Measuring Relative Humidity - Science Fair Projects

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psychrometer
A vintage psychrometer. These devices have been used for more than 100 years to measure the moisture content, or relative humidity, in the air. (Image credit: Andrey Savin | Shutterstock)

You may have noticed that humidity (the amount of water vapor in the air) makes a difference in how comfortable you feel outside. When the humidity is high in the summer we say it is "muggy." When it is cool and humidity is high we may say that it feels "clammy." Humidity can make a warm day feel warmer and a cool day feel cold. You can observe the difference that humidity makes.

In this experiment, you will determine the relative humidity using wet-bulb and dry-bulb thermometers. Such a device is called a psychrometer.

What you need:

  • 2 identical alcohol thermometers (Celsius)
  • Heavy cardboard rectangle, about 10 inches long
  • Strong tape
  • Fan
  • Cotton balls
  • Rubber band
  • Warm water

What to do:

1. Use tape to secure the thermometers to the cardboard rectangle about 5 inches apart. Make sure the bulb ends of the thermometers hang below the edge of the cardboard by at least 1 inch and the numbered sides of the thermometers can be easily read.

2. Set the thermometers in front of a small fan set to a moderate speed for 5 minutes and record the temperature readings. This is to verify that your thermometers are calibrated and show the same temperature under control conditions.

3. Saturate several cotton balls with warm water and use the rubber band to fix them around the bulb of one thermometer. Set the thermometers in front of the fan as before. After 5 minutes record the “wet bulb” temperature reading and the “dry bulb” temperature reading.

4. To determine relative humidity subtract the lower “wet bulb” temperature from the higher “dry bulb” temperature to determine the difference and compare to the chart.

