This activity will give students the opportunity to measure the amount of water vapor in the air and to determine the relative humidity.

Time requirement: 50 minutes as an activity, 20 minutes as a demonstration.



  1. If a psychrometer is not available, make one by wrapping the bulb of one of the thermometers with cotton.
  2. Wet the sock (or cotton) on the wet bulb thermometer
  3. Sweep the wet bulb thermometer back and forth in the air, and observe the temperature of the wet bulb thermometer. Continue to sweep the thermometer until the temperature stops dropping. Record the temperature after it stops falling.
  4. Read the temperature off the dry bulb thermometer.
  5. Calculate the difference between the wet and dry bulb temperatures and determine the the relative humidity from the humidity index.
  6. Try to repeat the procedure both indoors and outdoors to see if there is a difference in the two environments.

Observation questions

  1. Why is the wet bulb temperature lower than the dry bulb temperature?
  2. Why did you sweep the wet bulb thermometer through the air?
  3. What was the difference in relative humidity indoors and outdoors?
  4. How do you account for this difference if one exists?
  5. What would happen if the relative humidity were 100%?

Last updated: September 05, 1997
Joe Twicken /
Rob Wigand