Definition
A unit of pressure measurement equal to the pressure exerted by a column of mercury one inch tall at standard temperature and gravity. In aviation it is the standard unit for reporting manifold pressure, altimeter settings, and atmospheric pressure. Standard sea-level atmospheric pressure is 29.92 "Hg.
Plain English
A way of measuring pressure by comparing it to how high it would push a column of mercury in a tube. The taller the column, the higher the pressure. Aviation uses this unit on altimeters and on the manifold pressure gauge in airplanes with constant-speed propellers.
Context Anchor
Seen when discussing manifold pressure, engine power settings, and constant-speed propeller operation.
Derivation
Mercury is the liquid metal historically used in barometers because it is dense and easy to read. Early scientists measured atmospheric pressure by how high the air could push mercury up a sealed glass tube. The height of that column, in inches, became the unit. The symbol "Hg uses Hg from the Latin hydrargyrum, meaning 'liquid silver.'
Why Pilots Care
Lets a pilot set and monitor exact engine power on piston aircraft.
Grounding Statement
When the throttle is opened, the pressure in the engine intake usually rises, and the manifold pressure reading in inches of mercury goes up.
Intuition Check
Do not read “inches” here as a length you are measuring on the airplane, and do not think mercury is inside the engine. “Inches mercury” is a pressure unit that comes from an old mercury-column measuring method.
Example Sentence 1
The current altimeter setting is 30.05 inches of mercury.
Example Sentence 2
Standard sea-level pressure equals 29.92 inches mercury.