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4
The absolute humidity
is understood to be the water content in the dry air unit volume and is
expressed in grams (g) of the water per a kilogram (kg) of the dry air.
The relative humidity
is understood to be the ratio between the water percent currently
contained in the dry air and the maximum possible water content at certain temperature and
pressure and is expressed in percents (%).
When the air absorbs the maximum possible water volume the air becomes "saturated" and
the relative humidity becomes 100%.
The ability of air to absorb water vapor increases with temperature increasing. This
means that the maximum (absolute) water content increases with temperature
increasing.
Water vapor condensation
The maximum water vapor volume that can be absorbed increases when the air is
heated
,
but the water vapor content remains however the same and this results in relative humidity
reduction.
In contrast, when the air is
cooled
, the maximum water vapor volume that can be absorbed
continuously reduces, the water vapor volume contained in the air remains however the same
and the relative humidity increases.
If the air temperature falling continues the capacity to absorb the water vapor volume is
continuously reduced until the air becomes saturated which is equal to the maximum water
vapor content in the air. This temperature is called the dew-point temperature.
When the air is cooled below the dew-point temperature, the water vapor content becomes
higher than the maximum possible water vapor content.
Excess water vapor displacement starts. The vapor condenses to water and thus is removed
from the air.
A misted window glass in winter or a misted bottle with a cold drink are common
examples of condensing.
The higher the relative air humidity the higher is the dew-point temperature,
and thus it is easier to fall below the dew-point temperature.
-5
+10
+15
+20
+25
+30
1,3
3,8
5,1
6,9
9,2
12,9
1,9
5,6
7,7
10,4
13,8
18,2
2,6
7,5
10,2
13,8
18,4
24,3
3,3
9,4
12,8
17,3
23,0
30,3
40%
60%
80%
100%
3
Water vapor content in g/m in the air at the
humidity of
Temp. °C
