Barometric Altimetry Explained

This article closes the recommended trio of Elevation & Vertical Datums supports with an aviation-specific topic that ties into /learn/elevation-vs-altitude-vs-height. Barometric altimetry is the dominant aviation altitude reference and remains so despite GPS — for reasons of reliability, certification, and integration with global air traffic management.

How the instrument works

A barometric altimeter is a precise barometer with a display calibrated to show altitude rather than pressure.

Inside the instrument:

• An aneroid capsule (a sealed metal bellows) expands or contracts with the surrounding air pressure.
• The capsule's motion drives a mechanical linkage that rotates pointers on the altimeter face.
• A small adjustable subscale (the Kollsman window) displays the current altimeter setting in inches of mercury (inHg) or hectopascals (hPa).
• The setting knob adjusts the reference pressure point used in the conversion.

The instrument is mechanically simple, requires no electrical power, and is reliable across decades of operation. Backup altimeters in airliners are typically self-contained mechanical altimeters with internal lighting — operating after total power loss.

The conversion: pressure to altitude

The conversion uses the International Standard Atmosphere (ISA):

• Sea-level standard pressure: 1013.25 hPa = 29.92 inHg.
• Sea-level standard temperature: 15°C.
• Tropospheric lapse rate: 6.5°C/km (≈2°C per 1,000 ft).
• Tropopause: 11,000 m (36,089 ft), -56.5°C.
• Stratosphere: isothermal at -56.5°C up to 20,000 m (65,617 ft), then temperature increases.

The pressure-altitude relationship for the troposphere:

h = (T_0 / L_T) · [1 - (p / p_0)^(R · L_T / g)]

Where T_0 = 288.15 K, L_T = 0.0065 K/m, p_0 = 1013.25 hPa, R = 287.05 J/(kg·K), g = 9.80665 m/s². The formula is roughly logarithmic.

Approximations:

• Below 5,000 ft: ~30 ft per 1 mbar pressure decrease.
• Around 18,000 ft: ~50 ft per 1 mbar.
• Around 30,000 ft: ~70 ft per 1 mbar.

The altimeter applies this conversion mechanically through the aneroid capsule's pressure-deflection characteristic — no math is needed by the pilot during operation.

The three altimeter settings

ICAO defines three altimeter setting conventions, identified by Q-codes (an old aviation radio shorthand):

QNH: regional pressure setting

The most common setting. QNH is the local sea-level pressure for the region, corrected for temperature. With QNH set on the altimeter:

• On the ground at an airport: the altimeter reads the airport's elevation above MSL (the published field elevation).
• In flight: the altimeter reads approximate true altitude above MSL (subject to non-standard-temperature errors).

QNH is broadcast by ATIS (Automatic Terminal Information Service) and by ATC. Pilots set QNH for departure and arrival, transitioning between regions as they fly through different pressure regions.

QNH is used below transition altitude.

QFE: airfield-zero setting

QFE is the pressure at the airfield. With QFE set on the altimeter:

• On the runway: the altimeter reads zero.
• In flight: the altimeter reads height above the airfield.

QFE is convenient for circuit work (training flights, approach practice) because the altimeter directly shows altitude above the runway.

QFE is used primarily in Eastern Europe and some military applications. The US and most of Western Europe use QNH instead.

QNE: standard pressure setting

QNE is the standard pressure: 29.92 inHg / 1013.25 hPa, regardless of local conditions. With QNE set on the altimeter:

• The altimeter reads pressure altitude — the height that would correspond to the measured pressure in a standard atmosphere.
• All aircraft worldwide using QNE read the same altitude for the same actual pressure.

QNE is used above transition altitude for flight levels. The phrase “Flight Level 350” (FL350) means pressure altitude 35,000 ft.

Why QNE above transition? Standardized vertical separation: when all aircraft use the same setting, ATC can rely on aircraft at adjacent flight levels being separated by the assigned altitude difference regardless of local pressure. If aircraft used QNH at high altitudes, an aircraft flying through a low-pressure region while another stayed in high pressure could lose separation.

Transition altitude and level

The transition altitude is where pilots switch from QNH to QNE going up; the transition level is the corresponding flight level on the way down (often slightly above the transition altitude).

| Country / region | Transition altitude | | ---------------- | ------------------- | | United States | 18,000 ft (FL180) | | Canada | 18,000 ft | | Russia | 1,000 m (3,300 ft) | | Most of Western Europe | 5,000 or 6,000 ft | | UK | 3,000-18,000 ft varies by airspace | | Mexico | 18,500 ft | | Iceland | 7,000 ft |

The low European transition altitudes reflect smaller controlled-airspace structure and shorter typical climb profiles. The high US transition altitude reflects the larger continental airspace and longer high-altitude cruise segments.

Transition procedure: ATC instructs the pilot to climb through transition altitude and use the standard setting (29.92). The pilot rotates the Kollsman window to 29.92 / 1013.25 and continues climbing on pressure altitude. On descent, the pilot resets to local QNH at the transition level.

