Air Density and Geographic Altitude

Written by john csiszar
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Air Density and Geographic Altitude
Although air cannot be seen, its effects are felt by both humans and machines. (air cock image by Galyna Andrushko from Fotolia.com)

Air density is defined by the mass of air for a given volume, while geographic altitude refers to the height of an object relative to another one, with sea level generally being the point of reference. The relationship between air density and altitude is an inverse one. Lowered air density has a number of different effects, some positive and some negative.

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Air and Altitude Relationship

As altitude increases, air density tends to decrease. While allowing for atmospheric fluctuations due to thermal energy from the ground, the higher you go, the less air pressure you feel. This relationship carries a host of supplementary effects, such as the decreased availability of oxygen at higher altitudes, and the inability of air to retain heat, resulting in lower temperatures.

Classifications of Altitude

For land-based observations, altitude is generally referred to as elevation. Three specific altitude regions are defined by mountain medicine. High elevation ranges from 5,000 to 11,500 feet, very high altitude runs from 11,500 feet to 18,000 feet and extreme altitude refers to elevations above 18,000 feet.

Altitude and Air Effects on Humans

While the percentage of oxygen in the air remains constant at about 21 per cent all the way to 70,000 feet in altitude, above 10,000 feet, air density begins to decline precipitously, and as a result oxygen density does as well. Climbers and those who frequent alpine areas can begin feeling effects of low air density as low as 8,000 feet, where mental and physical alertness can begin to deteriorate.

Altitude and Air Relationships in Aviation

The decrease in air density at higher geographic altitudes works to the advantage of aeroplanes, as lower air resistance on the body of the aircraft means that less energy or jet fuel is required to propel the plane forward. Similarly, increased drag and air resistance as a plane descends to the ground helps to slow the plane for landing, and to provide extra lift at lower speeds.

Altitude and Air Relationships in Sport

Athletes performing at high altitudes encounter both positives and negatives from the reduced air density. The decreased air resistance at altitude is of particular benefit to sprinters, to the point that the International Association of Athletic Federations disallows records achieved at an altitude greater than 1,000 meters. But the decreased oxygen concentration at altitude is a hindrance to athletes in endurance events, such as swimming, cycling and distance running.

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