Definition
An instrument that measures atmospheric pressure by the height of a column of mercury supported in a vertical glass tube whose lower end sits in an open reservoir of mercury. The weight of the surrounding air pressing down on the reservoir pushes mercury up the tube, and the height of the column — typically expressed in inches or millimeters of mercury — indicates the current atmospheric pressure. Standard sea-level pressure on this instrument reads 29.92 inches of mercury (1013.2 millibars).
Plain English
A pressure-measuring device that uses a tube of mercury standing in a small pool. The heavier the air outside, the higher it pushes the mercury up the tube, and you read the pressure off the height of that column.
Context Anchor
Seen in discussions of atmospheric pressure, standard pressure, and how pressure information is used in aviation weather and altimeter settings.
Derivation
Mercurial' comes from mercury, the liquid metal used in the tube. 'Barometer' comes from the Greek baros (weight) and metron (measure) — literally a 'weight measurer,' which is exactly what it does: it measures the weight of the air above it.
Why Pilots Care
Precise pressure readings from this type of barometer allow correct altimeter calibration and reliable weather assessment before flight.
Analogy
Think of air pressure as a hand pushing down on a liquid. The harder the push, the higher the mercury column can be held up in the tube.
Grounding Statement
Picture a tall glass tube standing upright in a small dish of mercury — air pressing on the dish pushes mercury up the tube, and you simply measure how tall the column stands.
Intuition Check
Mercurial does not mean “changeable” here. In this aviation context, it means the barometer uses mercury to measure air pressure.
Example Sentence 1
The mercurial barometer in the weather office read 29.95, slightly above standard sea-level pressure.
Example Sentence 2
During preflight, the instructor compared the mercurial barometer reading to the ATIS report for verification.