Definition
An instrument that measures atmospheric pressure by balancing the weight of the air against a column of mercury in a sealed glass tube. The height of the mercury column, typically measured in inches or millimeters, indicates the current atmospheric pressure. Standard sea-level pressure supports a mercury column 29.92 inches (760 mm) high.
Plain English
A pressure-measuring tool that uses a tall tube of mercury. The heavier the air pressing down, the higher the mercury rises in the tube. You read the pressure by measuring how tall the mercury column is.
Context Anchor
Seen in weather reports, altimeter-setting discussions, and older explanations of why aviation pressure is often given in inches of mercury.
Derivation
Barometer comes from the Greek 'baros' meaning weight, and 'metron' meaning measure -- literally a 'weight measurer.' It measures the weight of the atmosphere pressing down on the mercury.
Why Pilots Care
The mercury barometer is the reason altimeter settings are given in 'inches of mercury.' When a controller says 'altimeter 29.92,' that number originally came from how high the air pressure could push a column of mercury at a weather station.
Analogy
Think of air pressure as an invisible hand pushing on a liquid in a tube. Stronger pressure pushes the mercury higher; weaker pressure lets it drop lower.
Grounding Statement
Picture a tall glass tube, closed at the top, sitting upside-down in a small dish of mercury. Air pushing down on the dish forces mercury up into the tube. Higher pressure pushes the mercury higher; lower pressure lets it fall.
Intuition Check
A mercury barometer is not a barometer used on the planet Mercury. It is a pressure-measuring instrument that uses the liquid metal mercury.
Example Sentence 1
Before electronic sensors became standard, weather stations relied on a mercury barometer to report local atmospheric pressure.
Example Sentence 2
Older training materials described how a mercury barometer was read to confirm local pressure.