (e

### Specific humidity

Specific humidity is the ratio of water vapor to air (including water vapor and 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:

$SH = {m_w \over m_a+m_v}$

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

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

### Humidity during rain

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. A cloud is a visible mass of droplets or frozen crystals floating in the atmosphere above the surface of the Earth or another Planetary body Rain is Liquid precipitation. On Earth it is the condensation of atmospheric Water vapor into drops heavy enough to fall often making it to 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 necessarily 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. The dew point (sometimes spelled dewpoint) is the Temperature to which a given parcel of Air must be cooled at constant Barometric pressure, Although that would indeed raise the relative humidity to 100%, the water lost from the air (as dew) would also lower the absolute humidity. Relative humidity is a measurement of the amount of Water vapor that exists in a gaseous mixture of air and water Humidity is the amount of water vapor in the air In daily language the term "humidity" is normally taken to mean Relative humidity.

## 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). The dew point (sometimes spelled dewpoint) is the Temperature to which a given parcel of Air must be cooled at constant Barometric pressure, 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. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Rain is Liquid precipitation. On Earth it is the condensation of atmospheric Water vapor into drops heavy enough to fall often making it to "Snowfall" redirects here For other uses see Snow (disambiguation or Snowfall (disambiguation. Frost is the solid deposition of Water vapor from saturated air Formation Water will condense into droplets depending on the temperature 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. Temperature and layers The temperature of the Earth's atmosphere varies with altitude the mathematical relationship between temperature and altitude varies among five

## Measuring and regulating humidity

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

There are various devices used to measure and regulate humidity. Hygrometers are instruments used for measuring Humidity. A simple form of a hygrometer is specifically known as a "psychrometer" and consists of two Thermometers A device used to measure humidity is called a psychrometer or hygrometer. Hygrometers are instruments used for measuring Humidity. A simple form of a hygrometer is specifically known as a "psychrometer" and consists of two Thermometers A humidistat is used to regulate the humidity of a building with a de-humidifier. A humidifier is a household Appliance that increases Humidity (moisture in a single room or in the entire home These can be analogous to a thermometer and thermostat for temperature control. The thermometer is a device that measures Temperature or Temperature gradient using a variety of different principles it comes from the Greek roots A thermostat is a device for regulating the Temperature of a System so that the system's temperature is maintained near a desired setpoint

## 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. Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14 Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Isaac Newton discovered this phenomenon and wrote about it in his book Opticks. Sir Isaac Newton, FRS (ˈnjuːtən 4 January 1643 31 March 1727) Biography Early years See also Isaac Newton's early life and achievements [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. The ideal gas law is the Equation of state of a hypothetical Ideal gas, first stated by Benoît Paul Émile Clapeyron in 1834 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.

## 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. Convective heat transfer is a mechanism of Heat transfer occurring because of bulk motion (observable movement of fluids Thermal radiation is Electromagnetic radiation emitted from the surface of an object which is due to the object's Temperature. 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. Blood is a specialized Bodily fluid that delivers necessary substances to the body's cells such as nutrients and oxygen—and transports Waste products In Medicine, hyperpyrexia is an excessive and unusual elevation of set Body temperature greater than or equal to 41 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. Muscle (from Latin musculus, diminutive of mus "mouse" is contractile tissue of the body and is derived from the The brain is the center of the Nervous system in animals All Vertebrates and the majority of Invertebrates have a brain In Biology, an organ ( Latin: organum, "instrument tool" from Greek όργανον - organon "organ instrument Physical strength declines and fatigue occurs sooner than it would otherwise. Physical strength is the ability of a person or animal to exert Force on physical objects using muscles. Alertness and mental capacity also may be affected. This resulting condition is called heat stroke or hyperthermia. 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

### 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. The American Society of Heating Refrigerating and Air Conditioning Engineers ( ASHRAE; pronounced 'ash'-'ray' is an international technical society for all individuals The house dust mite (sometimes abbreviated by Allergists to HDM) is a cosmopolitan guest in Human habitation 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. The term air conditioning refers to the cooling and dehumidification of indoor air for Thermal comfort. 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.

## 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. Condensation is the change of the physical state of aggregation (or simply state of matter from gaseous phase into liquid phase 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. Short Circuit is a 1986 comedy Science fiction film starring Ally Sheedy and Steve Guttenberg and directed by Such short circuits may cause substantial permanent damage if the equipment is powered on before the condensation has evaporated. Evaporation is the process by which Molecules in a Liquid state (e 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.

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.