Definition
A unit of pressure measurement equal to the pressure exerted by a column of mercury one inch tall at a standard temperature and gravity. In aviation, it is used to express manifold pressure (engine intake pressure) and atmospheric pressure (altimeter setting). Standard sea-level atmospheric pressure is 29.92 "Hg.
Plain English
A way of measuring pressure by how tall a column of mercury that pressure can hold up. Pilots see it on the manifold pressure gauge and on altimeter settings.
Context Anchor
Seen on manifold pressure gauges and in power-setting instructions for airplanes with adjustable-pitch or constant-speed propellers.
Derivation
The unit dates from early barometers, where atmospheric pressure was measured by how high it pushed a column of liquid mercury inside a sealed tube. Mercury was used because it is dense, so the column stays a manageable height. The aviation symbol "Hg comes from the chemical symbol for mercury (Hg, from the Latin hydrargyrum, meaning "liquid silver").
Why Pilots Care
Manifold pressure is read in inches of mercury and determines available engine power for takeoff, climb, and cruise settings.
Grounding Statement
When the throttle is opened, the manifold pressure reading in inches mercury usually rises because the engine intake is receiving more air pressure.
Intuition Check
Do not read “inches” here as a distance the airplane travels or a physical amount of mercury in the engine. It is a pressure reading expressed on a mercury-based scale.
Example Sentence 1
After takeoff, the pilot reduced power by setting the throttle to 25 "Hg of manifold pressure.
Example Sentence 2
Standard sea-level pressure is 29.92 inches mercury.