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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 United Kingdom 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 to avoid ambiguity.

QNH differs from QFE, which refers to the altimeter setting that will cause the altimeter to read the height above a specific aerodrome or ground level, and therefore read zero on landing. While using QFE is convenient while flying in the 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 – 1,013.25 hPa (29.92 inHg). When 1013 hPa (mbar) is set on an altimeter subscale the aircraft's vertical position (in feet, divided by one hundred) is referred to as a Flight level instead of an altitude.

The abbreviation QNH originates from the days when voice modulated radio was often difficult to receive, and communication was done by Morse Code. To avoid the need for long Morse transmissions, many of the most commonly asked questions were incorporated into a Q code. When the operator sent the letter 'Q' it indicated that he had a question to ask. To ask for atmospheric pressure at sea-level (i.e., at zero altitude) the letters 'QNH' would be transmitted. A common mnemonic for QNH is "No Height", (whereas the mnemonic often used for QFE is "Field Elevation").

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 might be:-

• Pilot: Golf Whiskey Alpha Charlie Foxtrot, request Cotswold QNH
• ATC: Golf Charlie Foxtrot, Cotswold QNH one-zero-one-six
• Pilot: QNH one-zero-one-six, Golf Charlie Foxtrot

Here, the pilot of G-WACF requests the regional air pressure, which is given as 1016 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 is required.

In most parts of the world, QNH is given in millibars (or hectopascals). In North America, QNH is given in hundredths of inches of mercury (in the example, ATC would say "Golf Charlie Fox, altimeter three zero zero one" meaning 30.01 inches of mercury).

[edit] Meteorological model

The letters QNH may also refer to the Quasi-Non-Hydrostatic meteorological model.

[edit] See also

• Flight level
• QFE
• QFF
• Pressure altitude