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Very old barometers

Homemade Barometer

The air pressure around us greatly affects our weather. Notice how your weatherman always mentions various pressure systems (low pressure system, high pressure system, etc.) and how they will affect tomorrow’s weather. In this experiment, we will create a tool that lets you gauge the pressure of the air around you.

  1. Fill the measuring cup or glass with water and add some colored dye to it.
  2. Flip the empty soda bottle upside down into the glass measuring cup.
  3. Make sure that you use a bottle that is just the right size. The thicker body of the bottle should fit snugly in the measuring cup so that the mouth of the bottle does not touch the bottom of the cup.
  4. Make sure that the level of the water extends into the neck of the bottle.
  5. Mark a line on the cup to indicate the water level within the bottle.
  6. Reexamine the bottle in a few days.

Notice the change in the water level?  The amount of air within the bottle is fixed and cannot change since the water extended into the bottle acts as a ‘plug’.  Hence, you can consider the amount of air trapped in the bottle as an indicator of the air pressure on the day you plugged the bottle.  The pressure on the surface of the water depends on the current air pressure.  When the air pressure increases (as it does in drier weather), the pressure on the surface of the water is greater and the water is forced up into the bottle changing the level of the water.  If you see the water level drop, find your umbrella…

Experiment Supplies

Supplies: Plastic bottle, Food dye, Glass jar


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