Altitude

Altitude

Discussion

Encyclopedia
Altitude or height is defined based on the context in which it is used (aviation, geometry, geographical survey, sport, and more). As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context. Although the term altitude is commonly used to mean the height above sea level of a location, in geography
Geography
Geography is the science that studies the lands, features, inhabitants, and phenomena of Earth. A literal translation would be "to describe or write about the Earth". The first person to use the word "geography" was Eratosthenes...

the term elevation
Elevation
The elevation of a geographic location is its height above a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface ....

is often preferred for this usage.

Vertical distance measurements in the "down" direction are commonly referred to as depth.

Altitude in aviation and in spaceflight

In aviation
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...

, the term altitude can have several meanings, and is always qualified by either explicitly adding a modifier (e.g. "true altitude"), or implicitly through the context of the communication. Parties exchanging altitude information must be clear which definition is being used.

Aviation altitude is measured using either Mean Sea Level (MSL) or local ground level (Above Ground Level, or AGL) as the reference datum.

Pressure altitude
Pressure altitude
In aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed baseline pressure setting. The baseline pressure is 1013.25 hPa, equivalent to 1013.25 millibar, or 29.92 inches of mercury. This setting is equivalent to the air pressure at mean sea level in the...

divided by 100 feet (30m) is referred to as the flight level
Flight level
A Flight Level is a standard nominal altitude of an aircraft, in hundreds of feet. This altitude is calculated from the International standard pressure datum of 1013.25 hPa , the average sea-level pressure, and therefore is not necessarily the same as the aircraft's true altitude either...

, and is used above the transition altitude (18000 feet (5,486.4 m) in the US, but may be as low as 3000 feet (914.4 m) in other jurisdictions); so when the altimeter reads 18,000 ft on the standard pressure setting the aircraft is said to be at "Flight level 180". When flying at a Flight Level, the altimeter is always set to standard pressure (29.92 inHg / 1013.25 mbar).

On the flight deck, the definitive instrument for measuring altitude is the pressure altimeter
Altimeter
An altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.-Pressure altimeter:...

, which is an aneroid barometer with a front face indicating distance (feet or metres) instead of atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

.

There are several types of aviation altitude:
• Indicated altitude is the reading on the altimeter when the altimeter is set to the local barometric pressure at Mean Sea Level.
QNH
QNH is one of the many Q codes. It is defined as, "barometric pressure adjusted to sea level." It is a pressure setting used by pilots, air traffic control , and low frequency weather beacons to refer to the barometric setting which, when set on an aircraft's altimeter, will cause the altimeter to...

• Absolute altitude is the height of the aircraft above the terrain over which it is flying. Also referred to feet/metres above ground level
Above ground level
In aviation and atmospheric sciences, an altitude is said to be above ground level when it is measured with respect to the underlying ground surface. This is as opposed to above mean sea level , or in broadcast engineering, height above average terrain...

(AGL).
• True altitude is the elevation above mean sea level. In UK aviation radiotelephony usage, the vertical distance of a level, a point or an object considered as a point, measured from mean sea level; this is referred to over the radio as altitude.(see QNH
QNH
QNH is one of the many Q codes. It is defined as, "barometric pressure adjusted to sea level." It is a pressure setting used by pilots, air traffic control , and low frequency weather beacons to refer to the barometric setting which, when set on an aircraft's altimeter, will cause the altimeter to...

)
• Height
Height
Height is the measurement of vertical distance, but has two meanings in common use. It can either indicate how "tall" something is, or how "high up" it is. For example "The height of the building is 50 m" or "The height of the airplane is 10,000 m"...

is the elevation above a ground reference point, commonly the terrain elevation. In UK aviation radiotelephony usage, the vertical distance of a level, a point or an object considered as a point, measured from a specified datum; this is referred to over the radio as height, where the specified datum is the airfield elevation (see QFE)
• Pressure altitude
Pressure altitude
In aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed baseline pressure setting. The baseline pressure is 1013.25 hPa, equivalent to 1013.25 millibar, or 29.92 inches of mercury. This setting is equivalent to the air pressure at mean sea level in the...

is the elevation above a standard datum air-pressure plane (typically, 1013.25 millibars or 29.92" Hg and 15°C). Pressure altitude and indicated altitude are the same when the altimeter is set to 29.92" Hg or 1013.25 millibars.
• Density altitude
Density altitude
Density altitude is the altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density at the place of observation, or, in other words, the height when measured in terms of the density of the air rather than the distance from the ground...

is the altitude corrected for non-ISA International Standard Atmosphere
International Standard Atmosphere
The International Standard Atmosphere is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes. It has been established to provide a common reference for temperature and pressure and consists of tables of...

atmospheric conditions. Aircraft performance depends on density altitude
Density altitude
Density altitude is the altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density at the place of observation, or, in other words, the height when measured in terms of the density of the air rather than the distance from the ground...

