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The Physics of Humidification

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Relative Humidity

What does "Relative Humidity" really mean ?

One of the major properties of the air around us is the ability to contain water in the form of vapour.
The air surrounding us is never really dry - it contains more or less water vapour. This amount of water, measured usually in grams per cubicmeter or in grams per weight (g/kg) depends strongly on the temperature of the air.
Hot air can take up more water vapour, cold air less.
The maximal amount of water vapour which air of a certain temperature can contain is called the "Saturation Amount". This means that air at a certain temperature carries 100% of the amount of water is can possibly contain - it is "saturated".
The saturation amount depends on the temperature. Values for the saturation amount have been determined experimentally and can be looked up in so-called "Water-Tables".

Example 1:
1 m³ air at a temperature of +30°C has a saturation amount of 30,39 g water / m³, whereas 1m³ water at a temperature of +3° can carry only 5,9 g water.
Usually the air around us is not really saturated; sometimes it is crisp and dry, sometimes hot and humid.
The well-being of all organic life - plants, animals and people - depends on the degree of saturation of the surrounding air. In all cases the amount of moisture in the air can be given only as a relative figure - depending on the saturation amount of the current temperature.

Example 2 from real life:
Weather forecast Vienna, Austria, 30. August, 12:00 hrs:
Air temperature +30°C, relative moisture 65%

Air at +30°C has a saturation amount of 30,39 g/m³, 65% of this value are currently there,
which means 30,39 x 0,65 = 19,75 g/m³

Some recommended values derived from practical experiences: 



Relative Humidity

Propagation of cuttings in Forestry ( see Vegetative Propagation )

96% to 60%

Climatization in conservatories of Botanical Gardens
(warm- and cold houses)


Cold-storage of Fruit and Vegetable (see Cool Storage )

90 - 94%

Wintergardens, private Gardening ( see Wintergardens )

75 - 85%

for people indoors - for instance Hospitals

52 - 55%

Print Shops, Paper Storage

55 - 65%



The Physics of Humidification

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last modified JAN 2020
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