Altitude

Altitude is a measurement of vertical distance between a reference datum and a point or object. Its exact definition depends on the context, which may include aviation, geometry, surveying, mountaineering, sport or atmospheric science. Although the term is commonly used in everyday language to describe height above sea level, geography often prefers the term elevation for this static measurement. In contrast, altitude is widely used in dynamic contexts, notably in aviation, where accurate height information is crucial for safe navigation, separation, and flight operations. In fields such as trekking, sport performance and medicine, understanding altitude is vital because reduced atmospheric pressure at higher elevations affects oxygen availability and physiological responses.
Altitude may also refer to the downward vertical direction, in which case the measurement is more commonly described as depth. Whether expressed as height or depth, the principle remains a vertical distance measurement referenced to a specific datum.

Altitude in aviation

In aviation, altitude is essential for ensuring separation between aircraft and maintaining obstacle clearance. Commercial airliners cruise in the stratosphere, where the air is stable and turbulence is reduced compared with the troposphere below. The stratosphere contains the ozone layer and represents an upper limit for conventional jet aircraft and meteorological balloons, with air density being only a fraction of its value at lower levels.
Aviation terminology includes several types of altitude, each defined by a particular reference datum:

• Indicated altitude is the value shown on the aircraft’s pressure altimeter when set to the local QNH, the atmospheric pressure reduced to mean sea level.
• Absolute altitude is the vertical distance of the aircraft above the terrain immediately below. It may be measured using a radar or absolute altimeter and is often expressed in feet or metres Above Ground Level (AGL).
• True altitude is the actual height above mean sea level (MSL), obtained by correcting indicated altitude for non-standard temperature and pressure.
• Height is the vertical distance above a specified point, frequently the airfield elevation in operational radiotelephony contexts.
• Pressure altitude is the altitude referenced to a standard pressure datum of 1013.25 millibars or 29.92 inHg. This value is used to assign flight levels, which standardise vertical separation in controlled airspace.
• Density altitude expresses altitude in terms of the density of the air in the International Standard Atmosphere (ISA). It is a critical factor in aircraft performance calculations: high density altitude—caused by high temperature, high humidity or low pressure—can significantly reduce lift and engine power, sometimes preventing safe take-off at high-elevation airports.

These different categories can be regarded as complementary ways of expressing the aircraft’s vertical position relative to varying reference frames. Above the transition altitude—18,000 feet in the United States, but lower in many countries—pilots set their altimeters to the standard pressure setting. Altitude is then reported as a flight level (for instance, FL310), ensuring international consistency at cruise altitudes.
The primary instrument used on the flight deck is the pressure altimeter, derived from the aneroid barometer but calibrated to display distance rather than pressure. For low-level height measurement, radar altimeters provide direct readings of absolute altitude, particularly during approach and landing.

Altitude in surveying and geometry

In geometry and topographic science, altitude refers to the vertical dimension used to map terrain features accurately. Surveyors use altitude data to produce contour maps, digital elevation models and geodetic reference frames. These measurements help assess landform gradients, watershed characteristics and infrastructure planning. Elevation above mean sea level remains the foundational datum for global mapping systems.

Altitude in atmospheric studies

In meteorology and atmospheric science, altitude defines the vertical structure of the atmosphere, which is divided into several distinct layers. Although boundaries vary with latitude and season, approximate ranges are widely accepted:

• Troposphere: from the surface to between 8 km at the poles and about 18 km at the Equator, ending at the tropopause.
• Stratosphere: up to approximately 50 km.
• Mesosphere: up to around 85 km.
• Thermosphere: up to several hundred kilometres.
• The Kármán line, at about 100 km above sea level, is commonly taken as the boundary between the atmosphere and outer space.

The upper mesosphere, thermosphere and exosphere are often conventionally described as space due to the extremely low air density.

Atmospheric pressure, temperature and lapse rates

Atmospheric pressure decreases with altitude because gravity compresses air molecules near the surface while heat energy causes expansion. The interaction between radiation and convection shapes the atmosphere’s temperature profile. Solar radiation warms the surface, which transfers heat to the air above. Hot air expands, rises and cools according to adiabatic processes. The dry adiabatic lapse rate is about 9.8°C per kilometre, while the moist adiabatic lapse rate is lower—around 5.5°C per kilometre—due to latent heat released when water vapour condenses. The International Standard Atmosphere adopts a temperature lapse rate of 6.49°C per kilometre for general reference. Significant vertical convection occurs primarily within the troposphere; the stratosphere is notably more stable because ozone absorption of ultraviolet radiation warms this layer from above.

High altitude and physiological effects

Regions and atmospheric levels significantly above mean sea level are described as high altitude. Medical definitions vary, but altitude effects begin to appear in humans above roughly 2,400 metres. Chronic habitation has not been recorded above approximately 5,800 metres for periods longer than two years. Reduced atmospheric pressure at high altitude lowers the partial pressure of oxygen, affecting respiration and leading to a range of altitude-related illnesses, including altitude sickness, high-altitude pulmonary oedema (HAPE) and high-altitude cerebral oedema (HACE).
The human body can acclimatise through increased breathing rate, elevated heart rate and alterations in blood chemistry. Acclimatisation requires several days or weeks, but above about 8,000 metres—the so-called death zone—long-term adaptation becomes impossible. Interestingly, long-term population studies show lower overall mortality rates among permanent residents of higher altitudes, and research suggests a dose–response relationship between increased elevation and decreased obesity prevalence. Emerging hypotheses also propose potential protective effects of high-altitude living against certain neurodegenerative conditions.

Altitude in satellite orbits

In orbital mechanics, altitude defines the distance between an orbiting object and a reference planetary surface. Satellite orbits are categorised by altitude, such as low Earth orbit, medium Earth orbit and geostationary orbit. Altitude affects atmospheric drag, orbital lifetime and the operational functions of satellites.