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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




Dew and Frost
On cool, calm nights, the temperature of the ground and
objects on the surface can cause temperatures of the
surrounding air to drop below the dew point. When this
occurs, the moisture in the air condenses and deposits itself on
the ground, buildings, and other objects like cars and aircraft.
This moisture is known as dew and sometimes can be seen on
grass in the morning. If the temperature is below freezing, the
moisture is deposited in the form of frost. While dew poses no
threat to an aircraft, frost poses a definite flight safety hazard.
Frost disrupts the .ow of air over the wing and can drastically
reduce the production of lift. It also increases drag, which,
when combined with lowered lift production, can adversely
affect the ability to take off. An aircraft must be thoroughly
cleaned and free of frost prior to beginning a flight.

Fog is a cloud that begins within 50 feet of the surface. It
typically occurs when the temperature of air near the ground
is cooled to the air's dew point. At this point, water vapor in
the air condenses and becomes visible in the form of fog. Fog
is classified according to the manner in which it forms and
is dependent upon the current temperature and the amount
of water vapor in the air.

On clear nights, with relatively little to no wind present,
radiation fog may develop. [Figure 11-21] Usually, it forms
in low-lying areas like mountain valleys. This type of fog
occurs when the ground cools rapidly due to terrestrial
radiation, and the surrounding air temperature reaches its
dew point. As the sun rises and the temperature increases,
radiation fog lifts and eventually burns off. Any increase in
wind also speeds the dissipation of radiation fog. If radiation
fog is less than 20 feet thick, it is known as ground fog.
When a layer of warm, moist air moves over a cold surface,
advection fog is likely to occur. Unlike radiation fog, wind
is required to form advection fog. Winds of up to 15 knots
allow the fog to form and intensify; above a speed of 15 knots,
the fog usually lifts and forms low stratus clouds. Advection
fog is common in coastal areas where sea breezes can blow
the air over cooler landmasses.

Upslope fog occurs when moist, stable air is forced up sloping
land features like a mountain range. This type of fog also
requires wind for formation and continued existence. Upslope
and advection fog, unlike radiation fog, may not burn off with
the morning sun, but instead can persist for days. They can
also extend to greater heights than radiation fog.

Steam fog, or sea smoke, forms when cold, dry air moves over
warm water. As the water evaporates, it rises and resembles
smoke. This type of fog is common over bodies of water
during the coldest times of the year. Low-level turbulence
and icing are commonly associated with steam fog.

Ice fog occurs in cold weather when the temperature is
much below freezing and water vapor forms directly into
ice crystals. Conditions favorable for its formation are
the same as for radiation fog except for cold temperature,
usually –25 °F or colder. It occurs mostly in the arctic
regions, but is not unknown in middle latitudes during the
cold season.

Radiation fog.
Figure 11-21. Radiation fog.