, which is affected by barometric pressure, humidity and temperature. On a very hot day, density altitude at an airport (especially one at a high elevation) may be so high as to preclude takeoff, particularly for helicopters or a heavily loaded aircraft.

These types of altitude can be explained more simply as various ways of measuring the altitude:
• Indicated altitude -- the altimeter
Altimeter
An altimeter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth underwater.-Pressure altimeter:...

• Absolute altitude -- altitude in terms of the distance above the ground directly below it
• True altitude -- altitude in terms of elevation above sea level
• Height
Height
Height is the measurement of vertical distance, but has two meanings in common use. It can either indicate how "tall" something is, or how "high up" it is. For example "The height of the building is 50 m" or "The height of the airplane is 10,000 m"...

-- altitude in terms of the distance above a certain point
• Pressure altitude
Pressure altitude
In aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed baseline pressure setting. The baseline pressure is 1013.25 hPa, equivalent to 1013.25 millibar, or 29.92 inches of mercury. This setting is equivalent to the air pressure at mean sea level in the...

-- altitude in terms of the air pressure
• Density altitude
Density altitude
Density altitude is the altitude in the International Standard Atmosphere at which the air density would be equal to the actual air density at the place of observation, or, in other words, the height when measured in terms of the density of the air rather than the distance from the ground...

-- altitude in terms of the density of the air

Altitude regions

The Earth's atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

is divided into several altitude regions:
• Troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

— surface to 8000 metres (5 mi) at the poles – 18000 metre at the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

, ending at the Tropopause.
• Stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

— Troposphere to 50 kilometres (31.1 mi)
• Mesosphere
Mesosphere
The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. In the mesosphere temperature decreases with increasing height. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring...

— Stratosphere to 85 kilometres (52.8 mi)
• Thermosphere
Thermosphere
The thermosphere is the biggest of all the layers of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the middle of the thermosphere, between...

— Mesosphere to 675 kilometres (419.4 mi)
• Exosphere
Exosphere
The exosphere is the uppermost layer of Earth's atmosphere. In the exosphere, an upward travelling molecule moving fast enough to attain escape velocity can escape to space with a low chance of collisions; if it is moving below escape velocity it will be prevented from escaping from the celestial...

— Thermosphere to 10000 kilometres (6,213.7 mi)

High altitude and low air pressure

Regions on the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's surface (or in its atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

) that are high above mean sea level are referred to as high altitude. High altitude is sometimes defined to begin at 8000 feet (2,438.4 m) above sea level.

At high altitude, atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

is lower than that at sea level. This is due to two competing physical effects: gravity, which causes the air to be as close as possible to the ground; and the heat content of the air, which causes the molecules to bounce off each other and expand.

Because of the lower pressure, the air expands as it rises, which causes it to cool. Thus, high altitude air is cold, which causes a characteristic alpine climate
Alpine climate
Alpine climate is the average weather for a region above the tree line. This climate is also referred to as mountain climate or highland climate....

. This climate dramatically affects the ecology at high altitude.

Relation between temperature and altitude in Earth's atmosphere

The environmental lapse rate (ELR), is the rate of decrease of temperature with altitude in the stationary atmosphere at a given time and location. As an average, the International Civil Aviation Organization (ICAO) defines an international standard atmosphere
International Standard Atmosphere
The International Standard Atmosphere is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes. It has been established to provide a common reference for temperature and pressure and consists of tables of...

