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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




At the time of departure from Pittsburgh, the weather is VFR
with three miles visibility in smoke and a scattered layer of
clouds at 3,500 feet. As the flight progresses westward to
Columbus and closer to the oncoming cold front, the clouds
show signs of vertical development with a broken layer at
2,500 feet. The visibility is six miles in haze with a falling
barometric pressure. Approaching Indianapolis, the weather
has deteriorated to overcast clouds at 1,000 feet, and three
miles visibility with thunderstorms and heavy rain showers.
At St. Louis, the weather gets better with scattered clouds at
1,000 feet and a ten mile visibility.

A pilot using sound judgment based on the knowledge of
frontal conditions would most likely remain in Indianapolis
until the front had passed. Trying to fly below a line of
thunderstorms or a squall line is hazardous, and flight over
the top of or around the storm is not an option. Thunderstorms
can extend up to well over the capability of small airplanes
and can extend in a line for 300 to 500 miles.

Comparison of Cold and Warm Fronts
Warm fronts and cold fronts are very different in nature as
are the hazards associated with each front. They vary in
speed, composition, weather phenomenon, and prediction.
Cold fronts, which move at 20 to 35 mph, move very quickly
in comparison to warm fronts, which move at only 10 to
25 mph. Cold fronts also possess a steeper frontal slope.
Violent weather activity is associated with cold fronts, and
the weather usually occurs along the frontal boundary, not in
advance. However, squall lines can form during the summer
months as far as 200 miles in advance of a severe cold front.
Whereas warm fronts bring low ceilings, poor visibility,
and rain, cold fronts bring sudden storms, gusty winds,
turbulence, and sometimes hail or tornadoes.

Cold fronts are fast approaching with little or no warning, and
they make a complete weather change in just a few hours.
The weather clears rapidly after passage and drier air with
unlimited visibilities prevail. Warm fronts, on the other hand,
provide advance warning of their approach and can take days
to pass through a region.

Wind Shifts
Wind around a high pressure system rotates in a clockwise
fashion, while low pressure winds rotate in a counterclockwise
manner. When two pressure systems are adjacent,
the winds are almost in direct opposition to each other at
the point of contact. Fronts are the boundaries between two
areas of pressure, and therefore, wind shifts are continually
occurring within a front. Shifting wind direction is most
pronounced in conjunction with cold fronts.

Stationary Front
When the forces of two air masses are relatively equal, the
boundary or front that separates them remains stationary and
influences the local weather for days. This front is called a
stationary front. The weather associated with a stationary
front is typically a mixture that can be found in both warm
and cold fronts.

Occluded Front
An occluded front occurs when a fast-moving cold front
catches up with a slow-moving warm front. As the occluded
front approaches, warm front weather prevails, but is
immediately followed by cold front weather. There are two
types of occluded fronts that can occur, and the temperatures
of the colliding frontal systems play a large part in defining
the type of front and the resulting weather. A cold front
occlusion occurs when a fast moving cold front is colder
than the air ahead of the slow moving warm front. When
this occurs, the cold air replaces the cool air and forces the
warm front aloft into the atmosphere. Typically, the cold
front occlusion creates a mixture of weather found in both
warm and cold fronts, providing the air is relatively stable.
A warm front occlusion occurs when the air ahead of the
warm front is colder than the air of the cold front. When this
is the case, the cold front rides up and over the warm front. If
the air forced aloft by the warm front occlusion is unstable,
the weather is more severe than the weather found in a cold
front occlusion. Embedded thunderstorms, rain, and fog are
likely to occur.

Figure 11-28 depicts a cross-section of a typical cold front
occlusion. The warm front slopes over the prevailing cooler
air and produces the warm front type weather. Prior to the
passage of the typical occluded front, cirriform and stratiform
clouds prevail, light to heavy precipitation is falling, visibility
is poor, dew point is steady, and barometric pressure is
falling. During the passage of the front, nimbostratus and
cumulonimbus clouds predominate, and towering cumulus
may also be possible. Light to heavy precipitation is falling,
visibility is poor, winds are variable, and the barometric
pressure is leveling off. After the passage of the front,
nimbostratus and altostratus clouds are visible, precipitation
is decreasing and clearing, and visibility is improving.