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1% Other Gases
21% Oxygen
Atmosphere
Figure 9-2. The composition of the atmosphere.
9-3
in this layer. The lower part of the troposphere interacts
with the land and sea surface, providing thermals,
mountain waves, and sea-breeze fronts. Although temperatures
decrease as altitude increases in the troposphere,
local areas of temperature increase (inversions)
are common.
The top of the troposphere is called the tropopause.
The pressure at this level is only about ten percent of
MSL (0.1 atmospheres) and density is decreased to
about 25 percent of its sea-level value. Temperature
reaches its minimum value at the tropopause, approximately
-55° Celsius (−67°F). For pilots this is an
important part of the atmosphere because it is associated
with a variety of weather phenomena such as
thunderstorm tops, clear air turbulence, and jet
streams. The vertical limit altitude of the tropopause
varies with season and with latitude. The tropopause is
lower in the winter and at the poles; it is higher in the
summer and at the equator.
The tropopause separates the troposphere from the
stratosphere. In the stratosphere, the temperature tends
to first change very slowly with increasing height.
However, as altitude increases the temperature
increases to approximately 0° Celsius (32°F) reaching
its maximum value at about 160,000 feet MSL. Unlike
the troposphere where the air moves freely both vertically
and horizontally, the air within the stratosphere
moves mostly horizontally.
Gliders have reached into the lower stratosphere using
mountain waves. At these altitudes, pressurization
becomes an issue, as well as the more obvious breathing
Dry air behaves almost like an “ideal” gas, meaning it
obeys the gas law given by P/DT = R, where P is pressure,
D is density, T is temperature, and R is a constant.
This law states that the ratio of pressure to the product
of density and temperature must always be the same.
For instance, at a given pressure if the temperature is
much higher than standard, then the density must be
much lower. Air pressure and temperature are usually
measured, and using the gas law, density of the
air can be calculated and used to determine aircraft
performance under those conditions.
STANDARD ATMOSPHERE
Using a representative vertical distribution of these
variables, the standard atmosphere has been defined
and is used for pressure altimeter calibrations. Since
changes in the static pressure can affect pitot-static
instrument operation, it is necessary to understand
basic principles of the atmosphere. To provide a
common reference for temperature and pressure, a
definition for the standard atmosphere, also called
International Standard Atmosphere (ISA), has been
established. In addition to affecting certain flight
instruments, these standard conditions are the basis
for most aircraft performance data. At sea level, the
standard atmosphere consists of a barometric pressure
of 29.92 in. Hg., (1013.2 millibars) and a temperature
of 15°C (59°F). This means that under the standard
conditions, a column of air at sea level weighs
14.7 lb./in.2.
Since temperature normally decreases with altitude, a
standard lapse rate can be used to calculate temperature
at various altitudes. Below 36,000 feet, the standard
temperature lapse rate is 2°C (3.5°F) per 1,000
feet of altitude change. Pressure does not decrease linearly
with altitude, but for the first 10,000 feet, 1
in.Hg. for each 1,000 feet approximates the rate of
pressure change. It is important to note that the standard
lapse rates should only be used for flight planning
purposes with the understanding that large
variations from standard conditions can exist in the
atmosphere. [Figure 9-3]
LAYERS OF THE ATMOSPHERE
The Earth’s atmosphere is divided into four strata or
layers: troposphere, stratosphere, mesosphere, and
thermosphere. These layers are defined by the temperature
change with increasing altitude. The lowest layer,
called the troposphere, exhibits an average decrease in
temperature from the Earth’s surface to about 36,000
feet above mean sea level (MSL). The troposphere is
deeper in the tropics and shallower in Polar Regions. It
also varies seasonally, being higher in the summer and
lower in the winter months.
Almost all of the Earth’s weather occurs in the troposphere
as most of the water vapor and clouds are found
Temperature (°C)
-55 -35 -15 0 +15
Sea Level
10,000
　
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