ページを印刷するThe relative air humidity
The ambient air is a gas mixture that always contains a certain amount of water in the form of
water vapor.
This water percentage is expressed in g per kg of dry air (absolute water content).
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1 m of dry air weighs about 1,2 kg at 20° C.
Depending on the temperature each kg of the air is only able to absorb a certain amount of
water vapor. When this absorptive capacity is reached the air becomes "saturated" and the
relative humidity becomes 100%.
The relative humidity is understood to be the ratio between the water vapor percent currently contained
in the air and the maximum water vapor percent at the same temperature.
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
reduced until it is equal to the water vapor content. 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.
Water vapor displacement starts.
The vapor condenses to water. Thus the moisture is removed from the air.
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R
Water vapor content in g/m in the air at the
Temp. °C
40%
-5
1,3
+10
3,8
5,1
+15
6,9
+20
9,2
+25
12,9
+30
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humidity of
60%
80%
100%
1,9
2,6
3,3
5,6
7,5
9,4
7,7
10,2
12,8
10,4
13,8
17,3
13,8
18,4
23,0
18,2
24,3
30,3
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