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Humidity and Dew-Point Instruments

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The following text is reproduced with kind permission

from the National Physical Laboratory. It was originally

published in a booklet,

A Guide to the Measurement of

Humidity.

1 What is humidity?

The word ‘humidity’ denotes the presence of water

vapour in air or other gases. 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 nearby, 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).

Deﬁ nition and explanations of the most important of

these terms are given in the next Section.

1.1 Humidity terms and deﬁ nitions

Some vocabulary speciﬁ c-to-humidity and other

common words with specialised meanings in this

context are as follows:

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 ‘wet-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 humidity, other more speciﬁ c 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 non-reactive 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

A Guide to the Measurement of Humidity

Figure 1. Saturation vapour pressure of water increases with temperature

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