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Pilot's Handbook of Aeronautical Knowledge
Weather Theory
Wind and Currents

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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




Altitude and the Human Body
As discussed earlier, nitrogen and other trace gases make
up 79 percent of the atmosphere, while the remaining 21
percent is life sustaining, atmospheric oxygen. At sea level,
atmospheric pressure is great enough to support normal
growth, activity, and life. By 18,000 feet, the partial pressure
of oxygen is reduced and adversely affects the normal
activities and functions of the human body.

The reactions of the average person become impaired at an
altitude of about 10,000 feet, but for some people impairment
can occur at an altitude as low as 5,000 feet. The physiological
reactions to hypoxia or oxygen deprivation are insidious and
affect people in different ways. These symptoms range from
mild disorientation to total incapacitation, depending on
body tolerance and altitude. Supplemental oxygen or cabin
pressurization systems help pilots fly at higher altitudes and
overcome the effects of oxygen deprivation.

Wind and Currents

Air flows from areas of high pressure into areas of low
pressure because air always seeks out lower pressure. Air
pressure, temperature changes, and the Coriolis force work in
combination to create two kinds of motion in the atmosphere—
vertical movement of ascending and descending currents,
and horizontal movement in the form of wind. Currents and
winds are important as they affect takeoff, landing, and cruise
flight operations. Most importantly, currents and winds or
atmospheric circulation cause weather changes.

Wind Patterns
In the Northern Hemisphere, the .ow of air from areas of
high to low pressure is deflected to the right and produces
a clockwise circulation around an area of high pressure.
This is known as anticyclonic circulation. The opposite
is true of low-pressure areas; the air flows toward a low
and is deflected to create a counterclockwise or cyclonic
circulation. [Figure 11-10]

High pressure systems are generally areas of dry, stable,
descending air. Good weather is typically associated with
high pressure systems for this reason. Conversely, air flows
into a low pressure area to replace rising air. This air tends
to be unstable, and usually brings increasing cloudiness and
precipitation. Thus, bad weather is commonly associated
with areas of low pressure.

A good understanding of high and low pressure wind patterns
can be of great help when planning a flight, because a pilot
can take advantage of beneficial tailwinds. [Figure 11-11]
When planning a flight from west to east, favorable winds
would be encountered along the northern side of a high
pressure system or the southern side of a low pressure system.
On the return flight, the most favorable winds would be along
the southern side of the same high pressure system or the
northern side of a low pressure system. An added advantage
is a better understanding of what type of weather to expect
in a given area along a route of flight based on the prevailing
areas of highs and lows.

Circulation pattern about areas of high and low pressure.
Figure 11-10. Circulation pattern about areas of high and low

While the theory of circulation and wind patterns is accurate
for large scale atmospheric circulation, it does not take into
account changes to the circulation on a local scale. Local
conditions, geological features, and other anomalies can
change the wind direction and speed close to the Earth's

Convective Currents
Different surfaces radiate heat in varying amounts. Plowed
ground, rocks, sand, and barren land give off a large amount of
heat; water, trees, and other areas of vegetation tend to absorb
and retain heat. The resulting uneven heating of the air creates
small areas of local circulation called convective currents.

Convective currents cause the bumpy, turbulent air sometimes
experienced when flying at lower altitudes during warmer
weather. On a low altitude flight over varying surfaces,
updrafts are likely to occur over pavement or barren places,
and downdrafts often occur over water or expansive areas
of vegetation like a group of trees. Typically, these turbulent
conditions can be avoided by flying at higher altitudes, even
above cumulus cloud layers. [Figure 11-12]

Convective currents are particularly noticeable in areas with
a land mass directly adjacent to a large body of water, such
as an ocean, large lake, or other appreciable area of water.
During the day, land heats faster than water, so the air over the
land becomes warmer and less dense. It rises and is replaced
by cooler, denser air flowing in from over the water. This
causes an onshore wind, called a sea breeze. Conversely, at
night land cools faster than water, as does the corresponding
air. In this case, the warmer air over the water rises and is
replaced by the cooler, denser air from the land, creating an
offshore wind called a land breeze. This reverses the local
wind circulation pattern. Convective currents can occur
anywhere there is an uneven heating of the Earth's surface.
[Figure 11-13]