Many terms specific to humidity are derived from the ancient Greek words hydor (water), hygros (wet) and also psychros (cold).

Meanings of general terms relevant to humidity:

Absorption (of water vapour) – retention (of water vapour) by penetration into the bulk of a material

Adsorption (of water vapour) – retention (of water vapour) as a surface layer on a material

Condensate – condensed material, e.g. liquid water or ice

Desorption – release of adsorbed or absorbed substance

Desiccant – any substance which exerts a drying action by chemically absorbing water vapour

Dry-bulb temperature – measured air temperature, usually paired with a “web-bulb” temperature to derive a value of relative humidity.

Humidity – the presence of water vapour in air or other gas. Some people use “humidity” to mean relative humidity only. Strictly speaking, “humidity” also refers to all kinds of absolute indications of humidity. For very low humidities, other more specific terms tend to be used.

Hygrometer – any instrument for measuring humidity

Hygrometry – the subject of humidity measurement

Hygroscopic – tending to absorb water vapour

Inert gas – chemically nonreactive gas, such as nitrogen, helium, argon, etc.

Moisture – commonly used to refer to liquid water or water vapour in any form, “moisture” is also the term particularly used to mean water that is absorbed or bound into any material

Probe – the part of an instrument that houses the sensor remotely from the main body of the instrument, e.g. at the end of a connecting electrical lead. In some situations the word “probe may be used to refer to an entire hygrometer. Also loosely used interchangeably with “sensor” and “transmitter”. “Probe” may also refer to a tube used to extract gas for measurement.

Sensor- the active or sensing part of a measuring instrument. There are some cases where a whole hygrometer is referred to as a “sensor”. Also loosely used interchangeably with “probe” and “transmitter”.

Transmitter – Instrument which normally gives an electrical output (analogue or digital) rather than a displayed result. The sensing head may be an integral part of the transmitter or may be connected via an external cable. Also loosely used interchangeably with “probe” and “sensor”.

Wet-bulb temperature – temperature indicated by a thermometer sheathed in wet wicking, and influenced by the rate of evaporation from the wicking. Usually paired with a “dry-bulb” temperature to derive a value of relative humidity.

Definitions of measured quantities

Many of the following definitions are based on those in the British Standard BS 1339: 1965 (confirmed 1981, under revision in 1996), “Definitions, formulae and constants relating to the humidity of the air”.

 

As given below, the definitions are explanatory rather than rigorous. For rigorous definitions, BS 1339 or other definitive documents should be consulted.

 

In practice, the usage of some terms varies according to the context: for example the terms in the field of air-conditioning are sometimes different from the terms used in meteorology for the same quantities. In each case a preferred term is given below, but qualifying notes indicate where there are common alternatives in use.

Unites of measurement for expressing the quantities are given, and may have alternative forms, e.g. “grams per cubic metre” is given by “ g m-3”, alternatively written “g/m3”.

Formulae inter-relating some of these quantities are given in Section 10 (Tables, charts and formulae).

Absolute humidity – The mass of water vapour present in unit volume of moist air of a given temperature and pressure. SI (metric) units are grams of water per cubic metre of air (g m-3). Older references may be in terms of pounds per million cubic feet (lb ft-6) or in grains per cubic foot

(gr ft-1). (One grain ? 0.0648 gram.).

Note: In chemical engineering the preferred term for this concept is “volumetric absolute humidity”, while “absolute humidity” is used to denote the quantity referred to in this document as “mixing ratio”. In meteorology the preferred term is “vapour concentration”. Other terms such as “vapour density”, “mass concentration” and “moisture content by volume” are also sometimes used to mean the same thing.

Usage: It is important not to confuse the particular quantity “absolute humidity” with the general category of “absolute measurements of humidity”.

Dew point (or dew-point temperature) – the temperature at which dew, or condensation, forms, on cooling a gas. This is, in effect, the temperature at which air becomes saturated in equilibrium with water. Expressed in degrees Celsius (°C) or occasionally in other units of temperature (e.g. in degrees Fahrenheit (°F) in USA). See also frost point.

