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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




Wind and Pressure Representation on Surface
Weather Maps

Surface weather maps provide information about fronts, areas
of high and low pressure, and surface winds and pressures
for each station. This type of weather map allows pilots to
see the locations of fronts and pressure systems, but more
importantly, it depicts the wind and pressure at the surface
for each location. For more information on surface analysis
and weather depiction charts, see Chapter 12, Weather
Aviation Services.

Wind conditions are reported by an arrow attached to the
station location circle. [Figure 11-18] The station circle
represents the head of the arrow, with the arrow pointing
in the direction from which the wind is blowing. Winds
are described by the direction from which they blow, thus
a northwest wind means that the wind is blowing from the
northwest toward the southeast. The speed of the wind is
depicted by barbs or pennants placed on the wind line. Each
barb represents a speed of ten knots, while half a barb is equal
to five knots, and a pennant is equal to 50 knots.

Depiction of winds on a surface weather chart.
Figure 11-18. Depiction of winds on a surface weather chart.

The pressure for each station is recorded on the weather chart
and is shown in mb. Isobars are lines drawn on the chart to
depict areas of equal pressure. These lines result in a pattern
that reveals the pressure gradient or change in pressure over
distance. [Figure 11-19] Isobars are similar to contour lines
on a topographic map that indicate terrain altitudes and
slope steepness. For example, isobars that are closely spaced
indicate a steep wind gradient and strong winds prevail.
Shallow gradients, on the other hand, are represented by
isobars that are spaced far apart, and are indicative of light
winds. Isobars help identify low and high pressure systems
as well as the location of ridges, troughs, and cut-off lows
(cols). A high is an area of high pressure surrounded by
lower pressure; a low is an area of low pressure surrounded
by higher pressure. A ridge is an elongated area of high
pressure, and a trough is an elongated area of low pressure.
A col is the intersection between a ridge and a trough, or an
area of neutrality between two highs or two lows.

Isobars furnish valuable information about winds in the first
few thousand feet above the surface. Close to the ground,
wind direction is modified by the surface and wind speed
decreases due to friction with the surface. At levels 2,000 to
3,000 feet above the surface, however, the speed is greater
and the direction becomes more parallel to the isobars.
Therefore, the surface winds are shown on the weather map,
as well as the winds at a slightly higher altitude.

Isobars reveal the pressure gradient of an area of high- or low-pressure areas.
Figure 11-19. Isobars reveal the pressure gradient of an area of
high- or low-pressure areas.

Generally, the wind 2,000 feet above ground level (AGL) is
20° to 40° to the right of surface winds, and the wind speed
is greater. The change of wind direction is greatest over rough
terrain and least over .at surfaces, such as open water. In the
absence of winds aloft information, this rule of thumb allows
for a rough estimate of the wind conditions a few thousand
feet above the surface.

Atmospheric Stability

The stability of the atmosphere depends on its ability to
resist vertical motion. A stable atmosphere makes vertical
movement difficult, and small vertical disturbances dampen
out and disappear. In an unstable atmosphere, small vertical
air movements tend to become larger, resulting in turbulent
airflow and convective activity. Instability can lead to
significant turbulence, extensive vertical clouds, and severe

Rising air expands and cools due to the decrease in air
pressure as altitude increases. The opposite is true of
descending air; as atmospheric pressure increases, the
temperature of descending air increases as it is compressed.
Adiabatic heating and adiabatic cooling are terms used to
describe this temperature change.