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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




For aviation purposes, a ceiling is the lowest layer of clouds
reported as being broken or overcast, or the vertical visibility
into an obscuration like fog or haze. Clouds are reported as
broken when five-eighths to seven-eighths of the sky is
covered with clouds. Overcast means the entire sky is covered
with clouds. Current ceiling information is reported by the
aviation routine weather report (METAR) and automated
weather stations of various types.

Closely related to cloud cover and reported ceilings is
visibility information. Visibility refers to the greatest
horizontal distance at which prominent objects can be
viewed with the naked eye. Current visibility is also reported
in METAR and other aviation weather reports, as well as
by automated weather systems. Visibility information, as
predicted by meteorologists, is available for a pilot during a
preflight weather briefing.

Precipitation refers to any type of water particles that form in
the atmosphere and fall to the ground. It has a profound impact
on flight safety. Depending on the form of precipitation, it can
reduce visibility, create icing situations, and affect landing
and takeoff performance of an aircraft.

Precipitation occurs because water or ice particles in clouds
grow in size until the atmosphere can no longer support them.
It can occur in several forms as it falls toward the Earth,
including drizzle, rain, ice pellets, hail, snow, and ice.

Drizzle is classified as very small water droplets, smaller
than 0.02 inches in diameter. Drizzle usually accompanies
fog or low stratus clouds. Water droplets of larger size are
referred to as rain. Rain that falls through the atmosphere but
evaporates prior to striking the ground is known as virga.
Freezing rain and freezing drizzle occur when the temperature
of the surface is below freezing; the rain freezes on contact
with the cooler surface.

If rain falls through a temperature inversion, it may freeze as
it passes through the underlying cold air and fall to the ground
in the form of ice pellets. Ice pellets are an indication of a
temperature inversion and that freezing rain exists at a higher
altitude. In the case of hail, freezing water droplets are carried
up and down by drafts inside clouds, growing larger in size as
they come in contact with more moisture. Once the updrafts
can no longer hold the freezing water, it falls to the Earth in
the form of hail. Hail can be pea sized, or it can grow as large
as five inches in diameter, larger than a softball.

Snow is precipitation in the form of ice crystals that falls
at a steady rate or in snow showers that begin, change in
intensity, and end rapidly. Falling snow also varies in size,
being very small grains or large flakes Snow grains are the
equivalent of drizzle in size.

Precipitation in any form poses a threat to safety of flight.
Often, precipitation is accompanied by low ceilings and
reduced visibility. Aircraft that have ice, snow, or frost on
their surfaces must be carefully cleaned prior to beginning
a flight because of the possible airflow disruption and
loss of lift. Rain can contribute to water in the fuel tanks.
Precipitation can create hazards on the runway surface itself,
making takeoffs and landings difficult, if not impossible, due
to snow, ice, or pooling water and very slick surfaces.

Air Masses

Air masses are classified according to the regions where
they originate. They are large bodies of air that take on the
characteristics of the surrounding area, or source region. A
source region is typically an area in which the air remains
relatively stagnant for a period of days or longer. During
this time of stagnation, the air mass takes on the temperature
and moisture characteristics of the source region. Areas of
stagnation can be found in polar regions, tropical oceans, and
dry deserts. Air masses are generally identified as polar or
tropical based on temperature characteristics and maritime
or continental based on moisture content.

A continental polar air mass forms over a polar region and
brings cool, dry air with it. Maritime tropical air masses form
over warm tropical waters like the Caribbean Sea and bring
warm, moist air. As the air mass moves from its source region
and passes over land or water, the air mass is subjected to the
varying conditions of the land or water, and these modify the
nature of the air mass. [Figure 11-24]

An air mass passing over a warmer surface is warmed from
below, and convective currents form, causing the air to rise.
This creates an unstable air mass with good surface visibility.
Moist, unstable air causes cumulus clouds, showers, and
turbulence to form.

Conversely, an air mass passing over a colder surface does not
form convective currents, but instead creates a stable air mass
with poor surface visibility. The poor surface visibility is due
to the fact that smoke, dust, and other particles cannot rise
out of the air mass and are instead trapped near the surface.
A stable air mass can produce low stratus clouds and fog.