QNH

QNH is an aviation term referring to the altimeter setting that causes the instrument to indicate altitude above mean sea level (AMSL). It is the local pressure value, adjusted to sea level, that reflects the current atmospheric conditions. In practice, QNH is the pressure value provided by meteorological services and appears in weather reports such as METARs, typically expressed in hectopascals (hPa) or millibars.

The QNH value is derived from the observed surface pressure at a location and corrected to the sea level using standard atmosphere assumptions. This correction accounts for the height of the location and the vertical temperature profile, resulting in a sea-level pressure that would be observed if the air were at sea level under standard conditions. Because atmospheric pressure changes with weather, QNH is not constant and is updated with weather observations.

In pilots’ procedures, setting QNH on an altimeter makes the instrument indicate height above mean sea level. This is used for altitude reporting and for maintaining vertical separation when flying below the transition altitude. Above the transition altitude, pilots typically set a standard pressure (QNE), which is 1013.25 hPa, to read flight levels independent of local weather. QFE is a different setting that causes the altimeter to display height above the airfield rather than AMSL; their use depends on local regulations and procedures.

Notes: Temperature affects the accuracy of altimeter readings derived from QNH, and pilots should consider potential deviations in nonstandard conditions.