Shallow dish or an aluminum pie plate
Empty coffee can
Ice water
Water spray bottle
Cloud cutout (white cardboard or
foamcore)
Projector
Projection screen
Dark room
Procedure:
Part 1. Fill the coffee can with ice water and place it in the shallow dish or pie plate. Observe the outside of the can every minute or two. Water droplets from the air will begin to condense on the outside of the can.
Part 2. Darken the room and turn on the projector. Place some clean water in the spray bottle. Adjust the spray to a fine mist. Hold the bottle between the
projector and screen and spray. Observe the shadows on the screen cast by the fine water droplets.
Part 3. Simulate how clouds block visible radiation by holding up a cutout of a cloud between the projector and the screen.
Discussion:
The first demonstration shows that water is present in the atmosphere. The capacity to hold water is determined by the atmospheric temperature. Warm air can hold more water than cold air. Because the can is chilled by the ice water, the air immediately surround-ing the can cools. Lowering air temperature reduces its capacity to hold water, and so the excess water condenses on the outside of the can. The amount of water in the atmosphere at any one time, expressed as a percentage of complete saturation, is called the relative humidity. Humid air filters out