What's So Different About the Air 18,000 Feet Above Us?


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Surprisingly, high altitude pulmonary edema occurs most frequently at high altitudes. Very rarely are people afflicted with HAPE at an altitude lower than 8000 feet. This is due to the fact that atmospheric characteristics at a high altitude, such as pressure and the abundance of certain gases, differ greatly from those at sea level. So what exactly is so different about the air up there?CS4gascomposition.gif

CS4chart.gifCompared with the atmosphere at sea level, the air at 18,000 feet is vastly different. The most significant difference is the change in air pressure. Due to gravity, air is denser closer to the earth’s surface. Therefore, at very high altitudes, the air particles (nitrogen, oxygen, carbon dioxide, and trace gases) are much more dispersed. The percentage composition of gases stays relatively constant; air is composed of roughly 78% nitrogen, 21% oxygen, and 1% carbon dioxide and trace gases at any altitude. However, as stated earlier, there are fewer and fewer molecules of these gases as elevation increases. The pressure at sea level is 1 atmosphere (760 mm Hg). At 18,000 feet, the pressure is only 0.37 atmospheres (280 mm Hg).

Another important distinction between the air at higher elevations and the air at sea level is the temperature. The air is colder at high altitudes primarily because there is less of it. The air particles at a high elevation cannot collectively carry or transfer as much heat because they are fewer in number. At sea level, the average temperature is approximately 59 degrees Fahrenheit. At 18,000 feet above sea level, though, the average temperature is approximated to be -5 degrees Fahrenheit.


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