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Pilot's Handbook of Aeronautical Knowledge
Weather Theory
Atmospheric Stability

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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




Clouds are visible indicators and are often indicative of
future weather. For clouds to form, there must be adequate
water vapor and condensation nuclei, as well as a method by
which the air can be cooled. When the air cools and reaches
its saturation point, the invisible water vapor changes into
a visible state. Through the processes of deposition (also
referred to as sublimation) and condensation, moisture
condenses or sublimates onto miniscule particles of matter
like dust, salt, and smoke known as condensation nuclei. The
nuclei are important because they provide a means for the
moisture to change from one state to another.

Cloud type is determined by its height, shape, and behavior.
They are classified according to the height of their bases as
low, middle, or high clouds, as well as clouds with vertical
development. [Figure 11-22]

Low clouds are those that form near the Earth's surface and
extend up to 6,500 feet AGL. They are made primarily of
water droplets, but can include supercooled water droplets
that induce hazardous aircraft icing. Typical low clouds
are stratus, stratocumulus, and nimbostratus. Fog is also
classified as a type of low cloud formation. Clouds in this
family create low ceilings, hamper visibility, and can change
rapidly. Because of this, they influence flight planning and
can make visual flight rules (VFR) flight impossible.

Middle clouds form around 6,500 feet AGL and extend up to
20,000 feet AGL. They are composed of water, ice crystals,
and supercooled water droplets. Typical middle-level clouds
include altostratus and altocumulus. These types of clouds
may be encountered on cross-country flights at higher
altitudes. Altostratus clouds can produce turbulence and may
contain moderate icing. Altocumulus clouds, which usually
form when altostratus clouds are breaking apart, also may
contain light turbulence and icing.

High clouds form above 20,000 feet AGL and usually form
only in stable air. They are made up of ice crystals and pose
no real threat of turbulence or aircraft icing. Typical high level
clouds are cirrus, cirrostratus, and cirrocumulus.

Clouds with extensive vertical development are cumulus
clouds that build vertically into towering cumulus or
cumulonimbus clouds. The bases of these clouds form in
the low to middle cloud base region but can extend into high
altitude cloud levels. Towering cumulus clouds indicate areas
of instability in the atmosphere, and the air around and inside
them is turbulent. These types of clouds often develop into
cumulonimbus clouds or thunderstorms. Cumulonimbus

clouds contain large amounts of moisture and unstable air,
and usually produce hazardous weather phenomena, such
as lightning, hail, tornadoes, gusty winds, and wind shear.
These extensive vertical clouds can be obscured by other
cloud formations and are not always visible from the ground
or while in flight When this happens, these clouds are said to
be embedded, hence the term, embedded thunderstorms.

To pilots, the cumulonimbus cloud is perhaps the most
dangerous cloud type. It appears individually or in groups and
is known as either an air mass or orographic thunderstorm.
Heating of the air near the Earth's surface creates an air mass
thunderstorm; the upslope motion of air in the mountainous
regions causes orographic thunderstorms. Cumulonimbus
clouds that form in a continuous line are nonfrontal bands
of thunderstorms or squall lines.

Since rising air currents cause cumulonimbus clouds, they
are extremely turbulent and pose a significant hazard to flight
safety. For example, if an aircraft enters a thunderstorm, the
aircraft could experience updrafts and downdrafts that exceed
3,000 fpm. In addition, thunderstorms can produce large
hailstones, damaging lightning, tornadoes, and large quantities
of water, all of which are potentially hazardous to aircraft.

A thunderstorm makes its way through three distinct stages
before dissipating. It begins with the cumulus stage, in and
instability are present, the clouds continue to increase in
vertical height. Continuous, strong updrafts prohibit moisture
from falling. The updraft region grows larger than the
individual thermals feeding the storm. Within approximately
15 minutes, the thunderstorm reaches the mature stage, which
is the most violent time period of the thunderstorm's life
cycle. At this point, drops of moisture, whether rain or ice,
are too heavy for the cloud to support and begin falling in the
form of rain or hail. This creates a downward motion of the
air. Warm, rising air; cool, precipitation-induced descending
air; and violent turbulence all exist within and near the cloud.
Below the cloud, the down-rushing air increases surface
winds and decreases the temperature. Once the vertical
motion near the top of the cloud slows down, the top of the
cloud spreads out and takes on an anvil-like shape. At this
point, the storm enters the dissipating stage. This is when
the downdrafts spread out and replace the updrafts needed
to sustain the storm. [Figure 11-23]

Basic cloud types.
Figure 11-22. Basic cloud types.