What does "Relative Humidity" really
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
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