Definition
A combined reference describing the altimeter setting regime used below the transition altitude. QNH is the local barometric pressure setting (in inches of mercury or hectopascals/millibars) which, when set in the altimeter, causes it to display the aircraft's altitude above mean sea level. The transition altitude is the published altitude at or below which an aircraft's vertical position is controlled by reference to altitudes (using the local QNH setting). Above the transition altitude, the altimeter is reset to the standard pressure setting (29.92 in Hg / 1013.2 hPa) and vertical position is expressed as flight levels.
Plain English
Below a certain altitude, pilots set their altimeter to the local pressure reading reported by air traffic control or the airport. That setting is called QNH, and the altitude at which pilots switch from this local setting to the standard pressure setting is called the transition altitude.
Context Anchor
Seen in instrument procedures, international operations, and altimeter-setting discussions where pilots must know when to change between local altimeter settings and standard pressure.
Derivation
QNH comes from the old international Q-code system used in early radio communication, where three-letter codes starting with Q were shorthand for common questions and answers. QNH originally stood for the question 'What should I set on the subscale of my altimeter so that the instrument would indicate its elevation if my aircraft were on the ground at your station?' It has no literal English expansion — it is simply a code that has stuck. 'Transition' comes from Latin transire, meaning 'to cross over,' which captures the idea of crossing from one altimeter reference to another.
Why Pilots Care
Correct QNH prevents altitude errors that could lead to terrain impact or airspace busts below the transition altitude.
Intuition Check
Do not read QNH transition altitude as a kind of airport elevation. It is the altitude where the pilot changes from using the local altimeter setting to using the standard pressure setting.
Example Sentence 1
Climbing out of London Heathrow, the pilot maintained the QNH setting until passing the transition altitude of 6,000 feet, then reset the altimeter to 1013 and reported climbing to flight level eight zero.
Example Sentence 2
With the correct QNH set, the altimeter read 1,800 feet when we crossed the airport threshold.