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 used for altimeter settings and manifold pressure in piston engines. The international standard sea-level atmospheric pressure is 29.92 in. Hg.
Plain English
It is a way of measuring air pressure. The number tells you how tall a column of liquid mercury the air pressure could push up in a sealed tube, measured in inches.
Context Anchor
You may see inches of mercury during a before-takeoff check when setting the altimeter or checking engine instruments.
Derivation
Hg' comes from hydrargyrum, the Latin name for mercury, meaning 'liquid silver.' Mercury was used historically because it is dense and liquid at normal temperatures, which made it practical to seal in a glass tube and watch the air's pressure push it up or down.
Why Pilots Care
Accurate readings in inches of mercury are required to set the altimeter correctly and verify engine power for safe takeoff.
Grounding Statement
Higher air pressure can push a mercury column higher, and that height is read in inches.
Intuition Check
Do not read Hg as a heading, hour, or checklist code here. In this context, Hg refers to mercury and the term means a pressure measurement.
Example Sentence 1
Tower reported the altimeter setting as 30.05 in. Hg, so the pilot dialed 30.05 into the altimeter window before takeoff.
Example Sentence 2
During the engine run-up the manifold pressure gauge showed 24 inches of mercury.