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When molecules from the vapor above a liquid surface impinge on the
surface, they may be trapped there, so that a constant two-way traffic of
molecules to and from the liquid occurs. If the density of the vapor above the
liquid is sufficiently great, as many molecules return as leave it at any
time, a situation that is described by saying that the region is saturated with the
substance. The higher the temperature, the greater the maximum vapor density:
at 0° C the density of water vapor at saturation is 5 g/m3, at
20° C it is 17 g/m3, at 100° C it is 598 g/m3, and at
300° C it is all the way up to 45.6 kg/m3.
If for any reason (such as a sudden drop in temperature) the vapor
density exceeds the saturation value, condensation will be more rapid than
evaporation until equilibrium is reestablished. It is for this reason that on
a hot day moisture condenses on the outside of a glass that contains a cold
drink. The relative humidity
of a volume of air describes its degree of saturation with water vapor.
Relative humidities of 0, 50%, and 100% mean respectively that no water vapor
is present, that the air contains half as much moisture as the maximum
possible, and that the air is saturated. On a hot day the evaporation of sweat
from the skin is the chief means by which the human body dissipates heat, and
a high relative humidity is uncomfortable because it impedes the process. A
low relative humidity is also undesirable because it leads to the drying of
the skin and mucous membranes. The regulation of relative humidity is as
important a function of a heating or of an air-conditioning system as the
regulation of temperature.
We know that heating air decreases its relative humidity and cooling
air increases its relative humidity. For instance, between 10° C and 20° C
the saturated vapor density (which corresponds to 100% relative humidity) just
about doubles. This means that if outside air at 10° C whose relative
humidity is say, 70% is taken inside a house and heated to 20° C, the relative humidity
indoors will only be 35% since the actual vapor density stays the same. A way
to humidify heated air in winter is clearly desirable. If the outside air is
at 30° C with 70% relative humidity, then cooling it down to 24° C is enough
to bring it to saturation, which is 100% relative humidity; further cooling
will cause water to condense out. An air-conditioning system therefore should
incorporate means to remove water vapor from the air being cooled.
Relative humidity is water vapor density relative to saturation
density. Changing the temperature
of a body of air also changes its relative humiditiy. |
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Page Last Updated: Friday March 02, 2007 Webmaster: Larry Jones Pickens County School District |