level (MSL) elevation of the field. If the density altitude is higher than average for the field, this field
has a high density altitude. An example of this is an airfield at 5,000 feet MSL with a density altitude of
10,000 feet. Aircraft operating from this field would be in air of the same density that would usually be
found in the atmosphere at 10,000 feet. The efficiency of the aircraft can be seriously affected in high
density altitudes, especially when the aircraft is critically loaded. If the density altitude is lower than
average for a given altitude, the efficiency of the aircraft increased. A pilot operating from a field of
5,000 feet MSL with a density altitude of 1,000 feet is in the same air density field elevation that usually
exists at 1,000 feet. Further, the density altitude on any airfield varies from hour to hour and day to day.
1.6 SUMMARY
Nitrogen, oxygen, argon, carbon dioxide, and small amounts of other gases make up the air
surrounding the earth. The amount of water vapor in the air depends upon the temperature of the air.
Impurities in the air are important because of their effect on visibility and the fact that they are necessary
for forming clouds and condensing water vapor.
Air has weight and is in layers surrounding the earth. The layers, starting with the one next to the
earth, are the troposphere, stratosphere, ionosphere, and exosphere. The tropopause is a narrow area
between the troposphere and the stratosphere. The jet stream is near the upper portion of the
troposphere.
As altitude is increased, temperature decreases at an average lapse rate of 2C or 3 1/2F per
thousand feet. As temperature increases or altitude becomes higher, wings and rotor blades become less
efficient. Density altitude must be evaluated before takeoffs or landings can be accomplished safely.
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