You and Your Students!
Vicki Cobb, Education World Science Editor
Use the sun to launch a hot air balloon.
Weather (Convection Currents, Heat, Absorption), Color
Setting the Scene (Background)
Different weather patterns occur because of the sun’s unequal heating of the atmosphere. This little “play” demonstrates how the sun and air team up to make a garbage bag take off.
If your school is air-conditioned, you can start this experiment inside. Otherwise, find a large shady spot where the sun has not yet had a chance to warm the air. There you will collect the air for your “balloon.”
A flying garbage bag? Now that's a funny thought -- and a memorable way to impress kids that hot air rises!
Shake open a large black plastic garbage bag. "Inflate" the bag by pulling it through the air. (Note the stage direction above; do this activity in a large shady area.) When the bag is fully inflated, twist it closed and tie it with string or twist ties. Tie some string to the bag as a "leash." Then take the inflated bag outside to a spot where it is exposed to the sun and tie it to a pole or a chair. The bag should rest on the ground but have enough string so that it can "fly" while still attached.
Now, you must be patient. Check on the bag periodically as the sun warms it. As the sun warms the bag, it will expand and become lighter than the surrounding air. At first it will struggle to get off the ground. Eventually, though, you will get a rise out of your sun-warmed garbage bag.
BEHIND THE SCENES
Hot-air balloons rise because hot air weighs less than an equal volume of cool air. The key to heating your "balloon" is the black surface of the garbage bag. Dark colors absorb heat rays from the sun. (As a control experiment, you might do the activity with a white plastic trash bag of the same size and weight.) The air inside the black balloon gets warmer and expands. Because the same amount of material takes up more space, the air inside the garbage bag was less dense than the outside air. So the garbage bag was light enough to float.
Help students understand that as the balloon rises and moves upwards, air fills in its former position. (You might have heard it said: "nature abhors a vacuum." This is a good example of that expression.) Rising warm air creates an upward draft called a convection current.
The sun doesn't heat air directly as much as it heats the surface of the earth; then the warm earth's surface radiates heat into the air. Land absorbs heat from the sun more quickly than water does, so land heats up more rapidly than water. In contrast, at night land loses heat more rapidly than water does.
Ask: During the day where are the convection currents strongest, over land or over the sea? (over land) How does that explain how the wind moves from the sea to the land during the day producing a sea breeze? (Air from over the sea rushes toward land to replace the rising warm air -- thus creating a "sea breeze.") The reverse happens at night: the air over land loses heat more quickly than the air over the sea. So what happens to the wind direction? (It reverses from daytime -- creating a "land breeze.")
Find more fun outdoor activities in Don't Try This At Home: Science Fun for Kids on the Go, by Vicki Cobb and Kathy Darling.