What instrument measures atmospheric pressure?

The instrument that measures atmospheric pressure is a barometer.

Atmospheric pressure is the force exerted by the weight of the air above a location. A barometer detects changes in that pressure, which can help meteorologists analyze weather systems. Falling pressure often accompanies approaching unsettled weather, while rising pressure commonly occurs as air becomes more stable, although pressure alone cannot predict every local condition.

Evangelista Torricelli is traditionally credited with inventing the mercury barometer in 1643 or 1644. His design used a sealed glass tube filled with mercury and inverted into a reservoir. Air pressure pushed on the reservoir and supported a column of mercury; changes in pressure altered the column’s height.

Modern instruments may be aneroid or electronic. An aneroid barometer uses flexible, partially evacuated metal capsules rather than liquid. Digital pressure sensors are now common in weather stations, aircraft, and smartphones. A barometer is often confused with an altimeter: both can sense pressure, but an altimeter uses pressure changes primarily to estimate altitude.