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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




As hailstones fall through air whose temperature is above
0 °C, they begin to melt and precipitation may reach the
ground as either hail or rain. Rain at the surface does not
mean the absence of hail aloft. Possible hail should be
anticipated with any thunderstorm, especially beneath the
anvil of a large cumulonimbus. Hailstones larger than one half
inch in diameter can significantly damage an aircraft
in a few seconds.

Ceiling and Visibility
Generally, visibility is near zero within a thunderstorm
cloud. Ceiling and visibility also may be restricted in
precipitation and dust between the cloud base and the ground.
The restrictions create the same problem as all ceiling and
visibility restrictions; but the hazards are multiplied when
associated with the other thunderstorm hazards of turbulence,
hail, and lightning.

Effect on Altimeters
Pressure usually falls rapidly with the approach of a
thunderstorm, rises sharply with the onset of the first gust
and arrival of the cold downdraft and heavy rain showers,
and then falls back to normal as the storm moves on. This
cycle of pressure change may occur in 15 minutes. If the pilot
does not receive a corrected altimeter setting, the altimeter
may be more than 100 feet in error.

A lightning strike can puncture the skin of an aircraft
and damage communications and electronic navigational
equipment. Although lightning has been suspected of igniting
fuel vapors and causing an explosion, serious accidents due
to lightning strikes are rare. Nearby lightning can blind the
pilot, rendering him or her momentarily unable to navigate
either by instrument or by visual reference. Nearby lightning
can also induce permanent errors in the magnetic compass.
Lightning discharges, even distant ones, can disrupt radio
communications on low and medium frequencies. Though
lightning intensity and frequency have no simple relationship
to other storm parameters, severe storms, as a rule, have a
high frequency of lightning.

Engine Water Ingestion
Turbine engines have a limit on the amount of water they
can ingest. Updrafts are present in many thunderstorms,
particularly those in the developing stages. If the updraft
velocity in the thunderstorm approaches or exceeds the
terminal velocity of the falling raindrops, very high
concentrations of water may occur. It is possible that these
concentrations can be in excess of the quantity of water
turbine engines are designed to ingest. Therefore, severe
thunderstorms may contain areas of high water concentration
which could result in flameout and/or structural failure of
one or more engines.

Chapter Summary

Knowledge of the atmosphere and the forces acting within
it to create weather is essential to understand how weather
affects a flight. By understanding basic weather theories, a
pilot can make sound decisions during flight planning after
receiving weather briefings. For additional information on
the topics discussed in this chapter, see AC 00-6, Aviation
Weather For Pilots and Flight Operations Personnel;
AC 00-24, Thunderstorms; AC 00-45, Aviation Weather
Services; AC 91-74, Pilot Guide Flight in Icing Conditions;
and chapter 7, section 2 of the Aeronautical Information
Manual (AIM).