What change in weather does a barometer measure?

Air Pressure

A barometer is a vacuum tube sitting in a pool of mercury. The mass of the column of air above the mercury will apply a downward force (due to gravity) on the mercury pushing it up into the vacuum tube. The average air pressure will cause the mercury to rise 29.92 inches up in the vacuum tube. Any changes in the column of air above the mercury will change the force applied and push the mercury column less or more.