Name_________________________
Date: _______Period______
Unit Three: Meteorology
Lab Activity: How Can We Measure Relative Humidity?
The amount of water vapor in air compared with the maximum amount of water vapor that the
air can hold at a given temperature is called relative humidity. Relative humidity can be measured with
a sling psychrometer. A psychrometer makes use of the cooling effect of evaporation. As water
evaporates from a surface, it cools the surface. Water will evaporate more quickly into dry air than into
humid, moist air. The more a wet surface is cooled by evaporation, the more quickly evaporation is
taking place – the drier the air.
Objective: In this activity you will use a sling psychrometer
to determine the relative humidity of the air
Skills: measuring, interpreting a chart, comparing
Materials: sling psychrometer, water, timing device
Procedure:
1. The thermometer with the cloth is called a wet-bulb thermometer. Wet the cloth with room
temperature water.
2. Spin the psychrometer for 20 seconds. Read both thermometers. Record the temperatures in the
Table 1. Subtract the wet-bulb temperature from the dry bulb temperature and record the
difference in the Table 1.
3. Continue spinning the psychrometer and checking the temperatures until there is no further
change or until two minutes have past. Record those temperatures as final temperatures in
Table 1. Subtract the wet-bulb temperature from the dry-bulb temperature. Record the
difference in Table 1.
4. In Table 1, circle the two numbers that represents the final dry-bulb temperature the final
temperature difference.
5. On the humidity chart provided, locate the two circled values from table 1. Dry-bulb
temperature is on the left hand side and the temperature difference is along the top.
6. Follow the temperature difference down and the dry-bulb temperature to the right until the two
lines meet. Circle this value. This value represents the relative humidity of the air.
Observations:
Table 1 - Measuring Relative Humidity (Inside)
Dry-Bulb
Temperature (°C)
Wet-Bulb
Temperature (°C)
Temperature
Difference (°C)
After 20 Seconds
Final
(after 2 minutes)
Table 2 - Measuring Relative Humidity (Outside)
Dry-Bulb
Temperature (°C)
After 20 Seconds
Final
(after 2 minutes)
Wet-Bulb
Temperature (°C)
Temperature
Difference (°C)
7. Record your data for final dry-bulb temperature, final temperature difference, and relative
humidity in line #1 of Table 2.
8. Repeat steps 1 – 7 for outside measurement.
9. Record both sets of data from Table 2 onto the board. As other groups record their data on the
board record their information on the remaining lines on Table 2.
Table 2: Group Data (Inside)
Dry Bulb
Temperature
Temperature
Difference
Table 2: Group Data (Outside)
Relative
Humidity
Dry-Bulb
Temperature
#1
#1
#2
#2
#3
#3
#4
#4
#5
#5
#6
#6
#7
#7
#8
#8
#9
#9
# 10
# 10
# 11
# 11
# 12
# 12
# 13
# 13
Temperature
Difference
Relative
Humidity
Questions:
1. What was your value for relative humidity?
Inside: __________
Outside: _________
2. Calculate the average relative humidity of all groups. How close was this to your value?
Inside: ___________
Outside: _________
3. Did the temperature on the wet-bulb rise or fall? _________
4. What happens to the molecules of water on the cloth when the wet-bulb thermometer is placed
in dry air?
5. What happens to the molecules of water on the cloth when the wet-bulb thermometer is placed
in saturated air?
6. What was the purpose of spinning the psychrometer? What did the spinning psychrometer
simulate?
7. Why did relative humidity vary at the two test sights?
8. Does a great difference in temperature between dry-bulb and wet-bulb thermometers indicate a
high humidity or low humidity? Explain your answer.