Download lab 6-7: dewpoint and relative humidity

Name: _____________________________________________
UNIT 6: Meteorology
Date: ______________
Period: _____________ Partner _______________________________
LAB 6-7: DEWPOINT AND RELATIVE HUMIDITY
INTRODUCTION: As the air cools, it holds less water vapor. The dew point is the temperature to
which the air must cool to be saturated, or filled to capacity with water vapor. When this
temperature is reached, we notice condensation, water droplets from the air, forming on objects.
The relative humidity is determined by using wet and dry bulb thermometers. If the air is very
dry, the water evaporates rapidly and the wet thermometer becomes cool. But if the air is moist,
water evaporates slowly. Then the wet bulb thermometer does not cool down as much. By
comparing the wet bulb and dry bulb temperatures, you can find the relative humidity of the air.
The relative humidity tells us what percent of the air's capacity is filled with water vapor.
Clouds are composed of water droplets which condense out of the air. In order for this to
happen, two things must occur. First, the air must cool so that condensation can occur. Second,
there must be tiny particles, such as dust, present in the air. These particles, called
condensation nuclei, give the water droplets something to form on.
OBJECTIVE: In this lab you will determine both the dew point and relative humidity of the classroom.
VOCABULARY:
water vapor:
dry-bulb temperature:
relative humidity:
wet bulb temperature:
condensation nuclei:
MATERIALS:
Sling psychrometer
cloth
rubber band
LabQuest
2 temperature probes
cloud formation globe
1
smoke paper
matches
water
PROCEDURE A: DEWPOINT DETERMINATION USING ELECTRONIC PROBES
1. Obtain a LabQuest from the charging station.
2. Insert the temperature probes into Channel 1 and Channel 2 on the interface
3. Turn on the unit. Temperatures should appear in ºC.
4. Use a rubber band to secure a piece of cloth on the tip of one temperature probe. This will
be the wet bulb.
5. Moisten the cloth with room temperature water.
6. Shake the probe or fan it until the temperature of the wet bulb stops dropping.
7. Record both the dry bulb and wet bulb temperatures in the Report Sheet.
8. Disconnect the temperature probes from the LabQuest.
9. Return the unit to the charging station.
PROCEDURE B: DEWPOINT DETERMINATION USING A SLING PSYCHROMETER
1. Moisten the cloth attached to the sling psychrometer with room temperature water.
2. Spin the psychrometer until the temperature of the wet bulb stops dropping.
3. Record both the dry bulb and wet bulb temperatures in the Report Sheet.
4. Use your reference tables to determine the dew point and relative humidity of the
classroom.
REPORT SHEET
Readings with
Temperature Probes
Dry Bulb Temperature ºC
Web Bulb Temperature ºC
Depression (Difference) ºC
Dew Point ºC
Relative Humidity %
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Readings with Sling
Psychrometer
PROCEDURE C: USING REFERENCE TABLES
Use the Earth Science Reference Tables to complete the chart below.
Dry Bulb
Temperature (ºC)
Wet Bulb
Temperature (ºC)
14
12
Dew Point
(ºC)
28
18
8
-9
22
17
Relative
Humidity (%)
53
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PROCEDURE D: CLOUD FORMATION
1. Pull about a teaspoon of water into the cloud-forming globe
by squeezing and releasing the bulb while the tube end is in
some water.
2. Cut a long, thin strip of the smoke paper about ½ " wide. Light
the end of the paper.
3. Draw a tiny amount of smoke into the globe as you did in step 1.
You should barely be able to see the smoke inside.
4. Clamp the tube end of the globe using the clamp provided.
5. Squeeze the bulb, then release the bulb. The "fog" you see is the
formation of a cloud.
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DISCUSSION QUESTIONS: (Answer the following in complete sentences on a separate sheet
of lined paper. You may type your answers.)
1. Explain why the wet bulb temperature drops below the dry bulb temperature.
2. Why can warm air hold more water vapor than cold air?
3. Why does fog usually form during the morning hours?
4. Why is the moisture in the air more noticeable to people during the summer months?
5. Refer to the table for finding relative humidity. What would be the difference between the dry and
wet bulb temperatures if you had measured the relative humidity to be 100%?
6. People use humidifiers in winter to keep them warmer and dehumidifiers in summer to keep them
cool. Based on the principles studied in this lab, explain how this works.
7. Why must an air conditioner be placed in an outside window?
8. Based on this lab, why is an air conditioner more effective than a fan in keeping you comfortable
on hot, humid days?
9. Using the dew point you obtained from the sling psychrometer as your measured value, and the
dew point you obtained from the electronic probes as your accepted value, determine the
percentage deviation. SHOW ALL WORK.
10. How was the temperature of the air inside the cloud formation globe affected when you squeezed
and then released the bulb?
11. Explain why no cloud would form if there were absolutely no water vapor present.
12. What was the purpose of adding smoke to the globe?
CONCLUSION: Describe IN DETAIL, step-by-step, how you would determine the dew point and
relative humidity in a room.
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