QNH
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de:QNHQNH is a Q code used by pilots, air traffic control (ATC) and low frequency weather beacons to refer to the barometric altimeter setting which will cause the altimeter to read altitude above mean sea level within a certain defined region. This region may be fairly widespread, or apply only to the airfield for which the QNH was given. In the latter case, an altimeter calibrated to the QNH will read the runway elevation above sea level on landing - pilots will usually use the more appropriate QFE for determining height while 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 North America, QNH is given in inches of mercury.