Altitude
Vertical distance measurement with varied definitions across contexts.
Altitude measures how far up something is from a set reference point. The specific reference point changes depending on the field—like aviation, mapmaking, sports, or weather science. In geography, people usually say "elevation" instead of altitude when talking about height above sea level.
In aviation, altitude has several distinct meanings, and pilots must know which one they're using. It's often measured with a barometric altimeter set to either local sea-level pressure or a standard sea-level pressure. GPS and radio altimeters are also used, especially near the ground during landings. Barometric readings are approximate because pressure changes with location and time, but they're useful for keeping aircraft separated. The main types include: indicated altitude (the altimeter reading set to local sea-level pressure); absolute altitude (distance above the terrain below, measured by radar altimeter); true altitude (actual elevation above sea level, corrected for temperature and pressure); height (vertical distance above a specific point, like an airfield); pressure altitude (elevation above a standard pressure plane of 1013.25 millibars or 29.92" Hg, used for flight levels); and density altitude (pressure altitude corrected for non-standard temperature and humidity, which affects aircraft performance—on hot days, high density altitude can prevent takeoff).
The atmosphere is divided into layers based on altitude, with boundaries that shift by season and latitude. The troposphere runs from the surface to about 8,000 meters at the poles and 18,000 meters at the equator, ending at the tropopause. Above that: the stratosphere (up to 50 km), mesosphere (up to 85 km), thermosphere (up to 675 km), and exosphere (up to 10,000 km). The Kármán line at 100 km is the conventional boundary between atmosphere and space.
High altitude on Earth's surface or in its atmosphere is often defined as starting at 2,400 meters (8,000 feet) above sea level. At such heights, atmospheric pressure is lower because gravity pulls air downward while heat makes air molecules bounce apart. The temperature profile of the atmosphere results from sunlight heating the ground, which then warms the air above it. Vertical measurements in the "down" direction are called depth.
- field
- Measurement, Aviation, Geography, Atmospheric Science
- known_for
- Vertical distance measurement; types include indicated, absolute, true, pressure, and density altitude; atmospheric layers; effects on human physiology
Lore & Background
In aviation, altitude has several distinct meanings. Indicated altitude is the reading on the altimeter set to local barometric pressure at mean sea level. Absolute altitude is the vertical distance above the terrain, measured by radar altimeter. True altitude is actual elevation above mean sea level, corrected for non-standard temperature and pressure. Height refers to vertical distance above a specified datum, such as an airfield elevation. Pressure altitude is elevation above a standard datum air-pressure plane, used for flight levels. Density altitude is altitude corrected for non-ISA atmospheric conditions, affecting aircraft performance.
Reader's Guide
Altitude is a fundamental concept across multiple disciplines. In aviation, precise definitions ensure safe navigation and separation of aircraft. Barometric altimeters provide approximate readings useful for standardizing flight paths, while GPS and radio altimeters offer accuracy near the ground. The Earth's atmosphere is divided into altitude regions: troposphere, stratosphere, mesosphere, thermosphere, and exosphere, with the Kármán line at 100 km marking the boundary to space. High altitude, beginning around 2,400 meters, reduces atmospheric pressure and oxygen, causing altitude sickness and requiring acclimatization. Above 8,000 meters, acclimatization becomes impossible. Interestingly, permanent residents at higher altitudes show lower overall mortality and reduced obesity prevalence. The temperature profile of the troposphere follows an adiabatic lapse rate, averaging 6.49 °C per kilometer in the International Standard Atmosphere.
Did You Know?
- In aviation, indicated altitude is the reading on the altimeter when set to local barometric pressure at mean sea level.
- The Kármán line, at 100 km above sea level, conventionally defines the demarcation between atmosphere and space.
- High altitude is sometimes defined to begin at 2,400 meters (8,000 ft) above sea level.
Frequently Asked Questions
What is Altitude in this encyclopedia entry?
Altitude is the vertical distance measured between a chosen reference datum and a given point or object. It is not a single fixed value but shifts meaning depending on the field—aviation, geography, atmospheric science, or sport—each applying its own datum and conventions.
What types of Altitude are recognized in canon?
The canon lists five principal kinds: indicated, absolute, true, pressure, and density altitude. Each serves a distinct purpose, from instrument readouts in cockpits to correcting for air density when calculating performance.
How does Altitude differ from Elevation?
While the two terms are often used interchangeably in casual speech, geography and surveying typically reserve 'elevation' for height above mean sea level. 'Altitude,' by contrast, can reference any chosen datum, making it the broader, more flexible term.
Why does Altitude matter for human physiology?
As vertical distance increases, atmospheric pressure and oxygen partial pressure drop, which stresses the respiratory and cardiovascular systems. This is why altitude sickness becomes a real concern in mountaineering and high-altitude aviation.
In which fields does Altitude appear as a key concept?
Altitude is central to measurement science, aviation navigation, geographical surveying, and atmospheric science. It also shows up in sport (e.g., track-and-field records at different venues) and in defining the layers of the atmosphere.
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