Dew Drops: What Causes Dew?

Problem

What Causes Dew?

Materials

• Two drinking glasses
• Tap water
• Ice
• Paper towel
• Timer

Procedure

1. Fill one of the glasses with water.
2. Fill the second glass with ice, then add enough water to cover the ice.
3. Dry the outside of each glass with the paper towel.
4. Allow the glasses to sit undisturbed for 15 minutes in a draft-free area.
5. Observe the outside of each glass.

Results

The outside of the glass of water without ice remains dry, but the outside of the glass of ice water is covered with water drops.

Why?

When air is completely full of water vapor, it is said to be saturated. Air may be saturated by adding water vapor, but since less water vapor is needed to saturate cold air, air may become saturated by cooling it. When air is saturated, condensation (the change of a gas into a liquid due to a removal of heat energy) occurs.

The ice water cools the glass and the cold glass cools the air around it. Water vapor molecules in this chilled, saturated air clump together, forming tiny drops of visible water. These droplets cling to the outside of the glass and grow as more water condenses on the glass. The glass containing water without ice does not cool the air enough for it to become saturated, so the vapor in the air does not condense. When water vapor in the air comes in contact with cool surfaces, it condenses and forms water droplets called dew.

Let's Explore

1. Repeat the experiment, using containers made of different materials, such as glass, paper, and metal.
2.  
   1. The temperature at which dew forms is called the dew point. Determine the temperature at which the dew formed on the glass. Repeat the original experiment, using only the glass of ice water. Place a thermometer in the glass and watch the outside of the glass. Record the temperature when you first observe dew on the outside of the glass.
   2. Does humidity (the amount of water vapor in the air) affect the dew point? Repeat the previous experiment on different days of varying humidity. Find out the humidity from the local weather reports in the newspaper or on television, and record it for each day the experiment is performed.

Show Time!

1. Another way of testing how humidity affects dew point is to create a moist environment. Ask an adult to cut the bottom from a 2-liter soda bottle. Secure the cap on the bottle, and set the bottle in a saucer filled with water. Be sure the entire bottom edge of the bottle is below the water in the saucer. Allow the bottle to remain undisturbed overnight. The next day, lift the bottle and place a glass of ice water containing a thermometer in the saucer. Cover the glass with the bottle and observe the out side of the glass. Record the temperature at which the dew forms as in the previous experiments.
2. Determine changes in humidity by constructing an instrument used to measure humidity, called a hygrometer. Ask an adult to clean the oil from a 6-inch (15-cm) strand of straight hair. If you or your family members do not have straight hair, ask a friend or beautician for a strand. The adult can clean the hair strand by pulling it through 2 cotton balls moistened with nail polish remover. Use a small piece of tape to secure one end of the strand of hair to the center of a toothpick. Mark the pointed end of the toothpick with a marking pen. Tape the free end of the hair strand to the center of a pencil. Place the pencil across the mouth of a quart (liter) jar so that the toothpick hangs inside the jar. If the toothpick does not hang horizontally, add a drop of glue to one end to balance the toothpick. Place the jar where it will not be disturbed. For 1 week or more, make daily observations of the direction in which the toothpick points. In moist air, the hair lengthens, and in dry air, it shrinks. The stretching and shrinking of the hair pulls on the toothpick and causes it to move. From your results, determine how this hygrometer can be used to measure humidity. Repeat Let's Explore 2a. and b. to determine the effect of humidity on dew point, using your hygrometer to measure humidity.

    

   

Check it Out!

Dew does not fall from the sky like rain but forms on cooled surfaces. Find out more about dew. Why does dew usually form at night? How does the difference between daytime and nighttime temperature affect dew formation?

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