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What Causes Dew?

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Author: Beth Touchette
See in slideshow:
Dog Days of Summer Science

Have you ever woken up after a clear summer night to find the grass is wet, even though you know it didn’t rain overnight?  The moisture on the grass is called dew, which forms when water in the form of a gas, which scientists call water vapor, comes in contact with cool surfaces, like grass or the glass on a car parked outside. The term for the amount of water vapor in the air is called humidity. If the air contains a lot of water vapor, like in a rain forest, we say the humidity is high. If there is little water vapor in the air, like in a desert, the humidity is low. 

When a mass of air contains as much water vapor as it can hold, the air is saturated. Warm air can hold more water vapor than cold air, so if a mass of warm air is cooled, it can no longer hold some of its water vapor.  Some of the water vapor condenses, which means the water vapor changes back to a liquid. The liquid water can fog up your bathroom mirror, or make the morning dew.  The exact temperature at which water begins to condense from a mass of air is called the dew point.

Problem: How does dew form?

What factors affect dew formation?

Materials

A. Making Dew

  • 2 Identical clean glasses
  • Water
  • Ice
  • Towel
  • 1-2 Thermometers

B. Factors That Affect Dew Formation

  • 2 liter Soda bottle
  • A grown-up
  • Sharp knife
  • Sharp Scissors
  • Plate
  • Water
  • Drinking glass
  • Ice
  • Thermometer

Procedure

A. Making Dew

  1. Fill one glass with room temperature water.
  2. Fill the second glass 2/3 full with ice cubes, then add cold water until the water level is the same as that of the first glass.
  3. Wipe the outside of each glass, ensuring both are completely dry.
  4. Set both glasses in a non-windy place, and wait 15 minutes.
  5. Observe both glasses. Record temperature in both glasses.

B. Factors That Affect Dew Formation

  1. Ask a grown-up to use a knife to cut off the bottom of your soda bottle.
  2. Find a level place to set the plate. Pour water on the plate until it almost reaches the brim.
  3. Set the capped and cut 2 liter water bottle on top.
  4. Let sit at least eight hours.
  5. Fill the glass 2/3 full with ice cubes.
  6. Place the thermometer in the glass.  
  7. Add cool water until glass is full.
  8. Carefully place the glass of ice water under the glass bottle.
  9. Record the temperature when you first see condensation (dew) on the side of the glass. This is the dew point.

Results

A. Making Dew

After fifteen minutes, the glass containing room temperature water is likely to be dry on the outside, while the glass with the ice water will be wet on the outside, with a layer of condensation, also known as dew.  Your results could vary depending on the temperatures of both glasses of water and the general humidity.

Dew formed on the outside of the glass with ice water because the glass cooled enough to cause the water vapor in the air coming in contact with the glass to condense to a liquid form.  The room temperature water did not cool the glass enough to cause the air surrounding it to reach its dew point.

B. Factors That Affect Dew Formation

The actual dew point temperature you find will vary depending on the humidity inside the soda bottle.  If you did part A of this experiment, you are likely to notice that the point where you first noticed condensation on the side of glass is likely to be a higher temperature than that of the ice water in regular air. You are also likely to notice more condensation.

When you covered the air with the soda bottle and let it sit for at least eight hours, you increased the humidity of the air in the bottle to close to 100%. We could also say the air was saturated with water vapor.  When you brought in the ice water, it did not take much cooling for the water in the air to begin condensing.

Going Further

Do you like making dew?  Well, then you probably like making and investigating frost in this science project.

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