This image shows three common ways that atmospheric pressure is measured - using a mercurial barometer, an aneroid barometer or a barograph.
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Windows Original (Using images from FAA)
Barometer
A barometer is a weather instrument used to measure atmospheric pressure. The first barometer was invented in 1643 by Evangelista Torricelli, one of Galileo's assistants. This first barometer used mercury to measure the pressure. The atmosphere pushes downward on the mercury. If there is an increase in pressure, it forces the mercury to rise inside the glass tube and a higher measurement is shown. If atmospheric pressure lessens, downward force on the mercury lessens and the height of the mercury inside the tube lowers. A lower measurement would be shown. This type of instrument can be used in a lab or a weather station, but is not easy to move! Measurements from a mercury barometer are usually made in inches of mercury (in Hg).
An aneroid barometer can be used in place of a mercury barometer. It is easier to move and is often easier to read. This instrument contains sealed wafers that shrink or spread out depending on changes of atmospheric pressure. If atmospheric pressure is higher, the wafers will be squished together. If atmospheric pressure lessens, it allows the wafers to grow bigger. The changes in the wafers move a mechanical arm that shows higher or lower air pressure.
These days, many mechanical weather instruments have been replaced by electronic instruments that record atmospheric pressure onto a computer.
Atmospheric pressure can be measured and recorded in many different units. Usually high pressure means the weather will be clear, and low pressure usually means the weather will be stormy.
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