How your DIY Barometer Works

Your DIY barometer relies on changes in air pressure to change the volume of air trapped inside the jar. Because the jar is sealed, the air inside acts like a spring that responds to the “weight” of the atmosphere pushing down on it from the outside.

As you sealed the balloon tightly across the jar opening, you trapped a set amount of air molecules within the container. From this point on, two main things can occur:

1. Expansion (Low Pressure): If the air pressure outside the jar drops, there is less force pushing down on the balloon. This allows the air trapped inside the jar to expand more easily, pushing the balloon upwards. This movement causes the kebab stick to pivot downwards towards ‘cloudy’.
2. Contraction (High Pressure): If the air pressure outside the jar increases, the “heavier” outside air pushes the balloon inwards. This causes the air inside the jar to compress, making the kebab stick point upwards towards ‘sunny’.

Note: Because air volume is also affected by temperature (the Ideal Gas Law), your barometer will also react if the room gets very hot or cold, which is why scientists usually keep professional barometers at a constant temperature!

Variables to test

Find out more on variables here.

• Seal Tension
  What happens if the balloon is not stretched tightly across the jar opening? Does it become more or less sensitive to pressure changes?
• Indicator Material
  Does using a straw instead of a kebab stick make any difference to the stability or the movement of the pointer?
• Container Size
  Does having a different volume of jar or tin change the results? Consider if a larger volume of air creates a bigger “push”.
• Heat Influence
  If you heat the jar with a hair dryer, what happens? Does this simulate high or low pressure?
• Cooling Influence
  If you cool the jar with ice or iced water, what happens to the pointer?