Simple Barometer

Did you know you can tell what the weather will be without checking the forecast? In this experiment we make our own indoor weather-predicting device!


  1. Cut the neck off of the balloon (ask an adult to help) and stretch the balloon over the glass.
  2. Hold the straw on top of the balloon so that one end is in the middle of the balloon and the other is past the edge of the glass.
  3. Tape the straw to the balloon. Believe it or not this is the main bit done!
  4. Put the glass somewhere against a vertical surface like a wall somewhere where it won't get disturbed, like the back of a shelf. The straw should stick out horizontally and parallel to the wall.
  5. Stick the piece of card to the wall behind the straw with blu-tack.
  6. Mark a line on the card where the straw points, and write what the weather is right now (dry and sunny, raining and windy, etc) next to the line.
  7. Check the position of the straw over the next few days - draw a new line on the card for each type of weather and write what it is!
Balloon barometer
"Balloon Barometer" by Paul Swift

When the straw points upwards the weather will be good, and when it points downwards the weather will be bad!

So what's happening?

What we've made is a device called a barometer, which can tell us if the weather will be dry or rainy based on atmospheric air pressure. By placing a balloon over the glass, some air has become trapped inside.

When the air pressure outdoors becomes lower the air in the glass will expand and make the balloon bulge. When the air pressure outdoors is higher it presses in on the air in the jar more, and the balloon pulls inwards. Because the balloon bulges or shrinks it also moves the taped straw up and down in front of the piece of card.

When the weather's going to be bad the air pressure drops - and this makes our balloon bulge and the straw point downwards. When the weather's going to be good the air pressure goes up - making our balloon shrink and the straw point up!

Now you know what the weather's going to be you can do your own forecast each day!

What if?

What would happen if you:

  • Used a larger or smaller container to put the balloon on?
  • Placed the glass inside a sealed plastic box like a lunch box, would it still work?
  • Used tissue paper instead of the balloon?