You may not realize it, but we earthlings are under a lot of pressure. Air pressure, that is. The nine miles of air above us (really 460, if you include the thin atmosphere that extends to the boundary of space) may seem light as a feather, but when you add it up, it really weighs a ton. At sea level, an area of about 1 square foot, about the size of a washcloth, is under about 2,000 pounds of air pressure.

You can show your kids this powerful, invisible force in action by making your own barometer, a device that reveals changes in air pressure. The barometer works best when assembled under the low pressure of a rainy day, making it a great project when foul weather has your kids cooped up inside.

What you'll need
Water
Food coloring
Plastic cup
Plastic bottle
Marker



How to make it
Build It: Have your child pour a few inches of water and a couple of drops of food coloring into a plastic cup. Set a plastic bottle upside down inside the cup. It should be a snug fit but not airtight, and the mouth of the bottle should be well below the waterline but not touching the bottom of the cup. Mark the water level on the outside of the cup with a marker, then set the barometer where it won't be disturbed.
Check It: Look at the barometer after a day or two. If the weather has cleared up, you should notice that the water level in the cup has dropped, while the water in the neck of the bottle has actually risen.
What Happened: When the air pressure increased, as it usually does when clear weather moves in, it pressed down on the water in the cup, forcing it up into the neck of the bottle.

source: spoonful.com