Air pressure is caused by the weight of the air pressing down on the Earth, the ocean and the air below. Air pressure is also known as atmospheric pressure and barometric pressure.
Air pressure is the force per unit area exerted by an atmospheric column, which is the entire body of air above a specified area. Air pressure can be measured with a mercury barometer, which indicates air pressure with the height of a column of mercury that balances the weight of the column of atmosphere over the barometer. Atmospheric pressure can also be measured using an aneroid barometer, which contains one or more hollow, partially evacuated and corrugated metal disks that are supported by an inside or outside spring to prevent it from collapsing.