QNH

QNH is a Q code which is an acronym for "Question Nil Height". 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.

 
History and origin of the term
Student pilots sometimes 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.
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").
 
Altitude above mean sea level
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 (millibar) 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.
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: <aircraft callsign>, request Cotswold QNH
• ATC: <aircraft callsign>, Cotswold QNH one zero one six
• Pilot: QNH one zero one six, <aircraft callsign>

Here, the pilot requests the regional air pressure, which is given as 1016 millibars for the Cotswold Altimeter Setting Region ("ASR") (one of twenty ASRs into which UK Lower Airspace is divided). The pilot is required to read back the safety critical part of the transmission (in this case, the QNH value).
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 "<aircraft callsign>, altimeter three zero zero one" meaning 30.01 inches of mercury).
 
The unit is named after Blaise Pascal, the eminent French mathematician, physicist, and philosopher noted for his experiments with a barometer, an instrument to measure air pressure. The name pascal was adopted for the SI unit newton per square metre by the 14th CGPM in 1971.
 
Standard atmospheric pressure is 101,325 Pa = 101.325 kPa = 1013.25 hPa = 1013.25 mbar = 760 Torr
 
Atmospheric air pressure is often given in millibars where "standard" sea level pressure (1 atm) is defined as 1013.25 mbar (hPa), equal to 1.01325 bar. Despite millibars not being an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars. For example, the weather office of Environment Canada uses kilopascals and hectopascals on their weather maps. In contrast, Americans are familiar with the use of the millibar in US reports of hurricanes and other cyclonic storms.