LAB: Finding Dew Point And Relative Humidity
January 1, 2016
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.
One instrument which is used to determine relative humidity is the sling psychrometer. This is an instrument consisting of
two thermometers, one of which is plain (the dry-bulb thermometer) and the other of which has a wick or piece of cloth wrapped
around the bulb or bottom (the wet-bulb thermometer). The amount of moisture in the air and the temperature of the air both
influence how rapidly water will evaporate or change into water vapor and enter the air. As water evaporates, it removes heat to
make this change of state. This heat is then stored in the water vapor in the air to be released later when the water vapor returns to
its liquid or its solid state by the processes of condensation. The sling psychrometer makes use of these principles. The wick is
dampened and as water evaporates from this cloth, heat is taken with it. Thus, the wet-bulb thermometer will register the wet-bulb
temperature which is usually lower than the air temperature (the dry-bulb temperature), registered by the dry-bulb thermometer
where no evaporation has taken place. This difference in temperature is called the depression. The dry-bulb temperature is always
higher than the wet-bulb temperature except for when the air is saturated or the relative humidity is 100%. In this situation, as
water evaporates from the wet-bulb, an equal amount of condensation is returning the heat which was lost. Therefore, the two
temperatures will be the same. After both thermometers have registered their temperatures and the depression has been
determined.
As air is cooled and no moisture is added, the relative humidity will increase until the air reaches saturation (100% RH). The
temperature of the air when this occurs is called the dew point temperature. Further cooling of the air will result in condensation
because the air is no longer capable of holding all of that moisture. Once this air begins to warm again, though, it will no longer have
as much water vapor in it as previously, so the relative humidity and dew point temperature will have changed. It is possible to find
the dew point temperature of the air by using a sling psychrometer and a table similar to the one used to find the relative humidity.
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
and outside.
DATA TABLE
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 _______ %
LAB: Finding Dew Point And Relative Humidity
inside 2nd results
January 1, 2016
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 _______ %
ANALYSIS: 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.
LAB: Finding Dew Point And Relative Humidity
January 1, 2016
____________________________________________________________________________________________________________
____________________________________________________________________________________________________________
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.
____________________________________________________________________________________________________________
____________________________________________________________________________________________________________