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
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".
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
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