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Pilot's Handbook of Aeronautical Knowledge
Weather Theory

Altitude and Atmospheric Pressure

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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




Mercurial barometer.
Figure 11-6. Mercurial barometer.

An aneroid barometer is an alternative to a mercurial
barometer; it is easier to read and transport. [Figure 11-7] The
aneroid barometer contains a closed vessel, called an aneroid
cell that contracts or expands with changes in pressure. The
aneroid cell attaches to a pressure indicator with a mechanical
linkage to provide pressure readings. The pressure sensing
part of an aircraft altimeter is essentially an aneroid
barometer. It is important to note that due to the linkage
mechanism of an aneroid barometer, it is not as accurate as
a mercurial barometer.

Aneroid barometer.
Figure 11-7. Aneroid barometer.

To provide a common reference, the International Standard
Atmosphere (ISA) has been established. These standard
conditions are the basis for certain flight instruments and
most aircraft performance data. Standard sea level pressure
is defined as 29.92 "Hg and a standard temperature of 59 °F
(15 °C). Atmospheric pressure is also reported in millibars
(mb), with 1 "Hg equal to approximately 34 mb. Standard sea
level pressure is 1,013.2 mb. Typical mb pressure readings
range from 950.0 to 1,040.0 mb. Constant pressure charts and
hurricane pressure reports are written using mb.

Since weather stations are located around the globe, all local
barometric pressure readings are converted to a sea level
pressure to provide a standard for records and reports. To
achieve this, each station converts its barometric pressure by
adding approximately 1 "Hg for every 1,000 feet of elevation.
For example, a station at 5,000 feet above sea level, with a
reading of 24.92 "Hg, reports a sea level pressure reading of
29.92 "Hg. [Figure 11-8] Using common sea level pressure
readings helps ensure aircraft altimeters are set correctly,
based on the current pressure readings.

By tracking barometric pressure trends across a large area,
weather forecasters can more accurately predict movement
of pressure systems and the associated weather. For example,
tracking a pattern of rising pressure at a single weather station
generally indicates the approach of fair weather. Conversely,
decreasing or rapidly falling pressure usually indicates
approaching bad weather and, possibly, severe storms.

Altitude and Atmospheric Pressure

As altitude increases, atmospheric pressure decreases. On
average, with every 1,000 feet of increase in altitude, the
atmospheric pressure decreases 1 "Hg. As pressure decreases,
the air becomes less dense or "thinner." This is the equivalent
of being at a higher altitude and is referred to as density
altitude (DA). As pressure decreases, DA increases and has
a pronounced effect on aircraft performance.

Differences in air density caused by changes in temperature
result in a change in pressure. This, in turn, creates motion in
the atmosphere, both vertically and horizontally, in the form
of currents and wind. The atmosphere is almost constantly
in motion as it strives to reach equilibrium. These neverending
air movements set up chain reactions which cause a
continuing variety in the weather.