QNH is a Q code used by pilots, air traffic control (ATC) and low frequency weather beacon s to refer to the barometric altimeter setting which will cause the altimeter to read altitude above mean sea level in the vicinity of an airfield. An altimeter set to QNH will therefore read runway elevation on landing. The lowest forecast value of QNH for an altimeter setting region is called the "regional pressure setting" and may be used to ensure safe terrain separation when cruising at lower altitudes.ATC may update pilots with the QNH on a regular basis.
A typical radio conversation might go:
- Pilot: Golf Whiskey Alpha Charlie Foxtrot, requesting regional QNH
- ATC: Golf Charlie Fox, Cotswold QNH one-zero-one-three
- Pilot: QNH one-zero-one-three, Golf Charlie Fox
Here, the pilot of G-WACF requests the regional air pressure, which is given as 1013 millibars for the Cotswold region.
The pilot acknowledges, as he is required to do.
In most parts of the world, QNH is given in millibars (or, expressed more correctly, hectopascals, which is the SI unit).
In the USA, QNH is given in inches of mercury.
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