Pressure altitude vs density altitude

Two concepts related to barometric altitude but distinct.

Pressure altitude

What the altimeter reads with the standard QNE setting. Reflects actual atmospheric pressure but doesn't correct for temperature.

Density altitude

Pressure altitude corrected for non-standard temperature. The standard atmosphere assumes 15°C at sea level with a 2°C/1000ft lapse. Real days are hotter or colder than standard.

Hot day: actual air is less dense than standard; density altitude is higher than pressure altitude. The aircraft's engines and wings perform as if at a higher altitude.

Cold day: actual air is denser; density altitude is lower than pressure altitude. Performance is better than at standard.

Computation:

density altitude ≈ pressure altitude + 120 ft × (OAT - ISA temp at that altitude)

Where OAT is the outside air temperature in °C.

Worked example: Denver airport (5,280 ft MSL, ~6,000 ft pressure altitude on a typical day), 30°C OAT. ISA temp at 6,000 ft is 15 - 12 = 3°C. Temperature deviation: 30 - 3 = 27°C. Density altitude: 6,000 + 120 × 27 = 9,240 ft.

An aircraft on the ground at Denver on a hot day performs like it's at over 9,000 ft. Many small aircraft cannot safely take off from Denver in summer heat.

See /learn/elevation-vs-altitude-vs-height for the broader altitude landscape.

Sources of altimeter error

Non-standard temperature

The ISA assumes 6.5°C/km lapse from a 15°C sea-level baseline. Real days deviate. Cold winter conditions cause the altimeter to read higher than the true altitude (because cold dense air has higher pressure gradient — the altimeter assumes standard rate but actual rate is steeper).

For high-precision flight (low-visibility approaches), cold temperature corrections are applied per ICAO Doc 8168 PANS-OPS. The correction is largest at high mountainous airports in cold weather: Denver in winter can require ~200 ft correction for an ILS approach.

Non-standard pressure

The QNH setting accounts for regional pressure variation, but persistent pressure gradients between the aircraft and the QNH source produce errors. The altimeter assumes the QNH source pressure applies everywhere; rapid pressure changes during flight introduce error.

Mechanical errors

• Instrument calibration drift: altimeters require periodic recalibration.
• Lag: rapid altitude changes can lag the actual altitude by 100-500 ft until the aneroid catches up.
• Hysteresis: the instrument may read slightly different during climb vs descent.
• Mechanical friction: vibration during flight generally improves accuracy by overcoming static friction.

Static port location

The altimeter measures pressure from the aircraft's static port (or alternate). Airflow around the aircraft can produce small pressure perturbations. Position error is calibrated and accounted for in the airspeed-altimeter system but can produce ±50-100 ft errors in some flight regimes.

Blocked static port

A blocked static port (insect blockage, icing) leaves the altimeter showing pressure from the moment of blockage. Pilots have alternate static sources (typically the cabin air pressure for unpressurized aircraft) for use after blockage detection. Multiple fatal accidents have resulted from blocked static ports + crew failure to recognize.

The “high-to-low, look out below” rule

A safety mnemonic warning pilots about altimeter errors:

“From high to low, look out below. From hot to cold, look out below.”

Translated:

• Flying from high-pressure to low-pressure regions without updating QNH: indicated altitude stays the same, but the air column is shorter — true altitude is lower than indicated. The ground is closer than the altimeter shows.
• Flying from hot to cold regions: cold air is denser; the altimeter under-corrects for the density change. True altitude is lower than indicated.

The combined “cold weather + low pressure” case is the dangerous one — particularly relevant for mountain flying in winter.

Mitigation: regular altimeter setting updates from ATC; cold-weather altimetry corrections during mountain operations; awareness of pressure-system movement.

The altimeter in modern aircraft

A modern airliner cockpit has multiple altitude indicators:

Primary barometric altimeter

The pilot's primary altitude reference. Mechanical plus electronic backup. Displayed on the primary flight display (PFD) in modern glass cockpits. Sets QNH or QNE.

Secondary / standby altimeter

A backup mechanical altimeter for use in primary display failure. Self-contained mechanical instrument with internal lighting.

Encoding altimeter / Mode-C transponder

Reports pressure altitude to ATC radar via Mode-C or Mode-S transponder. ATC's automated systems display this altitude (with corrections for the local altimeter setting if below transition).

Radar altimeter (RADALT)

A separate instrument measuring height above ground (AGL) using radar reflection. Active below ~2,500 ft typically; essential for autoland approaches and low- altitude operations.

GPS-derived geometric altitude

Modern flight management systems (FMS) display GPS-derived altitude alongside barometric. The two should agree within 100-200 ft under normal conditions; significant discrepancy indicates an altimeter error.

Inertial reference system

High-end aircraft have inertial reference units (IRUs) providing inertially derived altitude (with GPS corrections). Used as a backup and for system cross-check.

All five sources coexist in modern airline operations; each has specific uses.

Aviation altitude history

Early aviation

Pre-WWI aircraft used simple aneroid barometers as altitude indicators. Calibration was rough; errors of hundreds of feet were normal.

