Name:
Partner:
Period:
Due Date:
Dew Point & Relative Humidity
Introduction:
Relative Humidity is the percent of moisture (water vapor) in the air compared to the amount it can hold
at that temperature. If the relative humidity is 90% that means that the air contains 90% of the moisture (water
vapor) that it possibly can hold at that temperature. The temperature is important because warm air can hold
more water vapor than cold air. So, basically, it tends to feel more humid on warmer days then colder. Think
about July compared to January. You usually need to moisturize in the winter time compared to the summer.
This is because it is colder in winter and the air holds less moisture. Hence, the cooler the temperature, the less
the humidity.
The dew point is the temperature at which saturation occurs and condensation begins (forming fog,
clouds, or dew). Think about attending your family picnic in July. Your little sister Suzie sets the table with the
classic red and white checkered paper table cloth. Shortly after, your older brother Bobby grabs a Mountain
Dew from the cooler. Bobby dries off the can and puts it on the table cloth. Not even 5 minutes later, there is
condensation on the can and it leaves a ring on the table cloth. Suzie then sees what happened to the table cloth,
leading her to scream out, “WHY! Why did you ruin the table cloth!?!?” Bobby replies, “Well, you see what
happened is the air must be 100% saturated and the dew point must have been reached, which caused the
condensation to form on the can.” So, watch out, if the air is 100% saturated, you may ruin the picnic-don’t do
what Bobby did!
Purpose:
This exercise will deal with measuring relative humidity and dew point. Measurements will be taken
through the use of a sling psychrometer, which is a scientifically prepared device for measuring relative
humidity and dew point.
Materials:
-Sling Pyschrometer (wet-and-dry bulb hygrometer)
-Beaker/Cup of room temperature water
-Stopwatch (with second hand)
-ESRT
Safety:
Avoid breakage of Thermometers
Keep the sling psychrometer at least six inches from you and your partner
Procedure (Indoor and Outdoor Steps):
1. Record the Current Weather Data in the appropriate table on page 2.
2. Dip the end of the psychrometer into the cup/beaker of water to moisten the wick of the wet-bulb,
shaking off excess water. Press on wick to prevent spraying.
3. Hold the sling pyschrometer up above eye level (but not over your head), and sling for 90 seconds.
4. At the end of the 90 seconds, record both the wet-bulb and dry-bulb temperatures in the appropriate
table on page 2.
5. Repeat steps 2-4 for two additional Trials.
Adapted by C.Carroll 01/2012
Original from K.Kirsch
INDOOR Measurements
Trial
Number
Wet-Bulb
Reading
Dry-Bulb
Reading
Difference between
Wet Bulb & Dry Bulb
Dew Point
Temperature
Relative
Humidity
________°C
________°C
________°C
________°C
________%
________°C
________°C
________°C
________°C
________%
________°C
________°C
________°C
________°C
________%
1
2
3
OUTDOOR Measurements
Trial
Number
Wet-Bulb
Reading
Dry-Bulb
Reading
Difference between
Wet Bulb & Dry Bulb
Dew Point
Temperature
Relative
Humidity
________°C
________°C
________°C
________°C
________%
________°C
________°C
________°C
________°C
________%
________°C
________°C
________°C
________°C
________%
1
2
3
CURRENT Weather from Internet:
Current
Temperature
Dew Point
Temperature
Relative
Humidity
________°C
________°C
________%
Questions (Answer on separate sheet):
1. Determine the average relative humidity and dew point for both indoors and outdoors.
2. Why can we use the same data to compute both the relative humidity and the dew point?
3. Where was the humidity the lowest (indoors or outdoors)?
4a. Why do you believe the humidity was lowest at that location?
4b. If there is a difference between indoors and outdoors, what are the reasons?
5. Why is the wet bulb reading always cooler than the dry bulb reading?
6. If it was a rainy, overcast day. And the room felt sticky and damp, what results would you expect for both
the relative humidity and the dew point? Why?
Conclusions (Answer on separate sheet):
1. Why does your sweat evaporate on a dry day versus a humid day?
2. Human hair is a very sensitive hygrometer. Explain how human hair can be used as a scientific tool.
3. Compare your results to Current Weather Data and explain the similarity or difference.
Adapted by C.Carroll 01/2012
Original from K.Kirsch