Humidity, Relative Humidity, Dew Point and Sling Psychrometer Work
Package
What is always in the air, invisible and can make you wet, even if it’s not raining? Water.
Water vapor is the invisible, gaseous form of water that hangs around in our
atmosphere along with liquid water (cloud droplets, rain drops) and ice water (snow, hail
and sleet). When the air contains abundant water vapor, we say the air feels humid;
when the air has very little water vapor in it, we say the air feels dry or arid.
Where does water vapor come from? The primary source is from the Earth’s vast
reservoirs of liquid water, the oceans. The sun provides the energy to convert liquid
water into water vapor through the process of evaporation. Just as a puddle in the street
gradually disappears after the sun has warmed it up, the surface waters of the Earth’s
tropical oceans also evaporate each day. When humid air rises, it cools, and condenses
into water droplets we recognize as clouds or fog. This water vapor is carried along
Earth’s global winds and moving air masses, eventually to be released back to the
surface as precipitation.
Humidity, when simply expressed as a measure of how much water vapor is present at
a given temperature is called, absolute humidity. On warm, humid days, we sense this
by observing how muggy or sticky the air feels. Water or sweat form your skin cannot
evaporate easily because the air already carries much water vapor. More commonly,
though, humidity is reported as relative humidity. For a given temperature, relative
humidity is expressed as a percentage of how much moisture the air actually contains
relative to how much moisture the air could contain. In other words, air like a sponge,
could “absorb” that much volume of water.
For example, at 30C, warm air can hold a maximum of 26 g/m3 (grams per cubic
meter) of air. If, in our example, that much water was indeed present, the air would be
“filled” with water vapor and no more evaporation could take place. At this point, the air
becomes saturated, condensation begins to take place, and water droplets form on tiny
particles in the air or on cool surfaces along the ground. The temperature at which the
air becomes saturated with water vapor is called the dew point. In our example 30C is
the dew point and the relative humidity is 100% because the air is holding all the
moisture it can. If the same air only contains 13 g/m3, the relative humidity would be
50%. When the air temperature cools to the dew point, dew, fog and clouds can form.
Frost forms when the air temperature is low enough that the water vapor turns directly
into ice.
A psychrometer is used to measure both the dew point and relative humidity. It is based
on two important principles:
1.
When water evaporates, heat is required to change water into water vapor. As a
result, evaporation has a definite cooling effect. This cooling effect is a familiar
2.
one. How do you feel when you get out of a pool on a warm day? That “chilled”
feeling is the cooling effect of water evaporating from your skin.
Warm air can hold more moisture than cool air. In fact, air at near room
temperature, 20C, holds twice as much water vapor as air at 10C. That is why
during sticky summer days, a sudden drop in temperature often brings welcome
relief from the high humidity.
A psychrometer, therefore, is based on the fact that the more humid the air, the less
evaporative cooling can take place. Notice that your psychrometer has two
thermometers, a dry-bulb and a wet-bulb. The dry-bulb thermometer simply records the
temperature of the air while the wet-bulb thermometer is used to record how much
evaporation takes place at that temperature. If the air is dry, lots of evaporation can take
place, causing a marked cooling effect on the wet-bulb thermometer. If, however, the air
is humid (but not saturated), the wet-bulb will not cool as much and the difference
between the two thermometers will not be as great.
Investigation 1: Determining the Dew Point
Attach a 1 inch piece of muslin around the bulb of one of the two glass thermometers.
This will serve as the wet-bulb and must be soaked with distilled water just before the
measurement is made. The other thermometer without the soaked muslin will serve as
the dry-bulb. This simply records the air temperature.
Wearing a pair of goggles, carefully whirl the two thermometers in the air for about 30
seconds (or until the wet-bulb temperature stops falling and remains constant).
Immediately after whirling the “sling” psychrometer read and record the temperature
indicated on both thermometers in a table like the one below. The difference in the
readings between the wet-bulb and dry-bulb is a measure of the amount of heat energy
that is needed at the wet-bulb temperature to produce air saturated with water vapor.
This measure is called the wet-bulb depression.
Table 1: Dry-bulb Reading, Wet-bulb Depression and Dew Point
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
To determine the dew point temperature, find the dry-bulb or air temperature along the
left column of the Dew Point Chart (Figure 1). Record this on the table. Next, find the
vertical column that corresponds to the wet-bulb depression (difference between drybulb and wet-bulb temperatures). Record this on the table. Follow the row containing
the dry-bulb temperature until you reach the intersection of the column containing the
wet-bulb depression. This value is the dew point. Record the on the table.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Investigation 2: Determining Relative Humidity
From your previous investigation, re-record the values of the dry-bulb or air temperature
and the wet-bulb depression onto a table like the one below. Using the Relative
Humidity Chart (Figure 2) follow the row containing the dry-bulb temperature until you
reach the intersection of the column containing the wet-bulb depression. The resulting
value is the relative humidity. Record this on the table.
Table 2: Dry-bulb Reading, Wet-bulb Depression and Relative Humidity
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Using pages 557-561 in your textbook, or the above information, answer the following:
1.
Define humidity:
2.
What is another term used to describe humidity?
3.
What is the name of the instrument that measures the humidity of the air?
4.
Define relative humidity:
5.
What is the mathematical formula for calculating relative humidity?
6.
What does the term saturated air mean?
7.
Define dew:
8.
Define dew point:
9.
What is a psychrometer?
10.
Explain the two temperature readings you get when using a psychrometer.
11.
What is the wet-bulb depression?
12.
If you have ever worn eye glasses and walked into a warm house on a cool
winter evening, or have opened the oven with your face a little too closely, you
have noticed that your glasses have begun to fog up. Explain why this happens.
13.
When does dew form in your bathroom? Explain why this happens.
Answer questions 2-4, 8 on page 561 in your textbook.