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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".
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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
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