Name ______________________________________________ Date_______________ Per__________ Lab#_________
Relative Humidity Lab
Objective – to use a psychrometer to measure relative humidity.
Background Information
Have you ever heard the word “muggy” used to describe the weather? How about the phrase
“hazy, hot and humid”? These phrases are used to describe times when there is a considerable
amount of moisture in the air. Of course, there is always moisture in the air, but the amount varies. If
the air is warm and dry, it is easier for water to evaporate and enter the air as water vapor. If the air is
cold and already holds a lot of water vapor, much less will evaporate into the air
At every temperature there is a limit to the amount of water vapor that the air can hold. The
warmer the air is, the higher the limit is. When air is holding all the water vapor it can, it is said to be
saturated. Usually the air is unsaturated.
Relative humidity is a measure of how much water vapor present compared to how much it
can hold at that temperature. On a muggy day relative humidity can be high, as much as 80 – 90%.
When the relative humidity is high, perspiration does not easily evaporate. This means our bodies’
cooling mechanisms are less effective. As a result, we feel uncomfortable.
Saturated air has a relative humidity of 100%. Clouds or fog form when, and where, is
saturated. In desert areas there is little water to enter the air and relative humidity is low. In this lab,
we will measure the relative humidity using an instrument called a sling psychrometer.
Data Table
Location
Time Of Day
Dry Bulb Temp.
(°C)
Wet Bulb Temp.
(°C)
Difference
(°C)
Relative Humidity
(%)
Calculating Relative Humidity
A sample of air ar 25C has 15 g/m3. The maximum capcity for air at 25C is 22 g/m3. What is the
relative humidity of the air? (Show Work, Box Answer)
Determining Relative Humidity
Measuring the amount of water is not the only way to determine relative humidity. The relative
humidity can be determined by using a sling psychrometer.
2 thermometers (one wet and one dry) are needed. The wet thermometer will evaporate water
until the air is saturated. Because evaporation is a cooling process the temperature of this
thermometer will decrease proportionally to the amount of water that evaporated. The difference
between these thermometers is then compared to determine the relative humidity.
If the wet bulb had a reading of 15C and the dry thermometer had a reading of 20C then what is the
relative humidity?
1. Find the difference of the two thermometers. (Show Work, Box Answer)
2. Refer to Table 1 to use the differences from above and the dry bulb temperature to determine
the relative humidity. (This # should be a %, Box Answer)
Materials
-
1 Sling Psycrometer
Room temperature water
Procedure
Inside:
1. Put a drop or two of water on the sock wrapped thermometer.
2. Swing the psychrometer in circles 50 times
4. Record the wet bulb and dry bulb temperatures on the data table.
5. Complete the data table.
 Relative humidity can be determined by using the Relative Humidity Chart.
Outside:
Repeat the procedures above outside of the classroom.
Questions and Conclusions
1. What did you notice about the wet bulb temperature as you swung the psychorometer? Explain
this observation.
2. Is evaporation a cooling process or a heating process? Explain.
3. Was there a difference between the classroom and the outside relative humidity? If there was,
how can you explain the difference?
4. Which do you think is more stable the inside relative humidity or the outside? Explain your
reasoning.
5. Do water puddles evaporate faster when the humidity is high or low? Explain your reasoning.
6. Sometimes people say, “It’s the humidity that makes us feel so hot, not the heat?” What do you
think they mean? (Think about your answer to # 2 and #5 in relation to sweating)
7. What would the relative humidity be if the wet and dry bulb thermometers were the same
temperature? Explain why there is no change in temperature in this situation.