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Pilot's Handbook of Aeronautical Knowledge
Weather Theory
Air Masses

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Pilot's Handbook of Aeronautical Knowledge



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




Occluded front cross-section with a weather chart depiction and associated METAR.
Figure 11-28. Occluded front cross-section with a weather chart depiction and associated METAR.

For a thunderstorm to form, the air must have sufficient water
vapor, an unstable lapse rate, and an initial lifting action to
start the storm process. Some storms occur at random in
unstable air, last for only an hour or two, and produce only
moderate wind gusts and rainfall. These are known as air
mass thunderstorms and are generally a result of surface
heating. Steady-state thunderstorms are associated with
weather systems. Fronts, converging winds, and troughs aloft
force upward motion spawning these storms which often
form into squall lines. In the mature stage, updrafts become
stronger and last much longer than in air mass storms, hence
the name steady state. [Figure 11-29]

Knowledge of thunderstorms and the hazards associated with
them is critical to the safety of flight.

Weather can pose serious hazards to flight and a thunderstorm
packs just about every weather hazard known to aviation into
one vicious bundle. These hazards occur individually or in
combinations and most can be found in a squall line.

Squall Line
A squall line is a narrow band of active thunderstorms. Often
it develops on or ahead of a cold front in moist, unstable
air, but it may develop in unstable air far removed from
any front. The line may be too long to detour easily and too
wide and severe to penetrate. It often contains steady-state
thunderstorms and presents the single most intense weather
hazard to aircraft. It usually forms rapidly, generally reaching
maximum intensity during the late afternoon and the first few
hours of darkness.

The most violent thunderstorms draw air into their cloud bases
with great vigor. If the incoming air has any initial rotating
motion, it often forms an extremely concentrated vortex from
the surface well into the cloud. Meteorologists have estimated
that wind in such a vortex can exceed 200 knots with pressure
inside the vortex quite low. The strong winds gather dust
and debris and the low pressure generates a funnel-shaped
cloud extending downward from the cumulonimbus base. If
the cloud does not reach the surface, it is a funnel cloud; if
it touches a land surface, it is a tornado.