QNH is one of the many Q codes. It is defined as, "barometric pressure adjusted to sea level." It is a pressure setting used by pilots, air traffic control (ATC), and low frequency weather beacons to refer to the barometric setting which, when set on an aircraft's altimeter, will cause the altimeter to read altitude above mean sea level within a certain defined region. Within United Kingdom airspace, these are known as Altimeter Setting Regions (ASRs); these regions may be large areas, or apply only to the airfield for which the QNH was given. An airfield QNH will cause the altimeter to show airfield altitude, that is, the altitude of the centre point of the main runway above sea level 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.
Calculation of Transition level :- suppose QNH is 1002 & Transition altitude of an aerodrome is 4000 ft. then transition level is equal to 4000 + (1013.2 - 1002) x 30 equals to 4342 ft which is nearer to flight level 45 then transition level will be 1000 ft above the transition altitude i.e Fl 55.
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 "Query: Not Here" meaning it refers to the pressure setting that applies away from the airfield. This is to distinguish it from QFE ("Query: Field Elevation"), which typically refers to altitude relative to an airfield.
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 used communications were incorporated into a Q code. 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 "Query: Nautical Height", whereas the mnemonic often used for QFE is "Query: Field Elevation".
Altitude above mean sea level
In most parts of the world, QNH is given in hectopascals (in the past in millibars). In North America, QNH is given in hundredths of inches of mercury.