Lucien Vidi invented the aneroid barometer in 1844 — a sealed metal capsule that expands and contracts with pressure, allowing portable pressure measurement without mercury. The instrument was adopted for aviation use as soon as aviation existed.

Inter-war period

Standardized altimeter design emerged in the 1920s and 1930s. Paul Kollsman invented the adjustable barometric altimeter (the “Kollsman window”) in 1928 — the now-standard subscale for setting the altimeter to a specific reference pressure.

Standardization

ICAO standardized altimeter setting procedures in the 1940s and 1950s, establishing QNH, QFE, and QNE as the global conventions. The transition-altitude concept was added to formalize the QNH-to-QNE crossover.

Radio-derived altitude

Radio altimeters (now radar altimeters) emerged in the 1930s for low-altitude reference, particularly useful for approach-to-landing minimums in instrument flight.

GPS era

GPS altitude became available in civilian aircraft in the 1990s. WAAS-augmented GPS achieves vertical accuracy of ~1-3 m at low altitudes, more than adequate for non-precision approaches and Vertical Navigation (VNAV) en route. However, GPS has not replaced barometric altimetry because:

• Air traffic separation is built around pressure altitude.
• Type certification rules still mandate barometric.
• Backup considerations (GPS jamming, satellite outages).
• Mode-C transponders use pressure altitude.

Modern aircraft use both routinely.

Specific operational considerations

Mountain flying

In mountainous terrain, altimetry errors are particularly dangerous because terrain is close to indicated altitude. Best practices:

• Get current altimeter settings along the route.
• Apply cold-temperature corrections in winter at high mountain airports.
• Use radar altimeter for terrain awareness.
• Maintain extra margin above charted MEA (Minimum Enroute Altitude).

Polar operations

Above 80° N or S, GPS coverage can degrade due to poor satellite geometry, while barometric altimetry remains reliable. ICAO has specific procedures for polar flight operations.

Reduced Vertical Separation Minima (RVSM)

Above FL290 (FL300 in some regions), aircraft are separated by 1,000 ft vertical intervals (vs the 2,000 ft separation used below). RVSM requires precision altimetry equipment certification.

Standard pressure errors

The standard pressure 29.92/1013.25 isn't the actual average sea-level pressure. Annual mean sea-level pressure is about 1013-1015 hPa at most locations; the standard value is a convention. The small offset doesn't matter operationally.

Pressure altitude vs flight level

A common notation:

• Pressure altitude in feet: 18,000 ft, 25,000 ft, 35,000 ft.
• Flight Level (FL): pressure altitude in hundreds of feet, dropped to integers. So 18,000 ft → FL180; 35,000 ft → FL350.

The two are equivalent above transition altitude. FL notation is used in ATC and pilot conversation; the foot-based altitude is what appears on the altimeter.

Common misconceptions

“The altimeter shows true altitude.” No — it shows pressure-derived altitude with specific assumptions. Various corrections (temperature, pressure, ISA deviation) are needed to derive true altitude.

“GPS replaces the altimeter.” No — aviation type certification mandates barometric altimetry; ATC separation is built on pressure altitude; GPS is a complementary reference.

“Pressure altitude equals MSL.” Only when the day is exactly standard. Non-standard days have pressure altitude differing from true MSL altitude. Above transition, aircraft fly pressure altitude — fine for separation, less precise for terrain.

“The Kollsman window setting affects pressure measurement.” No — it adjusts the reference pressure for altitude display. The underlying pressure measurement is the same regardless of setting.

“Altimeter setting only matters near the ground.” Important at all altitudes below transition. Above transition, all aircraft use QNE regardless of local pressure.

“QFE is obsolete.” Still in use in Eastern Europe (Russia, Belarus, some Central Asian states), some military operations, and some flight training environments. The QNH/QFE choice is a national/regional convention.

“Cold temperature only affects altitude in extreme cases.” Even moderate cold (10°C below standard) at high mountain airports introduces ~100 ft of altimeter error — meaningful for IFR approaches.

“Pressure altitude is what the radar shows.” Yes — Mode-C and Mode-S transponders report pressure altitude to ATC, which then displays the QNH-corrected altitude on the controller's display.

“Altimeters fail silently.” They can, particularly with blocked static ports. Pilots are trained to cross-check altimeters against other sources (radar altimeter, GPS, IRU) regularly, especially during approach.

“ISA is realistic.” ISA is an average atmosphere; real days vary substantially in temperature, pressure, and lapse rate. The deviation from ISA is what introduces altimeter errors that pilots must compensate for.

“Density altitude only matters in performance.” Indeed, density altitude doesn't appear on the altimeter face. But it matters critically for takeoff, climb, and engine performance. Pilots compute density altitude in pre-flight planning, particularly at hot/high airports.

“Modern aircraft don't use Q-codes.” The Q-codes (QNH, QFE, QNE) are still standard in aviation despite predating modern aircraft by decades. The terminology is universal across English- and non-English-speaking aviation worldwide.