Altitude Totally Explained

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ť For other uses see Altitude (disambiguation)

Altitude is the elevation of a point or object from a known level or datum (plural: data). Common data are mean sea level, local ground level (Above Ground Level, or AGL), or the surface of the WGS-84 geoid, used by GPS. In aviation, altitude is measured in feet. For non-aviation uses, altitude may be measured in other units such as metres or miles. Atmospheric pressure decreases as altitude increases. This principle is the basis of operation of the pressure altimeter, which is an aneroid barometer calibrated to indicate altitude instead of pressure. It is the fall in pressure that leads to a shortage of oxygen (hypoxia) in humans on ascent to high altitude.

Altitude in aviation

In aviation, the term altitude can have several meanings, and is therefore qualified by either explicitly adding a modifier (for example "true altitude"), or implicitly through the context of the communication. Parties exchanging information concerning this topic must be clear which definition is being used.

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)
High altitude = 1500 m – 3500 m (5000 – 11,500 ft)
Very High altitude = 3500 m – 5500 m (11,500 – 18,000 ft)
Extreme altitude = 5500 m – above Travel to high altitudes can lead to medical problems, from the mild symptoms of acute mountain sickness to the potentially fatal high altitude pulmonary oedema (HAPE) and high altitude cerebral oedema (HACE). These conditions are caused by the profound hypoxia associated with travel to high altitudes.

The Earth's atmosphere is divided into several altitude regions:

Troposphere — surface to 5 miles (8 km) at poles – 11 miles (18 km) at equator, ending at the Tropopause.

Stratosphere — Tropopause to 31 miles (50 km)

Mesosphere — Stratopause to 53 miles (85 km)

Thermosphere — Mesopause to 420 miles (675 km)

Exosphere — Thermopause to 6200 miles (10,000 km)

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