Barometer

A barometer is a tool that tells us how much air pressure there is around us. Air is a type of gas all around us and it has weight just like anything else. When there is more air in one place, it pushes down more, and when there is less air in one place, it doesn't push down as much.

To measure air pressure, a barometer has a long glass tube filled with mercury (which is a heavy liquid). One end of the tube is closed and the other end is open. When the tube is turned upside-down and placed in a container of mercury, some of the mercury flows out of the tube and into the container.

The amount of mercury that stays inside the tube depends on how much pressure there is in the air outside the tube. When the air pressure is high, the mercury in the tube is pushed down, and there is less space for air to enter the tube. When the air pressure is low, the mercury is not pushed down as much, and there is more space for air to enter the tube.

The barometer has markings on it that tell us how high or low the air pressure is. If the barometer is showing a high pressure, we know that the weather is likely to be sunny and clear. If the barometer is showing a low pressure, we know that the weather might be cloudy and rainy.