We picked the perfect day to do this craft. Before the day was even over, it had turned from sunny to cloudy to raining (and later the tornado sirens went off).
The science behind this craft is fascinating.
To start, I explained atmospheric pressure. That's a pretty tough thing to visualize, but we had a little help from Bill Nye, the Science Guy. (Check out some of his video clips on YouTube.)
Once we understood what pressure was and that it changes with the weather, it was easier to understand how a barometer works.
Wide-mouth glass jar without a lid
Plastic drinking straw
Cardstock or posterboard
How to Make a DIY Barometer
Cut the neck of your deflated balloon off, making a straight cut about halfway down the balloon.
Stretch the balloon as tightly as possible over your glass jar, so the latex is taught. You want to make an air-tight seal, so add a rubberband or two to secure it.
Now trim a plastic drinking straw so it's about 6 or 7 inches in length. Then make angled cuts on both ends so each end has a point.
Using the corner of a small square of clear tape, attach the flat part of one end of the straw to the balloon in the center of the jar.
The other end of the straw is your pointer.
Now fold a piece of heavyweight paper in fourths lengthwise. Tape so it's in the shape of a triangle and stand on one end.
Take your barometer outside and let it acclimate. Set your triangular paper up at the end of the pointer, and with a pencil record where it touches and the weather conditions.
This is your baseline. Continue checking on the barometer and making recordings of the weather conditions. As the weather changes, the straw pointer will move up and down.
Once you have marks recorded for a variety of weather conditions, you'll see a trend. When the weather is sunny, it points upwards. When it's storming, it points downward. You can watch the DIY barometer and begin to make predictions based on the gauge you made.
How it Works
If the air pressure inside the jar is heavier than the air outside the jar, it will push up making the balloon convex (humped up), which will tip the straw pointer downward. This occurs when the weather is rainy or storming.
If the air pressure outside the jar is heavier, it will press down on the balloon (making it concave), tipping the straw pointer upwards. Expect this to occur when it's sunny.