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An aneroid barometer
A mercury barometer of 1890

A barometer is an instrument used to measure air pressure. The barometer measures air pressure in units called hectopascals (hPa). Air pressure and differences in pressure are important ways to describe or measure weather. Therefore the barometer is important for meteorologists.

There are various types of barometers such as the water barometer, aneroid barometer, and the mercury barometer. They were created by an Italian mathematician named Evangelista Torricelli in 1643. They were also 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. They were also used by miners in caves to determine the depth of a mine. The most used purpose of the barometer was measuring air pressure. The barometer is a good tool for predicting weather.