How do barometers work?

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A barometer is a special tool for measuring air pressure. Believe it or not, the air has a lot of weight! Air is pushing down on us all the time. We are very used to it, and we don’t really notice it too much.   

When the air is really heavy (high pressure,) it is settled and stops clouds from forming. When there is less pressure or low pressure, water droplets in the air can rise very high and condense into clouds. That’s why, at low pressure, we’re more likely to have storms.

By measuring the air pressure, we can figure out if we’re going to have a nice day or a stormy day. Many barometers use a glass tube filled with liquid (like mercury) to help us figure out the air pressure. The tube is open at the bottom, sitting in a cup of the same liquid. As high air pressure pushes the liquid in the cup downward, the liquid is pushed up into the tube.

When there is less pressure on the liquid in the cup, the liquid in the tube sinks—and we know that there is a drop in air pressure. It’s probably a good time to get an umbrella too!

Predict the Weather!

You can make a barometer at home! Here’s what you’ll need: an empty coffee can, a balloon, a rubber band, a straw, some tape, scissors and a piece of paper.

Have a grown-up help you. Ask your grown-up to cut out a large piece of the balloon and stretch it over the open top of the coffee can. Hold it in place with a rubber band. Next tape the straw in the center of the balloon, so that it is lying down and hanging over the edge.

As the air pressure changes (and this will work best outside), the straw will move up and down. On a piece of paper posted next to your barometer, mark where the straw is every day. When the straw points downward, what do you think is happening to the air pressure above your barometer? What will happen to the weather?