QNH is a Q code used by pilots, air traffic control (ATC) and low frequency weather beacons to refer to the current mean air pressure over a given region at sea level (if there is no sea, this is a virtual value by adjusting the value at the ground for its elevation), the "regional pressure setting". This value is used by pilots to calibrate the altimeter on board the aircraft, to ensure that the pilot is accurately aware of his actual flying height. The QNH allows a pilot to know his altitude with respect to mean sea level

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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