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Wikipedia

QNH is a Q code. It is a pressure setting 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. An airfield QNH will cause the altimeter to read field elevation on landing irrespective of the temperature. In the UK 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. In some parts of the world a similar procedure is adopted and this is known as "Regional QNH" however this name has been modified to the above in the UK for reasons of ambiguity.

QNH differs from QFE which refers to the altimeter setting that will cause the altimeter to read the height above a specific aerodrome, and therefore zero on landing. While using QFE is convenient while flying in a traffic circuit of an airfield, the most common procedure when flying "cross country" is to set the altimeter to either the local QNH or the standard pressure setting (1013.2 hPa). When 1013 hPa (mbar) is set on an altimeter subscale the aircraft's vertical position is referred to as a Flight level instead of an altitude.

A common mnemonic for QNH is "Nautical Height," and students and seasoned pilots alike differentiate it from QFE by the mnemonic "Field Elevation." Thus, if it's not QFE (providing the altitude above the field elevation, it's probably the QNH.

Some student pilots, particularly those in the UK, remember QNH as "Query Newlyn Harbour". Newlyn Harbour in Cornwall, UK is home to the National Tidal and Sea Level Facility which is a reference for mean sea level. Another way to remember is "Q - Not Here" meaning it refers to the pressure setting that applies away from the airfield. This is to distinguish it from QFE, which novices sometimes confuse.

Air Traffic Control will pass the QNH to pilots on clearing them to descend below the transition level, as part of air traffic control clearance, on request of the pilot or when the QNH changes. A typical radio conversation may be:-

Pilot: Golf Whiskey Alpha Charlie Foxtrot, requesting Cotswold QNH
ATC: Golf Charlie Fox, Cotswold QNH one-zero-one-three
Pilot: QNH one-zero-one-three, Golf Charlie Fox

Here, the pilot of GWACF requests the regional air pressure, which is given as 1013 millibars for the Cotswold region (one of twenty Altimeter Setting Regions into which UK Lower Airspace is divided). The pilot reads back the safety-critical part of the transmission (in this case the QNH), as it is required to do.

In most parts of the world, QNH is given in millibar (or hecto Pascal). In North America, QNH is given in inches and hundredths thereof inches of mercury (in the example, ATC would say "Golf Charlie Fox, altimeter two niner niner two" as in 29.92 inches of mercury).

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