Definition
QNH is the local barometric pressure value, corrected to mean sea level, that a pilot sets in the altimeter's Kollsman window so the instrument reads true elevation above mean sea level. With QNH set, the altimeter shows the airport's published field elevation when the aircraft is on the ground, and shows altitude above sea level when in flight.
Plain English
QNH is the pressure number you dial into your altimeter so it shows your height above sea level. Set it correctly and your altimeter will read the airport's elevation when you're sitting on the runway.
Context Anchor
Seen in altimeter setting procedures, international operations, ATIS or controller-issued altimeter settings outside many U.S. contexts, and instrument procedure discussions.
Derivation
QNH is one of the international Q-codes, a system of three-letter shorthand codes developed for radio communication in the early 1900s and adopted by aviation. The letters themselves don't stand for words — they're just an assigned code. QNH specifically refers to the sub-scale setting that gives altitude above sea level.
Why Pilots Care
Using the correct QNH keeps altitude readings accurate for terrain clearance and assigned flight levels.
Grounding Statement
If you set the correct QNH while parked on the runway, the altimeter should read close to the airport’s published elevation.
Intuition Check
QNH is not an altitude and not a radio frequency. It is a pressure setting you put into the altimeter so the altitude indication is based on sea level pressure.
Example Sentence 1
Approach told us, "Altimeter two niner niner two," so I set QNH to 29.92 in the Kollsman window before starting the descent.
Example Sentence 2
With the updated QNH entered, the altimeter now read 1800 feet above mean sea level over the valley.