(ISA) with a temperature lapse rate
Lapse rate
The lapse rate is defined as the rate of decrease with height for an atmospheric variable. The variable involved is temperature unless specified otherwise. The terminology arises from the word lapse in the sense of a decrease or decline; thus, the lapse rate is the rate of decrease with height and...

of 6.49 K(°C)/1,000 m (3.56 °F or 1.98 K(°C)/1,000 Ft) from sea level to 11 kilometres (36,089.2 ft). From 11 to 20 km (36,089.2 to 65,616.8 ft), the constant temperature is −56.5 C, which is the lowest assumed temperature in the ISA. The standard atmosphere contains no moisture. Unlike the idealized ISA, the temperature of the actual atmosphere does not always fall at a uniform rate with height. For example, there can be an inversion layer in which the temperature increases with height.

Effects of high altitude on humans

Medicine recognizes that altitudes above 1500 metres (4,921.3 ft) start to affect humans, and extreme altitudes above 5500–6000 m (18,044.6–19,685 ft) cannot be permanently tolerated by humans. As altitude increases, atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

decreases, which affects humans by reducing the partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....

of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

. The lack of oxygen above 8000 feet (2,438.4 m) can cause serious illnesses such as altitude sickness
Altitude sickness
Altitude sickness—also known as acute mountain sickness , altitude illness, hypobaropathy, or soroche—is a pathological effect of high altitude on humans, caused by acute exposure to low partial pressure of oxygen at high altitude...

, High altitude pulmonary edema
High Altitude Pulmonary Edema
High altitude pulmonary edema is a life-threatening form of non-cardiogenic pulmonary edema that occurs in otherwise healthy mountaineers at altitudes typically above ....

, and High altitude cerebral edema
High altitude cerebral edema
High altitude cerebral edema is a severe form of altitude sickness. HACE is the result of swelling of brain tissue from fluid leakage and almost always begins as acute mountain sickness...

. The higher the altitude, the more likely are serious effects.

The human body can adapt to high altitude by breathing faster, having a higher heart rate, and adjusting its blood chemistry. It can take days or weeks to adapt to high altitude. However, above 8000 metres (26,246.7 ft), (in the "death zone"), the human body cannot adapt and will eventually die.

There is a significantly lower overall mortality rate for permanent residents at higher altitudes. However, people living at higher elevations have a statistically significant higher rate of suicide. The cause for the increased suicide risk is unknown so far.

For athletes, high altitude produces two contradictory effects on performance. For explosive events (sprints up to 400 metres, long jump, triple jump) the reduction in atmospheric pressure means there is less resistance from the atmosphere and the athlete's performance will generally be better at high altitude. For endurance events (races of 5,000 metres or more) the predominant effect is the reduction in oxygen which generally reduces the athlete's performance at high altitude. Sports organisations acknowledge the effects of altitude on performance: the International Association of Athletic Federations (IAAF), for example, have ruled that performances achieved at an altitude greater than 1000 metres (3,280.8 ft) will not be approved for record purposes.

Athletes also can take advantage of altitude acclimatization to increase their performance. The same changes that help the body cope with high altitude increase performance back at sea level. These changes are the basis of altitude training
Altitude training
Altitude training is the practice by some endurance athletes of training for several weeks at high altitude, preferably over above sea level, though more commonly at intermediate altitudes due to the shortage of suitable high-altitude locations...

which forms an integral part of the training of athletes in a number of endurance sports including track and field, distance running, triathlon, cycling and swimming.

• Acronyms and abbreviations in avionics
Acronyms and abbreviations in avionics
-A:*ACARS: Aircraft Communications Addressing and Reporting System.*ACAS: Airborne Collision Avoidance System.*ACP: Audio Control Panel.*ACS: Audio Control System.*ADAHRS: Air Data and Attitude Heading Reference System.*ADC: Air Data Computer....

• Flight altitude record
Flight altitude record
These are the records set for going the highest in the atmosphere from the age of ballooning onward. Some records are certified by Fédération Aéronautique Internationale.-Fixed-wing aircraft:-Piston-driven propeller aeroplane:...

• High altitude wind power
High altitude wind power
High-altitude wind power has been imagined as a source of useful energy since 1833 with John Etzler's vision of capturing the power of winds high in the sky by use of tether and cable technology...

• Organisms at high altitude
Organisms at high altitude
Organisms can exist at high altitude in a habitat, while flying or gliding, or through man-made systems. Many animals have adapted to high altitude life and some have evolved to cope well with the problems of an environment with a reduced level of oxygen....