Usage: When used as a noun, the two-word expression is not hyphenated:

e.g. “… a dew point of 10 °C …”    “… dew points between -10 °C and 0 °C …”

When used as a modifier, or adjective, the expression has a hyphen:

e.g.  “… dew-point hygrometer …”

Usage as a single word is also widely accepted (in either context):

e.g.  “… dewpoint of 10 °C …”    “ … dewpoint hygrometer …”

Negative dew points, with respect to supercooled water below 0 °C, are always shown with a minus (-) sign. Where there is any risk of ambiguity, a plus (+) sign may also be used for for positive dew points: e.g.   “ … a range of dew points between -5 °C and +5 °C … “

The term “dew point” is often used generally to include “frost point” (see below). However in the range just below 0 °C, where either frost or dew (supercooled water ) can form, the values of dew point and frost point differ.

The use of initials (e.g. “dp”) is not a recognised abbreviation, but it occurs widely, and is used to distinguish clearly between dew-point temperatures and other values of (air) temperature. For example a dew point value might be expressed in the form “… 1- °C dp …”

Enthalpy (of humid gas) - Measure of the total energy in a humid gas.   Enthalpy is a function of the gas temperature and pressure, and of the moisture content, since water absorbs energy on changing from condensed state to vapour. Enthalpy is a useful concept in air conditioning, where it is important to know how much of the "stored" energy will be consumed, or released, when the temperature or water content is raised, or lowered.  Enthalpy of a gas can be defined as the sum of "sensible" and "latent" heat for each component in the gas. (See below for definitions of sensible heat and latent heat.) Values of enthalpy are conventionally expressed relative to a datum point (i.e. a zero or base line). For a dry gas, this is normally the heat content at 0 °C. For water vapour, the enthalpy is usually expressed relative to the heat content of liquid water at 0.01 °C. Expressed in terms of energy per quantity of dry gas, i.e. kilojoules per kilogram (kJ kg'1) (or other units, for example British thermal units per pound, Btu/lb).

Equilibrium relative humidity (ERH) (over a substance) - The value of relative humidity of the air at which            there is no net exchange of moisture with any nearby substance.  This is used for indirectly indicating or controlling the condition of moisture-sensitive substances such as paper. Expressed as a percentage (%). (See also water activity.)

Frost point (or frost-point temperature) - The temperature at which frost forms on cooling a gas. This is, in effect, the temperature at which air is saturated in equilibrium with ice. It is the exact counterpart to dew point (though values differ). Expressed in degrees Celsius, °C, or occasionally in other units of temperature, i.e. in degrees Fahrenheit (°F), in USA. (See also dew point.)

Humidity ratio - Mixing ratio

Latent heat - Heat stored in a substance but not directly related to its temperature. For example, heat is stored in a gas because this heat was originally supplied to evaporate it. "Latent" means "hidden". Expressed in energy per unit mass of substance, i.e. joules per kilogram (J kg"1). (See also enthalpy, sensible heat.)

Mixing ratio - Mass of water vapour per unit mass of dry air with which it is associated. It is a dimensionless ratio, but is often expressed in grams of water per kilogram of dry gas (g kg"1) or in other units of mass.

For low levels of moisture content, this may be expressed in parts per million by weight, i.e. mass of water vapour per million parts mass of dry gas (ppmw or ppm(w)).

NOTE: In chemical engineering this quantity is normally termed "absolute humidity" - but must not be confused with the definition of "absolute humidity" given above. Mixing ratio is also alternatively known as "humidity ratio".

Moisture content - A humidity term best reserved for general descriptive or qualitative use only. Use of this term to identify a measured quantity should be avoided, as there is a risk of confusion because "moisture content" has been used in the past to mean both mixing ratio and specific humidity. Moisture content is also a term particularly used to refer to the proportion of water held in liquids or solids.

