A barometer is an instrument used to measure air pressure. The barometer measures air pressure in various kinds of units including hectopascals (hPa) and millibars.

There are various types of barometers such as the water barometer, aneroid barometer, and the mercury barometer. The mercury barometer, the earliest barometer, was created by an Italian mathematician named Evangelista Torricelli in 1643.

They are used for measuring altitude, or height above the ground, such as the height of a mountain, and they were often used to measure altitude aboard a hot air balloon. Barometers are also used in modern aviation as altimeters. Miners sometimes use them to determine the depth of a mine. The most used purpose of the barometer is measuring air pressure. This helps meteorologists in predicting weather.

A barograph is a device which records barometric readings on paper.

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