National Weather Service United States Department of Commerce

Learning Lesson: Sweatin' to the Coldies


There are three states of matter; gas, liquid, and solid. Water in our atmosphere exists in these three states constantly. As the temperature of water vapor (a gas) decreases, it will reach the point at which it turns into a liquid (called the dew point or the point at which dew forms). This change of state from a gas to a liquid is called condensation.

Using some ice and a glass, the students will chill the glass to the point where water from the atmosphere will condense on the outside of the glass. This demonstrate the change of state of water vapor to liquid.

TOTAL TIME 30 minutes
SUPPLIES Glass cups or jars, Ice cubes
This can be done as a class demonstration or you can divide the students into pairs should you have enough glass jars. You can also shorten this experiment by using crushed ice instead of cubed ice. Crushed ice chills water quicker, causing condensation sooner.
SAFETY FOCUS Flash Flood Safety


  1. Fill the cups/jars with ice.
  2. Add cold water to the cups/jars.
  3. Let the cups/jar set for about 30 minutes.
  4. Observe the outside of the glass for condensation.


Ask the students where the water on the outside of the glass came from. The answer is from the atmosphere. As air comes in contact with the glass, it is cooled to below the dew point temperature and the water vapor contained in the air condenses onto the glass.

The amount of water on the side of the glass depends upon the humidity which is the ratio of dry air to moist air. The higher the humidity the more moisture that air contains. The greater the moisture, the greater the water that can condense.

As an aside, you can repeat the process with different types of drinking containers, such as plastic and Styrofoam. Ask the students why there is a difference in the amount of water condensing onto each type.

Building a Weather-Ready Nation

The amount of moisture in the atmosphere is indicated by the dew point temperature. As the dew point increases, so does the potential for the amount of rain produced by a thunderstorm increase. Stationary or slow-moving thunderstorms produce heavy rain over small areas and increase the risk of flash flooding.

Hilly and mountainous areas are especially vulnerable to flash floods, where steep terrain and narrow canyons can funnel heavy rain into small creeks and dry ravines, turning them into raging walls of water. Even on the prairie, normally-dry draws and low spots can fill with rushing water during very heavy rain.

Take time to develop a flood safety plan-for home, work, or school, and wherever you spend time during the summer. The National Weather Service has additional information about flood safety and a brochure "Floods and Flash Floods...The Awesome Power".

Preparations at home and work:
  • Determine if you are in a flood-prone area. If you are, know where to go if the water starts to rise. Have an escape route if you have to leave quickly.
  • Make a safety kit containing: A flashlight and extra batteries, battery-powered weather radio receiver and commercial radio, extra food and water, first-aid supplies, canned food and a can opener, water (three gallons per person), extra clothing, and bedding. Don't forget special items for family members such as diapers, baby formula, prescription or essential medications, extra eyeglasses or hearing aids, and pet supplies.
  • Know how and when to shut off utilities: Electricity, gas, and water.
  • Seek sources for obtaining local warning information such as from cable TV or the NOAA Weather Radio.