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Pilot's Handbook of Aeronautical Knowledge
Weather Theory

Atmosphere

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Pilot's Handbook of Aeronautical Knowledge

Preface

Acknowledgements

Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making

Appendix

Glossary

Index

At the top of the troposphere is a boundary known as the
tropopause, which traps moisture and the associated weather
in the troposphere. The altitude of the tropopause varies with
latitude and with the season of the year; therefore, it takes
on an elliptical shape, as opposed to round. Location of the
tropopause is important because it is commonly associated
with the location of the jet stream and possible clear air
turbulence.

Above the tropopause are three more atmospheric levels. The
first is the stratosphere, which extends from the tropopause to
a height of about 160,000 feet (50 km). Little weather exists
in this layer and the air remains stable although certain types
of clouds occasionally extend in it. Above the stratosphere
are the mesosphere and thermosphere which have little
influence over weather.

Atmospheric Circulation
As noted earlier, the atmosphere is in constant motion.
Certain factors combine to set the atmosphere in motion, but a
major factor is the uneven heating of the Earth's surface. This
heating upsets the equilibrium of the atmosphere, creating
changes in air movement and atmospheric pressure. The
movement of air around the surface of the Earth is called
atmospheric circulation.

Heating of the Earth's surface is accomplished by several
processes, but in the simple convection-only model used for
this discussion, the Earth is warmed by energy radiating from
the sun. The process causes a circular motion that results
when warm air rises and is replaced by cooler air.

Warm air rises because heat causes air molecules to spread
apart. As the air expands, it becomes less dense and lighter
than the surrounding air. As air cools, the molecules pack
together more closely, becoming denser and heavier than
warm air. As a result, cool, heavy air tends to sink and replace
warmer, rising air.

Because the Earth has a curved surface that rotates on a tilted
axis while orbiting the sun, the equatorial regions of the Earth
receive a greater amount of heat from the sun than the polar
regions. The amount of sun that heats the Earth depends on
the time of year and the latitude of the specific region. All of
these factors affect the length of time and the angle at which
sunlight strikes the surface.

Solar heating causes higher temperatures in equatorial areas
which causes the air to be less dense and rise. As the warm
air flows toward the poles, it cools, becoming denser, and
sinks back toward the surface. [Figure 11-3]

Circulation pattern in a static environment.
Figure 11-3. Circulation pattern in a static environment.

Atmospheric Pressure
The unequal heating of the Earth's surface not only modifies
air density and creates circulation patterns; it also causes
changes in air pressure or the force exerted by the weight
of air molecules. Although air molecules are invisible, they
still have weight and take up space.

Imagine a sealed column of air that has a footprint of one
square inch and is 350 miles high. It would take 14.7 pounds
of effort to lift that column. This represents the air's weight;
if the column is shortened, the pressure exerted at the bottom
(and its weight) would be less.

The weight of the shortened column of air at 18,000 feet is
approximately 7.4 pounds; almost 50 percent that at sea level.
For instance, if a bathroom scale (calibrated for sea level)
were raised to 18,000 feet, the column of air weighing 14.7
pounds at sea level would be 18,000 feet shorter, and would
weigh approximately 7.3 pounds (50 percent) less than at
sea level. [Figure 11-4]

The actual pressure at a given place and time differs with
altitude, temperature, and density of the air. These conditions
also affect aircraft performance, especially with regard to
takeoff, rate of climb, and landings.

 

11-3