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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




Tornadoes occur with both isolated and squall line
thunderstorms. Reports for forecasts of tornadoes indicate that
atmospheric conditions are favorable for violent turbulence.
An aircraft entering a tornado vortex is almost certain to
suffer structural damage. Since the vortex extends well into
the cloud, any pilot inadvertently caught on instruments in a
severe thunderstorm could encounter a hidden vortex.

Families of tornadoes have been observed as appendages of
the main cloud extending several miles outward from the area
of lightning and precipitation. Thus, any cloud connected to
a severe thunderstorm carries a threat of violence.

Potentially hazardous turbulence is present in all
thunderstorms, and a severe thunderstorm can destroy an
aircraft. Strongest turbulence within the cloud occurs with
shear between updrafts and downdrafts. Outside the cloud,
shear turbulence has been encountered several thousand feet
above and 20 miles laterally from a severe storm. A low-level
turbulent area is the shear zone associated with the gust front.
Often, a "roll cloud" on the leading edge of a storm marks the
top of the eddies in this shear and it signifies an extremely
turbulent zone. Gust fronts often move far ahead (up to 15
miles) of associated precipitation. The gust front causes a
rapid and sometimes drastic change in surface wind ahead of
an approaching storm. Advisory Circular (AC) 00-50A, Low
Level Wind Shear, explains in detail the hazards associated
with gust fronts. Figure 1 in the AC shows a schematic truss
section of a thunderstorm with areas outside the cloud where
turbulence may be encountered.

Updrafts in a thunderstorm support abundant liquid water
with relatively large droplet sizes. When carried above
the freezing level, the water becomes supercooled. When
temperature in the upward current cools to about –15 °C,
much of the remaining water vapor sublimates as ice crystals.
Above this level, at lower temperatures, the amount of
supercooled water decreases

Supercooled water freezes on impact with an aircraft. Clear
icing can occur at any altitude above the freezing level, but at
high levels, icing from smaller droplets may be rime or mixed
rime and clear ice. The abundance of large, supercooled
water droplets makes clear icing very rapid between 0 °C and
–15 °C and encounters can be frequent in a cluster of cells.
Thunderstorm icing can be extremely hazardous.

Thunderstorm icing can be extremely hazardous.
Thunderstorms are not the only area where pilots could
encounter icing conditions. Pilots should be alert for icing
anytime the temperature approaches 0 °C and visible moisture
is present.

Hail competes with turbulence as the greatest thunderstorm
hazard to aircraft. Supercooled drops above the freezing level
begin to freeze. Once a drop has frozen, other drops latch on
and freeze to it, so the hailstone grows—sometimes into a
huge ice ball. Large hail occurs with severe thunderstorms
with strong updrafts that have built to great heights.

Eventually, the hailstones fall, possibly some distance from
the storm core. Hail may be encountered in clear air several
miles from thunderstorm clouds.