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 the term elevation 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, 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 divided by 100 feet (30m) is referred to as the flight level, and is used above the transition altitude (18,000 feet (5,500 m) in the US, but may be as low as 3,000 feet (910 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, which is an aneroid barometer with a front face indicating distance (feet or metres) instead of atmospheric pressure.
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.
- Absolute altitude is the height of the aircraft above the terrain over which it is flying. Also referred to feet/metres above ground level (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)
- Height 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 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 is the altitude corrected for non-ISA International Standard Atmosphere atmospheric conditions. Aircraft performance depends on density altitude, 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 reading
- 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 -- altitude in terms of the distance above a certain point
- Pressure altitude -- altitude in terms of the air pressure
- Density altitude -- altitude in terms of the density of the air
- Troposphere — surface to 8,000 metres (5.0 mi) at the poles – 18,000 metres (11 mi) at the equator, ending at the Tropopause.
- Stratosphere — Troposphere to 50 kilometres (31 mi)
- Mesosphere — Stratosphere to 85 kilometres (53 mi)
- Thermosphere — Mesosphere to 675 kilometres (419 mi)
- Exosphere — Thermosphere to 10,000 kilometres (6,200 mi)
High altitude and low air pressure
Regions on the Earth's surface (or in its atmosphere) that are high above mean sea level are referred to as high altitude. High altitude is sometimes defined to begin at 2,400 metres (8,000 ft) above sea level.
At high altitude, atmospheric pressure 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. 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 (ISA) with a temperature lapse rate 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 (36,000 ft). From 11 to 20 kilometres (36,000 to 66,000 ft), the constant temperature is −56.5 °C (−69.7 °F), 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 1,500 metres (4,900 ft) start to affect humans, and extreme altitudes above 5,500–6,000 metres (18,000–20,000 ft) cannot be permanently tolerated by humans. As altitude increases, atmospheric pressure decreases, which affects humans by reducing the partial pressure of oxygen. The lack of oxygen above 2,400 metres (8,000 ft) can cause serious illnesses such as altitude sickness, High altitude pulmonary edema, and High altitude cerebral edema. 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 8,000 metres (26,000 ft), (in the "death zone"), the human body cannot adapt and will eventually die.
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 1,000 metres (3,300 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 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.
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- Downloadable ETOPO2 Raw Data Database (2 minute grid)
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