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In the description of "Relative Humidity" we have learned that air tries to take up water
in the form of water vapour until it is "saturated".
The saturation amount is dependent of the air-temperature.
If the saturation amount has been taken up - i.e. the saturation amount is
contained in the air - it is said that the "Saturation Point" is
reached.
In
the discussion so far we have taken for granted that we have knowledge
about the air temperature.
Now let us take an opposite point of view.
§ Suppose we have a
volume of air with a known amount of water vapour, yet do not know its
temperature.
§ Looking up the
"Steam Tables" we can deduct the temperature, at which the air
would be saturated.
§ This temperature
is called the "Saturation-Temperature" or "Dew-Point".
§ The dew-point is
therefore the temperature at which a volume of air with a certain amount of
water vapour has reached its maximal water contents - 100% .
§ This temperature
may be increased or decreased by heating or cooling.
§ If the
temperature increases (with constant water content) it will no longer be
saturated and we arrive at a relative moisture with values below 100%.
§ If the
temperature is decreases, e.g. by cooling, the humidity should increase; it
cannot, however exceed 100%, as the air was already saturated with water
vapour.
§ The volume of air
therefore cannot carry the total amount of water anymore and the
superfluous amount of water will fall out - it "condenses".
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