Humidity

Humidity is the amount of water vapour present in air. Water vapour, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. The amount of water vapour 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 vapour contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 grams of water per cubic metre of air at 30 °C, but only 8 grams of water per cubic metre of air at 8 °C. Humidity is the amount of water vapour present in air. Water vapour, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. The amount of water vapour 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 vapour contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 grams of water per cubic metre of air at 30 °C, but only 8 grams of water per cubic metre of air at 8 °C. Three primary measurements of humidity are widely employed: absolute, relative and specific. Absolute humidity describes the water content of air and is expressed in either grams per cubic metre or grams per kilogram. Relative humidity, 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 (sweating) 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, also known as a humidex. Absolute humidity 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 30 grams per cubic metre when the air is saturated at 30 °C (86 °F). Absolute humidity is the mass of the water vapor ( m H 2 O ) {displaystyle (m_{H_{2}O})} , divided by the volume of the air and water vapor mixture ( V n e t ) {displaystyle (V_{net})} , which can be expressed as: The absolute humidity changes as air temperature or pressure changes, if the volume is not fixed. This makes it unsuitable for chemical engineering calculations, e.g. in drying, where temperature can vary considerably. 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 (see 'specific humidity' below), which is better suited for heat and mass balance calculations. Mass of water per unit volume as in the equation above is also defined as volumetric humidity. Because of the potential confusion, British Standard BS 1339 suggests avoiding the term 'absolute humidity'. Units should always be carefully checked. Many humidity charts are given in g/kg or kg/kg, but any mass units may be used. The field concerned with the study of physical and thermodynamic properties of gas–vapor mixtures is named psychrometrics. The relative humidity ( R H {displaystyle (RH} or ϕ ) {displaystyle phi )} of an air-water mixture is defined as the ratio of the partial pressure of water vapor ( p H 2 O ) {displaystyle (p_{H_{2}O})} in the mixture to the equilibrium vapor pressure of water ( p H 2 O ∗ ) {displaystyle (p_{H_{2}O}^{*})} over a flat surface of pure water at a given temperature: