QNH

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

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

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: , request Cotswold QNH

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 ", altimeter three zero zero one" meaning 30.01 inches of mercury).

Meteorological model

References