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The atmosphere is a mixture of gases surrounding the
Earth. Without it, there would be no weather (wind,
clouds, precipitation) or protection from the sun’s
rays. Though this protective envelope is essential to
life, it is extraordinarily thin. When compared to the
radius of the Earth, 3,438 nautical miles, the vertical
limit of the atmosphere represents a very small distance.
Although there is no specific upper limit to the
atmosphere—it simply thins to a point where it fades
away into space—the layers up to approximately
164,000 feet (about 27 nautical miles) contain 99.9
percent of atmospheric mass. At that altitude, the
atmospheric density is approximately one-thousandth
the density of that at sea level. [Figure 9-1]
COMPOSITION
The Earth’s atmosphere is composed of a mixture of
gases, with small amounts of water, ice, and other particles.
Two gases, nitrogen (N2) and oxygen (O2), comprise
approximately 99 percent of the gaseous content
of the atmosphere; the other one percent is composed
of various trace gases. Nitrogen and oxygen are both
considered permanent gases, meaning their proportions
remain the same to approximately 260,000 feet.
Water vapor (H2O), on the other hand, is considered a
variable gas. Therefore, the amount of water in the
atmosphere depends on the location and the source of
the air. For example, the water vapor content over tropical
areas and oceans accounts for as much as four percent
of the gases displacing nitrogen and oxygen.
Conversely, the atmosphere over deserts and at high
altitudes exhibits less than one percent of the water
vapor content. [Figure 9-2]
Although water vapor exists in the atmosphere in small
amounts as compared to nitrogen and oxygen, it has a
significant impact on the production of weather. This is
because it exists in two other physical states: liquid
(water) and solid (ice). These two states of water contribute
to the formation of clouds, precipitation, fog,
and icing, all of which are important to aviation
weather. In addition, by absorbing the radiant energy
from the Earth’s surface, water vapor reduces surface
cooling, causing surface temperatures to be warmer.
Altitude (Feet)
18,000
Sea Level
53,000
164,000
99.9% of
Mass Below
This Level
90% of
Mass Below
This Level
50% of
Mass Below
This Level
Figure 9-1. Atmospheric mass by altitude.
9-2
PROPERTIES
The state of the atmosphere is defined by fundamental
variables, namely temperature, density, and pressure.
These variables change over time and, combined with
vertical and horizontal differences, lead to daily
weather conditions.
TEMPERATURE
The temperature of a gas is the measure of the average
kinetic energy of the molecules of that gas. Fast moving
molecules are indicative of high kinetic energy and
warmer temperatures. Conversely, slow moving molecules
reflect lower kinetic energy and lower temperatures.
Air temperature is commonly thought of in terms
of whether it feels hot or cold. For quantitative measurements,
the Celsius scale is used in aviation, although
the Fahrenheit scale is still used in some applications.
DENSITY
The density of any given gas is the total mass of molecules
in a specified volume, expressed in units of mass
per volume. Low air density means a fewer number of
air molecules in a specified volume while high air density
means a greater number of air molecules in the
same volume. Air density affects aircraft performance,
as noted in Chapter 5–Glider Performance.
PRESSURE
Molecules in a given volume of air not only posses a
certain kinetic energy and density, but they also exert
force. The force per unit area defines pressure. At the
Earth’s surface, the pressure exerted by the atmosphere
is due to its weight. Therefore, pressure is measured in
terms of weight per area. For example, atmospheric
pressure is measured in pounds per square inch
(lb./in.2). From the outer atmosphere to sea level, a typical
value of atmospheric pressure is 14.7 lb./in.2 This
force measured at sea level is commonly reported as
29.92 inches of mercury (in. Hg.) (from the level of
mercury at standard sea-level pressure in a mercurial
barometer). In aviation weather reports, the common
reporting unit is millibars. When 29.92 in. Hg. is converted,
it becomes 1013.2 millibars. The force created
by the moving molecules act equally in all directions
when measured at a given point.
260,000 ft.
78% Nitrogen
　
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