What is the physical quantity measured by a barometer?

A barometer is an instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or Mercury. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems, and frontal boundaries.

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A barometer measures air pressure.
A barometer is an instrument that is used to measure atmospheric pressure.