What is the meaning of the term "Dew-Point"
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
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"
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".
An example from Meteorology
A low-pressure area carries rain-clouds (saturated
with water vapour) northwards - from the Mediterranean Sea to the Alps. When the clouds
arrive at the southern slopes of the mountains they will be pushed upwards - into layers
of air which get cooler with increasing elevation. Due to the lowered temperature the dew-point
will be underrun, the superfluous amount of water will fall out as rain.
Rain will persist
until the air in the cloud cover again reaches saturation.
last modified MAR 2013
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