Humidity

Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the naked eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depends on the temperature and pressure of the system of interest. The same amount of water vapor results in higher relative humidity in cool air than warm air. A related parameter is the dew point. The amount of water vapor needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing water mass. The amount of water vapor contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 8 g of water per cubic metre of air at, and 28 g of water per cubic metre of air at
Three primary measurements of humidity are widely employed: absolute, relative, and specific. Absolute humidity is the mass of water vapor per volume of air. Relative humidity, often expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature. Specific humidity is the ratio of water vapor mass to total moist air parcel mass.
Humidity plays an important role for surface life. For animal life dependent on perspiration to regulate internal body temperature, high humidity impairs heat exchange efficiency by reducing the rate of moisture evaporation from skin surfaces. This effect can be calculated using a heat index table, or alternatively using a similar humidex.
The notion of air "holding" water vapor or being "saturated" by it is often mentioned in connection with the concept of relative humidity. This, however, is misleading—the amount of water vapor that enters a given space at a given temperature is almost independent of the amount of air that is present. Indeed, a vacuum has approximately the same equilibrium capacity to hold water vapor as the same volume filled with air; both are given by the equilibrium vapor pressure of water at the given temperature. There is a very small difference described under "Enhancement factor" below, which can be neglected in many calculations unless great accuracy is required.

Definitions

Absolute humidity

Absolute humidity usually denotes the volumetric absolute humidity, which is the total mass of water vapor present in a given volume or mass of air. It does not take temperature into consideration. Absolute humidity in the atmosphere ranges from near zero to roughly per cubic metre when the air is saturated at.
Absolute humidity is the mass of the water vapor, divided by the volume of the air and water vapor mixture, which can be expressed as:
In the equation above, the absolute humidity of a mass of air varies with changes in air temperature or pressure. This makes the absolute humidity unsuitable for some applications.
Air is a gas, and the volume of a specific amount varies with pressure and temperature, per Boyle's law. Absolute humidity is defined as water mass per volume of air, and a given mass of air will grow or shrink as the temperature or pressure varies; thus, the absolute humidity of a mass of air will vary due to changes in temperature or pressure, even when the proportion of water in that mass of air remains constant.
Because of the variability of absolute humidity with changes in air temperature or pressure, use of the absolute humidity as defined above is inappropriate for computations in chemical engineering, such as drying, where temperature variations might be significant. As a result, absolute humidity in chemical engineering may refer to mass of water vapor per unit mass of dry air, also known as the "humidity ratio" or "mass mixing ratio", which is better suited for heat and mass balance calculations. The ratio of the mass of water per unit volume as defined in the equation above is also called the "volumetric humidity", which may be the preferred term in such contexts. Because of the potential confusion, British Standard BS 1339 recommends avoiding the term "absolute humidity", deprecating it in favor of "volumetric humidity", "specific humidity", and "mixing ratio". In general, when using absolute humidity, units should always be carefully checked to determine whether the volumetric or specific humidity is being used; many humidity charts are given in g/kg or kg/kg, but any mass units may be used.

Relative humidity

Relative humidity is the ratio of how much water vapour is in the air to how much water vapour the air could potentially contain at a given temperature and pressure. More formally, relative humidity is defined as the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at the same temperature.
If a sample of humid air at temperature T₁ contains water vapour with partial pressure P_w the relative humidity RH is:
where P_s is the saturation pressure of water at temperature T₁.
Relative humidity varies with any change in the temperature or pressure of the air: colder air can contain less vapour, and water will tend to condense out of the air more at lower temperatures. So changing the temperature of air can change the relative humidity, even when the specific humidity remains constant. If two parcels of air have the same specific humidity and temperature but different pressures, the parcel at the higher pressure will have the higher relative humidity.
Cooling air increases the relative humidity. If the relative humidity rises to 100% and there is an available surface or particle, the water vapour will condense into liquid or deposit into ice. Likewise, warming air decreases the relative humidity. Warming some air containing a fog may cause that fog to evaporate, as the droplets are prone to total evaporation due to the lowering partial pressure of water vapour in that air, as the temperature rises.
Relative humidity only considers the invisible water vapour. Mists, clouds, fogs and aerosols of water do not count towards the measure of relative humidity of the air, although their presence is an indication that a body of air may be close to the dew point.
Relative humidity is normally expressed as a percentage; a higher percentage means that the air–water mixture is more humid. At 100% relative humidity, the air is saturated and is at its dew point. In the absence of a foreign body on which droplets or crystals can nucleate, the relative humidity can exceed 100%, in which case the air is said to be supersaturated. Introduction of some particles or a surface to a body of air above 100% relative humidity will allow condensation or ice to form on those nuclei, thereby removing some of the vapour and lowering the humidity.
In a scientific notion, the relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapor in air to the saturation vapor pressure of water at the same temperature, usually expressed as a percentage:
Relative humidity is an important metric used in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, a rise in relative humidity increases the apparent temperature to humans by hindering the evaporation of perspiration from the skin. For example, according to the heat index, a relative humidity of 75% at air temperature of would feel like.
Because wood changes shape with changes in humidity, relative humidity is used to evaluate moisture content and size changes in wood, such as making allowances for seasonal movement in wood floors.

Specific humidity

Specific humidity is the ratio of the mass of water vapor to the total mass of the air parcel. Specific humidity is approximately equal to the mixing ratio, which is defined as the ratio of the mass of water vapor in an air parcel to the mass of dry air for the same parcel. It is typically represented with the symbol ω, and is commonly used in HVAC system design.

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 in a non-condensable phase other than air.

Measurement

A device used to measure humidity of air is called a psychrometer or hygrometer. A humidistat is a humidity-triggered switch, often used to control a humidifier or a dehumidifier.
The humidity of an air and water vapor mixture is determined through the use of psychrometric charts if both the dry bulb temperature and the wet bulb temperature of the mixture are known. These quantities are readily estimated by using a sling psychrometer.
There are several empirical formulas that can be used to estimate the equilibrium vapor pressure of water vapor as a function of temperature. The Antoine equation is among the least complex of these, having only three parameters. Other formulas, such as the Goff–Gratch equation and the Magnus–Tetens approximation, are more complicated but yield better accuracy.
The Arden Buck equation is commonly encountered in the literature regarding this topic:
where is the dry-bulb temperature expressed in degrees Celsius, is the absolute pressure expressed in millibars, and is the equilibrium vapor pressure expressed in millibars. Buck has reported that the maximal relative error is less than 0.20% between when this particular form of the generalized formula is used to estimate the equilibrium vapor pressure of water.
There are various devices used to measure and regulate humidity. Calibration standards for the most accurate measurement include the gravimetric hygrometer, chilled mirror hygrometer, and electrolytic hygrometer. The gravimetric method, while the most accurate, is very cumbersome. For fast and very accurate measurement the chilled mirror method is effective. For process on-line measurements, the most commonly used sensors nowadays are based on capacitance measurements to measure relative humidity, frequently with internal conversions to display absolute humidity as well. These are cheap, simple, generally accurate and relatively robust. All humidity sensors face problems in measuring dust-laden gas, such as exhaust streams from clothes dryers.
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. 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 and in the development of weather forecasts.