FINDING THE DEW POINT

KEY CONCEPTS

  1. The dew point is the temperature to which air must be cooled before the water vapor in it condenses. Water vapor condenses on a surface when that surface is colder than the air. For example, when warm breath strikes the cold windshield, water vapor molecules cool, slow down, and join together to form small drops of water.
  2. Condensation is a key component of the water cycle.
 

MATERIALS

  • shiny metal can (label and lid removed)
  • thermometer
  • crushed ice and water
  • one tablespoon of salt
  • student journals
 
 

PROCEDURE

  
 
 

Activity

  1. Discuss student experiences with condensation (such as moisture accumulating on a mirror in a steamy bathroom or moisture appearing on the inside of a car window).
  2. Record the air temperature. Measure the temperature of water that is warmer than the air temperature and pour it into the can. (The can's exterior must be clean and dry).
  3. Add a slush mixture of crushed ice and salt into the water and slowly stir the mixture with the thermometer, keeping its reservoir in contact with the can's surface.
  4. Record the temperature at the precise moment small water drops first appear on the can's exterior.
  5. Conduct a classroom discussion on dew-point temperatures. How does the temperature of the air in direct contact with the can compare to that of the ambient (surrounding atmospheric air) air temperature? Define the term "dew point." How would wind affect a dew-point temperature?
 
 

Assessment Ideas

  • student accuracy in measuring the dew point temperature and answering questions
 
 
 

CROSS-CURRICULAR IDEAS

  • English language arts: creating a journal.
 
 

VOCABULARY

  • condense: turn or change to a liquid (for example, change from a vapor to a liquid); usually from cooling (molecules compact together).
  • dew point: the temperature to which air must be cooled before the water vapor in it condenses.
  
 

SOURCE

  • "Winds of Change" educational CD-ROM, Copyright Caltech and NASA/Jet Propulsion Laboratory