Atmospheric Pressure at Sea Level

What is QNH?

QNH is the atmospheric pressure at a station, corrected to mean sea level using the International Standard Atmosphere (ISA) temperature profile. When a pilot sets the QNH value on the aircraft's altimeter, the instrument reads altitude above mean sea level (AMSL). This is the standard altimeter setting used for all operations below the transition altitude — takeoff, landing, and low-level flight — because it allows the altimeter to approximate the aircraft's actual height above sea level, which is what matters for terrain clearance.

The term comes from the Q-code system, a set of three-letter abbreviations developed in the early days of radio communication. "QNH" specifically means "what should I set on my altimeter to read my elevation if I were on the ground at your station?" ATC broadcasts the current QNH with every ATIS (Automatic Terminal Information Service) update and in METAR weather reports. Pilots set this value before departure and update it as they receive new QNH values from stations along their route.

QNH is distinct from QFE (which gives height above a specific aerodrome) and from the standard setting of 1013.25 hPa used for flight levels above the transition altitude. The transition altitude — the point where pilots switch from QNH to the standard setting — varies by country. Below the transition altitude, accurate QNH is essential; above it, everyone uses the same standard reference, so relative vertical separation is maintained regardless of local pressure variations. QNH values are expressed in hectopascals (hPa) in most of the world and in inches of mercury (inHg) in North America.

Why It Matters for Airspace Risk

Altimetry accuracy is the foundation of vertical separation, and QNH errors directly translate to altitude errors. A 1 hPa error in the QNH setting results in approximately 30 feet of altitude error — seemingly small, but significant when RVSM separation is only 1,000 feet. In regions with sparse weather observation networks, QNH values may be based on distant stations and may not accurately represent local pressure conditions, introducing systematic altitude errors for all aircraft in the area.

GPS spoofing introduces a newer and more subtle altimetry threat. Modern aircraft use GPS-derived altitude as a cross-check for barometric altitude. When GPS spoofing feeds incorrect altitude data, it can create a discrepancy that confuses the aircraft's systems or, in more sophisticated scenarios, could theoretically be used to manipulate the aircraft's understanding of its vertical position. The interaction between barometric altimetry (based on QNH) and satellite-derived altitude (based on GPS) is an active area of concern in the context of the widespread GPS interference events occurring since 2022. In extreme weather events, rapidly changing pressure (such as during intense low-pressure systems) can also cause altimetry surprises if QNH updates are not received frequently enough.

Key Facts

• •A 1 hPa error in QNH causes approximately 30 feet (9 meters) of altitude error on the altimeter.
• •QNH is used below the transition altitude; above it, all aircraft switch to the standard setting of 1013.25 hPa for flight levels.
• •QNH values are expressed in hectopascals (hPa) globally and inches of mercury (inHg) in North America.
• •Sparse weather station networks in remote or conflict-affected regions can lead to inaccurate QNH data.
• •GPS spoofing can create conflicts between satellite-derived and barometric altitude, complicating altimetry cross-checks.

Related Terms