The word "humidity" denotes the presence of water vapor in air or other gas. Water vapour is the gaseous form of water, and can be thought of much like any other kind of gas. It is normally transparent, and makes up about one hundredth (or one percent) of the air around us.

     Humidity arises in practice because, in the same way that hot water gives off steam, so water at lower temperatures - including ice - also gives off water vapour. Wherever there is water or ice, there is evaporation (or its opposite, condensation). The extent to which this happens depends upon a number of factors, the most important of which is temperature. Similarly, other liquid or solid materials - most of which have some water content - will give off (or sometimes soak up) water vapour. Of course, water vapour can also be found in places where there is no liquid or solid mearby, for example in remote parts of the Earth's atmosphere.

     Air (or empty space, or any other gas) has a given capacity to absorb water vapour. This capacity depends mainly on temperature. Generally speaking, the hotter the air, the more water vapour it can hold. The graph in Figure 1 illustrates this. At any particular temperature, air that contains its full capacity of water vapour, is said to be "saturated". The "relative humidity" of the air expresses how fully saturated it is with water vapour. A variety of other ("absolute") measures are used to express how much water vapour is actually present (irrespective of temperature or level of saturation).

Humidity is the term for the amount of water that is contained in a gaseous environment. With most cases, measurement of humidity is for an air environment at atmospheric pressure and ambient or raised temperatures.

     Measurement of humidity is important for the optimisation of many processes. In many cases involving heating or colling of air, the latent heat contained by the water vapour represents a large energy flow. With air-conditioning problem, humidity levels are important for calculating latent heat loads on cooling coils. In the case of drying applications, humidity levels of exhausts indicate if they are carrying their full potential of water vapour from the process.

Humidity terms and definitions

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

     Some vocabulary specific to humidity (and other common words with specialised meaning in this context) are as follows:

Relative Humidity - the actual quantity of water vapour present in a given space expressed as a percentage of the quantity of water vapour that would be present in the same space under saturation conditions at the same temperature.

Absolute Humidity - the mass of water vapour per unit mass of dry air or the mass of water vapour present in unit volume of moist air of a given temperature and presure.

Dew point - the saturation temperature of the mixture at the corresponding vapour pressure. If the gas is cooled at constant pressure to the dew point, condensation of vapour will being.

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

Adsorption (of water vapour) - retention (of water vapour) as a surfacelayer on a material

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

Desorption - release of adsorbed or absorbed subtance

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

Dry-bulb temperature - measured air temperature, usually paired with a "wet-bulb" temperaturre 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. Strrictly 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 - any subject of humidity measurement

Hygroscopic - tending to absorb water vapour

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

Moisure - commonly used to refer liquid water or water vapour in any form, "moisture" is aloso 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 refert to a tube used to extract gas for measurement.

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

Transmitter - Instrument which normaslly 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 derived a value of elative humidity.