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Temperature, Dew Point, and Relative Humidity, Continued
Measuring Dew
As already mentioned, the amount of water vapor the atmosphere can
Point and
hold varies with the temperature. When the atmosphere contains all the
Relative
Humidity
water it can hold for a given temperature, the air is at the saturation
point or humidity is 100%. If it contains 50% of what it could hold at
that particular temperature, relative humidity is 50%. Relative humidity
and dew point are determined through the use of a psychrometer.
Using the Sling
A psychrometer is simply two ordinary thermometers mounted together
Psychrometer
on a single strip of material. The bulb of one thermometer is covered
by a water-soaked wick from which the water evaporates rapidly or
slowly, depending on the amount of water vapor in the surrounding
atmosphere.
Evaporation of water around the wet thermometer cools it. The amount
of cooling depends on the rate of evaporation. The reading on the wet
bulb is lower than the reading on the dry bulb except when the humidity
is 100%, at which time both readings coincide. The difference between
the wet-bulb and dry-bulb readings, when applied to tables developed
for that purpose, results in relative humidity and dew point temperature.
The dew point is the temperature to which air must be cooled at
constant pressure and constant water vapor content to reach saturation
(100% relative humidity). When air is cooled to its dew point
temperature, small water droplets condense on objects; that is, dew
forms.
A sling psychrometer (fig. 10-12) is sometimes used to speed up the
process of getting accurate wet-bulb and dry-bulb readings. The sling
psychrometer can be whirled around to rapidly bring the wet bulb into
contact with a great volume of air. This contact with air accelerates the
evaporation rate. The person using the sling psychrometer should face
the wind and should shield the instrument as much as possible from the
direct rays of the Sun. The whirling should be repeated until no further
change can be detected in the wet-bulb reading.
10-27
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