A mercury barometer is a tube of mercury with a nearly pure vacuum sealed above the column. What does it measure?

1. The pressure above the mercury column is essentially zero because that space is a vacuum. 2. The atmosphere pressing on the open reservoir supports the column, so h rho g for the mercury equals atmospheric pressure itself. 3. Because the reference is a vacuum rather than the surrounding air, the reading is an absolute pressure. 4. Rising and falling mercury then tracks changes in atmospheric pressure, which is why barometers guide weather forecasting. 5. A gauge reading would require the reference side to be open to the atmosphere, as in a manometer. _Source: OpenStax College Physics (CC BY 4.0), Ch 11 "Fluid Statics", section 11.6 Gauge Pressure, Absolute Pressure, and Pressure Measurement_