What you need:
- Clear plastic cup
- 2-liter plastic soda bottle
- Food coloring
What to do:
- This tool will work best if you put it together on a rainy day (when air pressure is lower). Invite your child to pour a few inches of water into a plastic cup. Add a few drops of food coloring to the water.
- Place a plastic bottle upside down inside the cup. (It should fit snugly, but should not be airtight.) The mouth of the bottle must be below the waterline but not touching the bottom of the cup. Use a marker to note the water level on the side of the cup.
- Check the barometer after a couple of days. If the weather has gotten clearer, the water level in the cup should have fallen and the level in the bottle should have risen.
- Explain to your child that air pressure is what causes the change in water level. When the weather is clear, air pressure is higher, but when a storm is coming, air pressure drops. As the weather cleared up, the air pressure rose, and this pushed down on the water in the cup. Thus, the water level in the cup fell and the level in the bottle rose.
- As you watch the weather reports on the news, invite your child to note what the weather forecaster says about the air pressure (or "barometric pressure"). If he says it will rise, does the water level in your homemade barometer also rise? What happens if he says the pressure is falling?