Mole - Amount of substance which contains as many elementary entities as there are atoms in 12 grams of carbon 12. Expressed in moles (symbol, mol).

NOTE: When the mole is used, the elementary entities must be specified as atoms, molecules, etc.

Mole fraction - The mole fraction of a component is the ratio of the amount (number of moles) of that component to the total amount of substance present. Expressed as a  dimensionless ratio.

Partial pressure (of water vapour) - The part of the overall pressure exerted by the water vapour component in a gas. Expressed in units of pressure such as pascals (Pa) or in multiples; kilopascals (kPa) or megapascals (MPa), with non-Si alternatives such as millibar (mbar) or millimetres of mercury (mmHg). 100 Pa = 1 mbar = 0.75 mmHg.

Parts per million - Abbreviated as "ppm", it must always be stated whether this is by mass (weight) or by volume, and whether the figure is the ratio of water vapour to dry gas, or to total (moist) gas.

Parts per million by volume (ppmv, ppm(v)) - Volume of water vapour per total volume of gas, for an ideal gas. Sometimes expressed relative to the total volume of moist gas (mole fraction times one million) or sometimes relative to the total dry gas. For small numbers of parts per million, the two are almost identical; at higher humidities they become significantly different.

Parts per million by weight, or mass (ppmw, ppm(w)) - Sometimes used to express the amount (mass) of water vapour relative to the total dry gas (mixing ratio times one million), but sometimes to express the amount relative to the total mo ist gas (specific humidity times one million). For small numbers of parts per million, the two are almost identical; at higher humidities they become significantly different.

Percentage saturation - The ratio of the actual mixing ratio to the saturation mixing ratio at the same temperature, expressed as a percentage (%).

NOTE. Under ordinary climatic conditions the percentage saturation is almost identical to the relative humidity.

Relative humidity - The ratio of the actual vapour pressure to the saturation vapour pressure over a plane liquid water surface at the same temperature, expressed as a percentage. This is commonly understood when the term "X percent relative humidity" is used.

USAGE: The phrase "relative humidity" is commonly abbreviated RH although this is not an a recognised abbreviation. Values of relative humidity are commonly expressed in units of percent relative humidity (%rh).

Care must be taken when expressing uncertainties, changes or fractional differences in relative humidity. For example, the difference between 50 %rh and 52 %rh is 2 %rh. This can also be expressed as a difference of 4% of value. It is important to distinguish clearly between these two kinds of statement.

Saturation vapour pressure (of water) - maximum pressure of water vapour that can exist at a given temperature. Expressed in units of pressure e.g. in pascals (Pa), or in non-Si units such as millibars (mbar) or millimetres of mercury (mm Hg).

Sensible heat (of a gas) - energy that resides in a gas according to its temperature. Expressed in terms of energy per mass of gas, e.g. in joules per kilogram (J kg '), or equivalent units. (See also enthalpy, latent heat.)

Specific humidity - mass of water vapour per unit mass of humid air. May be expressed as a dimensionless ratio, or in grams of water per kilograms of humid gas (g kg"1) or in kilograms per kilogram (kg kg"1)

Vapour pressure - that part of the total pressure contributed by the water vapour. Expressed in units of pressure e.g. in pascals (Pa), or in non-metric units such as millibars (mbar) or millimetres of mercury (mm Hg).

Water activity (of a substance) - Water activity (aw) is the relative humidity which is eventually reached in a closed space where a hygroscopic substance, such as a foodstuff, has been placed. It is the same as equilibrium relative humidity (ERH) except that it is expressed on a scale of 0 to 1 (no units), instead of 0% to 100%. Water activity is particularly used in connection with foodstuffs. (See also equilibrium relative humidity.)

*Source: A Guide to the Measurement of Humidity Nationa Physical Laboratory (NPL), The Institute of Measurement and Control, London, 1996

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