Definition
A unit of pressure measurement based on the height, in inches, of a column of mercury that a given pressure can support. In the International Standard Atmosphere, sea level pressure is defined as 29.92 inches of mercury. The symbol "Hg uses the chemical abbreviation for mercury (Hg).
Plain English
A way of measuring air pressure by comparing it to how high it can push a column of mercury inside a sealed tube. The higher the column, the greater the pressure.
Context Anchor
Seen in weather reports, altimeter settings, instrument flying discussions, and International Standard Atmosphere references.
Derivation
Comes from the original mercury barometer, invented in the 1600s. Air pressure pushes down on a pool of mercury, which forces mercury up into a sealed glass tube. The height of the mercury column, measured in inches, equals the pressure pushing it up. The unit stuck even though modern instruments no longer use actual mercury.
Why Pilots Care
Correct use of inches of mercury ensures accurate altimeter readings and safe altitude awareness.
Grounding Statement
Picture air pressure pushing on a pool of mercury in a glass tube: stronger pressure pushes the mercury higher, and weaker pressure lets it sit lower.
Intuition Check
Do not read “inches” here as a normal distance measurement by itself. In this term, inches describes the height of a mercury column used to measure pressure.
Example Sentence 1
Tower reported the altimeter setting as 30.05 inches of mercury, so I set 30.05 in the Kollsman window before takeoff.
Example Sentence 2
The pilot adjusted the altimeter to match the reported 30.05 inches of mercury before takeoff.