How do barometers work?

Look out of your window. Is the air moving? It is probably moving enough to make the branches on the trees sway slightly. The weight of this air pressing down all around the earth produces air pressure. If you live high up above sea level, or even if you go up to the top of a tall skyscraper, the air pressure is less. This is because there is less air above you, and therefore less air pressure.
On the surface of the earth, or near it, air always moves from areas of high pressure to areas of low pressure. Temperature has a great effect on air pressure. As warm air rises, it produces an area of low pressure near the ground. As the air pressure changes from place to place, and from day to day, it helps move air across the earth’s surface all the time. As cooler air moves down, it leaves behind an area of high pressure.
You can forecast what the weather will be like for the next few hours by measuring the air pressure.
The instrument used to measure changes in air pressure is called a barometer. If you have one at home, it is probably an aneroid barometer. There’s a needle behind a glass-covered dial marked “Fine,” “Fair” and “Stormy.” If you tap the glass lightly in the morning, the needle points to the weather you can expect that day.
Inside the aneroid barometer is a metal box, or chamber, from which almost all the air has been removed. The metal of the chamber is so thin that small changes in the air pressure outside it make the metal bend. This makes the needle turn round and point to a new position on the dial.
A mercury barometer measures the air pressure more accurately. A long, glass tube is filled with mercury, a silvery liquid metal. The tube is sealed at one end, and placed open end down in a small dish of mercury. Some of the mercury stays in the tube. It is held there by the air pressure at the surface of the mercury in the dish. As the air pressure changes, the column of mercury moves up or down. A scale beside the tube shows the pressure measurements.