LAB: Finding Dew Point And Relative Humidity
2014-15
PURPOSE: To determine the dew point temperature and relative humidity of air by use of a sling psychrometer. Relate your findings
to different scenarios which are a critical part of the study of weather.
BACKGROUND: The temperature of air does not change if the wind is blowing. However, the temperature that we feel on our skin
can change dramatically if the wind is blowing. The difference is given by weathermen when they discuss the wind chill during the
winter. Wind chill lets you know what the temperature will feel like to exposed skin. This can become extremely dangerous,
especially if you cannot stay dry. How comfortable you will feel in the summer is indicated when the weathermen talks about the
heat index. This is letting you know how uncomfortable the same temperature can feel given different amounts of water vapor in
the air. If you run 1 mile on a hot humid day in summer compared to running 1 mile on a hot dry day in summer, there is a great deal
of a difference in how you feel at the end of each run. Coaches of sports are required to take heat index into account when
scheduling practices, due to the possibility of heat stroke.
MATERIALS: room temperature water, sling psychrometer, reference sheet
PROCEDURE:
1. Make sure the temperatures on both sides of the sling psychrometer are the same.
2. Take a sling psychrometer and wet only the bulb covered with cheesecloth (use the room temperature water).
3. Carefully twirl the psychrometer until the wet bulb temperature stops dropping. Record the dry bulb and the wet bulb
temperatures.
4. Repeat steps 2-3 and record the results. Average these two sets of data.
5. Repeat steps 2-4 outside and record the results.
6. Use the relative humidity chart (on reference sheet) to determine the dew point temperature and relative humidity of the room.
DATA SHEET:
inside 1st results
outside 1st results
1. Dry bulb temperature in room _______ °C
1. Dry bulb temperature outside _______ °C
2. Wet bulb temperature in room _______ °C
2. Wet bulb temperature outside _______ °C
3. Dew point temperature in room _______ °C
3. Dew point temperature outside _______ °C
4. Relative humidity of room _______ %
4. Relative humidity outside _______ %
inside 2nd results
outside 2nd results
1. Dry bulb temperature in room _______ °C
1. Dry bulb temperature outside _______ °C
2. Wet bulb temperature in room _______ °C
2. Wet bulb temperature outside _______ °C
3. Dew point temperature in room _______ °C
3. Dew point temperature outside _______ °C
4. Relative humidity of room _______ %
4. Relative humidity outside _______ %
inside averaged results
outside averaged results
dew point temperature _______ °C
dew point temperature _______ °C
relative humidity _______ %
relative humidity _______ %
LAB: Finding Dew Point And Relative Humidity
2014-15
ANALYSIS: Answer all questions in complete sentences where there is space. HINT: Remember latent heat when answering these
questions.
1. What variables determine the dew point temperature ?
2. Was there a difference between the indoor and outdoor dew points in this lab? Why ?
3. What variables determine the relative humidity ?
4. Was there a difference between the indoor and outdoor relative humidity in this lab? Why ?
5. Why does the wet bulb temperature change when the dry bulb temperature remains constant ?
6. What variables determine how much the wet bulb temperature changes ?
7. If the temperature of the air in a room goes up 5 °C and the amount of water vapor in the air remains the same, does the relative
humidity go up, go down or remain the same ? Explain your answer.
8. If the dry bulb and the wet bulb temperatures on a sling psychrometer were the same, the relative humidity would be %.
9. As a rule, the relative humidity is very high at dawn and gets progressively lower as the day goes on. Explain why this happens.
10. Why do people stand in front of fans when the are sweating ? Explain your answer.
11. Why is it that when summertime temperatures in the Arizona desert reach well over 100 °F it is more comfortable than South
Florida's 90 °F ? Explain your answer.
12. As clouds form (water vapor condenses into water droplets) should the relative humidity of the surrounding air go up, go down,
or stay the same ? Explain your answer.
13. You are planning to hike up Old Rag Mountain in the winter (in temperatures below freezing). You realize that while going up
the steep slopes you are going to exerting a great deal of energy and your body is going to sweat. Additionally, as you hike along the
down slopes your energy output will be considerably less. With this in mind, what would you suggest wearing and what would you
do as you start to sweat ? Explain your answer.