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Let’s Dew It!

Measure the temperature you need to make fog form.

Goal: To discover how air temperature and humidity work together to make condensation, dew and fog. 

Time: 30 minutes to an hour

Space: Desk or table



  • Set up a work space for your child that includes the “Let’s Dew It!” handout, a metal can half-full of water, a thermometer, 6 to 10 ice cubes, a spoon and a magnifying lens.
  • Optional:  Preview the episode or clip, and if using them with your child, cue up the videos via the links above.


TIP: This experiment works best when the air has some moisture in it. If the air is very dry (there is static electricity in hair, sparks occur when scuffing feet and touching metal doorknobs, etc.), there is little water vapor in the air and you would have to drop the temperature in the room below freezing before dew or condensation would form. We suggest you try the experiment first to make sure your child will get satisfactory results.

  1. Talk about what fog looks like. Ask: Where have you seen fog? What was it like? Tell your child that today she’s going to do an experiment that will show what temperature it has to be in the room for moisture in the air to form fog.
  2. Explain that fog forms because warm air holds more moisture (water vapor) than cold air, and as air cools, the water gets   “squeezed out.”
  3. Optional: Use the episode or clip to further explore this idea.
  4. Oversee your child as she does the experiment, following the directions on the “Let’s Dew It!” handout. TIP: After each ice cube is added, your child should watch the thermometer to see the water temperature drop. That way, she will be ready to note the temperature once the water is cool enough for condensation to form on the can.
  5. Once your child has finished her experiment, talk about her findings. How much would you have to lower the temperature in the room to make fog form?

Take It Further

Share this activity’s Science, Technology, Engineering, Math (STEM) connections and invite your child’s comments:

Meteorologists—scientists who study the weather—measure and report the dew point so airplane pilots can predict visibility, rain, and other conditions that might make flying difficult. Engineers who work with air conditioners and furnaces watch for changes in the dew point because condensation can cause these machines to work less efficiently or even shut down.

Talk About it

Ask: What did you learn today that you didn’t know before?

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Produced by: Funding is provided by:
WNET logo   National Science Foundation logo Northrop Grumman logo Ernst & Young logo Corporation for Public Broadcasting logo
Additional funding provided by the Volckhausen Family

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