EARTH’S ATMOSPHERE


About 2.7 Billion years ago, oxygen began to appear in our atmosphere. Before that, it contained mostly CO2 and volcanic gasses.


Earth’s atmosphere stretches from the surface of the planet up to as far as 10,000 kilometers (6,214 miles) above. After that, the atmosphere blends into space. Not all scientists agree where the actual upper boundary of the atmosphere is, but they can agree that the bulk of the atmosphere is located close to Earth’s surface—up to a distance of around eight to 15 kilometers (five to nine miles).


While oxygen is necessary for most life on Earth, the majority of Earth’s atmosphere is not oxygen. Earth’s atmosphere is composed of about 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, and 0.1 percent other gases. Trace amounts of carbon dioxide, methane, water vapor, and neon are some of the other gases that make up the remaining 0.1 percent.


Atmospheric composition


Although the atmosphere contains many trace gases, the 2 primary components are Nitrogen at 78% and Oxygen at 21%.


The Nitrogen we breathe is an inert gas not used by the body. Instead, it is simply inhaled and exhaled with each breath. When the body is saturated with Nitrogen, the same amount is exhaled as is inhaled. A problem can occur when the pressure of gases in the atmosphere change quickly as we rapidly ascend or descend. Diffused Nitrogen in the body can change back into a gas and decompression sickness (DCS), can result.


Oxygen is essential for life. It is used to burn nutrients and for the production of heat and kinetic energy which generates cellular life.


An important point to remember is this: The composition, or percentage, of Nitrogen and Oxygen does not change with altitude. The percentages do not change; however, increased altitude does limit the availability of Oxygen to the body as a result of less pressure (at the higher altitude) to push the Oxygen through the cell walls in order to oxygenate the blood.


Atmospheric Temperature


Standard average temperature at sea level is 59° F. As altitude increases, temperature decreases. For instance, at 18,000 ft MSL the temperature is -5.1° F, at 34,000 ft MSL, -62° F, and at 36,000 ft MSL and higher the temperature remains -69.7° F.


Atmospheric Pressure


Atmospheric pressure or barometric pressure is literally the weight of the air pressing down on the Earth. There are areas in the atmosphere of low pressure as well as areas of higher pressure. Low pressure areas have a greater mass and as elevation increases there is less overlying mass. This means that as you ascend, the pressure gets lower and the closer you are to the ground, the higher the atmospheric pressure.


Mean sea level pressure (MSLP) is listed as the pressure in the atmosphere at sea level. The average sea-level pressure is 29.92 inches of Hg, 760 mm of hg or 14.7psi.


25,000


Altitude 40,000


Low Pressure


14,000 High Pressure Level Sea 2.7 5.4 Pressure


Generally, when weather reports are provided in the media, this is what is provided. Not necessarily the actual barometric pressure at your altitude but the MSLP.


Remember, the altimeter setting in an aircraft is a atmospheric pressure setting, not an actual altitude. High Altitude Awareness Training 6 8.6 14.7


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