You can explain facts about a barometer to children simply. Some young children may have difficulty understanding scientific tools, so speak in short sentences. Let children look at a barometer while you tell them about it, so they can understand your explanation. Tell them that when a barometer measures air pressure, mercury in the barometer rises or falls. If the mercury rises, it means the weather will be dry and sunny. When the mercury falls, the weather will be wet and cloudy.
If children can make their own mercury-free barometers, they will be able to understand and remember how a barometer works. Give each child a small coffee can and have them cover the tops of the cans tightly with plastic wrap. Ask them to tape a straw onto the plastic wrap so the straw lies across the can, and one end touches the edge of the can. Tape an index card to the edge of the can behind the end of the straw. The straw will move up and down against the index card as air pressure makes the plastic wrap rise or fall. Ask the children to mark where the straw touches the index card every fifteen minutes so they can see the air pressure change.
You can teach children about air pressure changes around the world by showing them weather maps. Show them weather maps that show high-pressure and low-pressure areas, marked by "H" and "L" respectively. Explain that a barometer tells meteorologists which areas have high or low pressure. Show children a map of where they live, and let them see whether there is high or low pressure over the area. Have them look at a barometer to confirm that the weather map is accurate. Ask them to look outside and confirm that the weather is dry when there is high pressure, or wet when there is low pressure.
If children record the weather and compare it to barometer readings, they will see how a barometer can predict the weather. Have each child record whether the weather is dry or wet each day for a week. Have them write down whether the barometer is rising or falling each day when they record the weather. They will learn that when the barometer falls they can expect wet weather, and when it rises they can expect dry weather.