In aviation, the term altitude can have several meanings. It is a fundamental tenet of flight safety that both parties exchanging information concerning this topic are absolutely 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.
- 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.
- Absolute altitude is the height of the aircraft above the terrain over which it is flying.
- Indicated altitude is the reading on the altimeter.
- Pressure altitude is the elevation above a standard datum plane (typically, 1013.2 millibars or 29.92" Hg and 15°C). Pressure altitude divided by 100 feet is referred to as the flight level; so when the altimeter reads 18,000 ft on the standard pressure setting the aircraft is said to be at "Flight level 180". Below FL180, altitudes are read in thousands, pronounced "one three thousand" for 13,000, "seven thousand" for 7,000 etc.
- Density altitude is the altitude corrected for non-ISA International Standard Atmosphere (ISA) conditions at which the air density is unequal to ISA conditions. Aircraft performance depends on density altitude, which is affected by barometric pressure and temperature. On a very hot day, density altitude at an airport may be so high as to preclude takeoff, particularly for helicopters or a heavily loaded aircraft.
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.
Mountain medicine recognizes three altitude regions:
- 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 leads to a range of medical problems, from the relatively 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