Definition
A unit of pressure measurement equal to the pressure exerted by a column of mercury one inch high at standard temperature and gravity. In aviation, it is the standard unit used to express atmospheric pressure for altimeter settings and manifold pressure in piston engines.
Plain English
A way of measuring air pressure. The number tells you how high a column of mercury that pressure can push up, measured in inches.
Context Anchor
Seen on altimeter settings, weather reports, and manifold pressure gauges during engine checks and before-takeoff checks.
Derivation
The double-quote symbol (") means inches. Hg is the chemical symbol for mercury, taken from the Latin hydrargyrum, meaning 'liquid silver.' Together, "Hg literally means 'inches of mercury' — a unit that dates back to early barometers, which measured air pressure by how high it pushed a column of mercury up a glass tube.
Why Pilots Care
Correct Hg values ensure the altimeter shows true altitude above sea level and allow the pilot to verify that the engine is producing expected power before takeoff.
Grounding Statement
When pressure goes up, the "Hg number goes up; when pressure goes down, the "Hg number goes down.
Intuition Check
Do not read the quote mark as punctuation here. In this aviation pressure unit, " means inches and Hg means mercury.
Example Sentence 1
Before takeoff, the pilot set the altimeter to 30.05 "Hg as reported by the tower.
Example Sentence 2
At idle the manifold pressure gauge read 22 inches Hg, confirming normal engine operation on the ground.