Question

What does barometric pressure measure?

Answer 1

A barometer is a scientific instrument used in meteorology to measureatmospheric pressure. 
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Answer 2

Barometer is a special kind of instrument which is used to find out the atmospheric pressure.

The atmospheric pressure varies with altitude and that is why its value can also change.

Therefore the instrument can give different values.

It was introduced in the year 1643 and the first one designed was the water barometer and not the atmospheric pressure one