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Whether preparing for a local flight or a long
cross-country, flight-planning decisions based on
weather can dramatically affect the safety of the flight.
A solid understanding of weather theory provides the
tools necessary to understand the reports and forecasts
obtained from a Flight Service Station weather
specialist and other aviation weather services.
This chapter is designed to help pilots acquire the
background knowledge of weather principles necessary
to develop sound decision making skills relating to
weather. It is important to note, however, that there is
no substitute for experience.
NATURE OF THE ATMOSPHERE
The atmosphere is a mixture of gases that surround the
Earth. This blanket of gases provides protection from
ultraviolet rays as well as supporting human, animal,
and plant life on the planet. Nitrogen accounts for 78
percent of the gases that comprise the atmosphere,
while oxygen makes up 21 percent. Argon, carbon
dioxide, and traces of other gases make up the remaining
1 percent. [Figure 10-1]
Within this envelope of gases, there are several
recognizable layers of the atmosphere that are defined
not only by altitude, but also by the specific
characteristics of that level. [Figure 10-2]
The first layer, known as the troposphere, extends
from sea level up to 20,000 feet (8 km) over the
northern and southern poles and up to 48,000 feet (14.5
km) over the equatorial regions. The vast majority of
weather, clouds, storms, and temperature variances
1% Trace Gases
21% Oxygen
78% Nitrogen
Figure 10-1. Composition of the atmosphere.
Troposphere-The layer of the atmosphere extending from the surface
to a height of 20,000 to 60,000 feet depending on latitude.
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occur within this first layer of the atmosphere. Inside
the troposphere, the temperature decreases at a rate of
about 2° Celsius every 1,000 feet of altitude gain, and
the pressure decreases at a rate of about 1 inch per
1,000 feet of altitude gain. At the top of the troposphere
is a boundary known as the tropopause, which traps
moisture, and the associated weather, in the