Swipe to scroll horizontally
Relative Humidity (%)
Dry-Bulb Temp (°C)Difference between Wet-Bulb and Dry-Bulb Temperatures (°C)
0123456789101112131415
-2010028Row 3 - Cell 3 Row 3 - Cell 4 Row 3 - Cell 5 Row 3 - Cell 6 Row 3 - Cell 7 Row 3 - Cell 8 Row 3 - Cell 9 Row 3 - Cell 10 Row 3 - Cell 11 Row 3 - Cell 12 Row 3 - Cell 13 Row 3 - Cell 14 Row 3 - Cell 15 Row 3 - Cell 16
-1810040Row 4 - Cell 3 Row 4 - Cell 4 Row 4 - Cell 5 Row 4 - Cell 6 Row 4 - Cell 7 Row 4 - Cell 8 Row 4 - Cell 9 Row 4 - Cell 10 Row 4 - Cell 11 Row 4 - Cell 12 Row 4 - Cell 13 Row 4 - Cell 14 Row 4 - Cell 15 Row 4 - Cell 16
-1610048Row 5 - Cell 3 Row 5 - Cell 4 Row 5 - Cell 5 Row 5 - Cell 6 Row 5 - Cell 7 Row 5 - Cell 8 Row 5 - Cell 9 Row 5 - Cell 10 Row 5 - Cell 11 Row 5 - Cell 12 Row 5 - Cell 13 Row 5 - Cell 14 Row 5 - Cell 15 Row 5 - Cell 16
-141005511Row 6 - Cell 4 Row 6 - Cell 5 Row 6 - Cell 6 Row 6 - Cell 7 Row 6 - Cell 8 Row 6 - Cell 9 Row 6 - Cell 10 Row 6 - Cell 11 Row 6 - Cell 12 Row 6 - Cell 13 Row 6 - Cell 14 Row 6 - Cell 15 Row 6 - Cell 16
-121006123Row 7 - Cell 4 Row 7 - Cell 5 Row 7 - Cell 6 Row 7 - Cell 7 Row 7 - Cell 8 Row 7 - Cell 9 Row 7 - Cell 10 Row 7 - Cell 11 Row 7 - Cell 12 Row 7 - Cell 13 Row 7 - Cell 14 Row 7 - Cell 15 Row 7 - Cell 16
-101006633Row 8 - Cell 4 Row 8 - Cell 5 Row 8 - Cell 6 Row 8 - Cell 7 Row 8 - Cell 8 Row 8 - Cell 9 Row 8 - Cell 10 Row 8 - Cell 11 Row 8 - Cell 12 Row 8 - Cell 13 Row 8 - Cell 14 Row 8 - Cell 15 Row 8 - Cell 16
-8100714113Row 9 - Cell 5 Row 9 - Cell 6 Row 9 - Cell 7 Row 9 - Cell 8 Row 9 - Cell 9 Row 9 - Cell 10 Row 9 - Cell 11 Row 9 - Cell 12 Row 9 - Cell 13 Row 9 - Cell 14 Row 9 - Cell 15 Row 9 - Cell 16
-6100734820Row 10 - Cell 5 Row 10 - Cell 6 Row 10 - Cell 7 Row 10 - Cell 8 Row 10 - Cell 9 Row 10 - Cell 10 Row 10 - Cell 11 Row 10 - Cell 12 Row 10 - Cell 13 Row 10 - Cell 14 Row 10 - Cell 15 Row 10 - Cell 16
-410077543211Row 11 - Cell 6 Row 11 - Cell 7 Row 11 - Cell 8 Row 11 - Cell 9 Row 11 - Cell 10 Row 11 - Cell 11 Row 11 - Cell 12 Row 11 - Cell 13 Row 11 - Cell 14 Row 11 - Cell 15 Row 11 - Cell 16
-2100795837201Row 12 - Cell 7 Row 12 - Cell 8 Row 12 - Cell 9 Row 12 - Cell 10 Row 12 - Cell 11 Row 12 - Cell 12 Row 12 - Cell 13 Row 12 - Cell 14 Row 12 - Cell 15 Row 12 - Cell 16
01008163452811Row 13 - Cell 7 Row 13 - Cell 8 Row 13 - Cell 9 Row 13 - Cell 10 Row 13 - Cell 11 Row 13 - Cell 12 Row 13 - Cell 13 Row 13 - Cell 14 Row 13 - Cell 15 Row 13 - Cell 16
210083675136206Row 14 - Cell 8 Row 14 - Cell 9 Row 14 - Cell 10 Row 14 - Cell 11 Row 14 - Cell 12 Row 14 - Cell 13 Row 14 - Cell 14 Row 14 - Cell 15 Row 14 - Cell 16
4100857056422714Row 15 - Cell 8 Row 15 - Cell 9 Row 15 - Cell 10 Row 15 - Cell 11 Row 15 - Cell 12 Row 15 - Cell 13 Row 15 - Cell 14 Row 15 - Cell 15 Row 15 - Cell 16
610086725946352210Row 16 - Cell 9 Row 16 - Cell 10 Row 16 - Cell 11 Row 16 - Cell 12 Row 16 - Cell 13 Row 16 - Cell 14 Row 16 - Cell 15 Row 16 - Cell 16
8100877462513928176Row 17 - Cell 10 Row 17 - Cell 11 Row 17 - Cell 12 Row 17 - Cell 13 Row 17 - Cell 14 Row 17 - Cell 15 Row 17 - Cell 16
1010088766554433324134Row 18 - Cell 11 Row 18 - Cell 12 Row 18 - Cell 13 Row 18 - Cell 14 Row 18 - Cell 15 Row 18 - Cell 16
121008878675748382819102Row 19 - Cell 12 Row 19 - Cell 13 Row 19 - Cell 14 Row 19 - Cell 15 Row 19 - Cell 16
1410089796960504133251681Row 20 - Cell 13 Row 20 - Cell 14 Row 20 - Cell 15 Row 20 - Cell 16
161009080716254453729211471Row 21 - Cell 14 Row 21 - Cell 15 Row 21 - Cell 16
1810091817264564840332619126Row 22 - Cell 14 Row 22 - Cell 15 Row 22 - Cell 16
201009282746658514436302317115Row 23 - Cell 15 Row 23 - Cell 16
22100928375686053464033272115104Row 24 - Cell 16
241009284766962554942363025201494
2610092857770645751453934282318139
28100938678716559534742363126211712
30100938679726661554944393429252016

Results:

Even though the air temperature did not change, you should have noticed a lower temperature reading from the wet-bulb thermometer. This happens because as the cotton balls were drying, some of the water evaporated (changed to water vapor.)

This change of state, from liquid water to water vapor, requires a bit of heat energy from the surrounding atmosphere. The same thing happens when ice melts into liquid water. On the other hand, a tiny bit of heat is released when water vapor condenses back into liquid form or when water freezes to form ice. This explains why you feel cold when you get out of a pool on a windy day, even though the air temperature is high, the water evaporating from your skin is pulling the evaporation heat from your body instead of the air!

Mary Bagley
LiveScience Contributor