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A barometer measures the pressure of the air around it. When air pressure drops, clouds and rain form. Higher pressure leads to clear skies. Barometers can help us forecast the weather in days ahead.

Topics and Connections:

• Air pressure
• Weather instruments
• Making measurements

Materials:

• scissors
• balloon (uninflated)
• wide-mouth jar
• rubber band
• drinking straw

Directions:

Teacher:
Air pressure is a very difficult concept for students to grasp. Ask them what they know about pressure, what things can have pressure and what the effects of pressure can be on them or other objects.

Students:

• Cut off the neck of the balloon.
• Cut off the tip of the straw diagonally so it comes to a point.
• Stretch the balloon around the jar very tightly, so the surface is very flat. Put the rubber band around the balloon and jar to keep the balloon from slipping.
• Tape the uncut end of the straw to the center of the balloon, so the tip extends far out from the jar. Try pushing on or pulling the balloon to see how the pointed end of the straw moves.
• To measure the straw’s position exactly, put the jar near a vertical piece of paper, so the straw is very close to the paper. You can draw a line on the paper to mark exactly where the straw is today, then check back over several days to see how it changes.

Teacher:
Explain that there is now air trapped in the jar, pushing out-ward, and air in the room pushing inward. When the air pressure in the room changes, it will push harder or less hard on the balloon. If the pressure increases, it will push down on the balloon more. If the air pressure decreases, it won’t be pushing down on the balloon as much as the air trapped inside is pushing up. The straw will move one way or the other.

Students:
Keep track of what the weather is like on various days, and where the straw was pointing. After enough data is collected (a few weeks) try to find a connection between your barometer reading and what the weather is—or what the weather is about to be. Why is this important?

Follow up:
Have students brainstorm how to 'calibrate' their barometer so they know exactly what the pressure is when they look at it.

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