Figure 12-10. Circulation pattern about areas of high and low
pressure.
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. The
combination of atmospheric pressure differences, Coriolis
force, friction, and temperature differences of the air near
the earth cause two kinds of atmospheric motion: convective
currents (upward and downward motion) and wind
(horizontal motion). 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 flow 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 12-10]
High-pressure systems are generally areas of dry, 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 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 12-11] 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.
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 surface.
Convective Currents
Plowed ground, rocks, sand, and barren land absorb solar
energy quickly and can therefore give off a large amount
of heat; whereas, water, trees, and other areas of vegetation
tend to more slowly absorb heat and give off 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 12-12]
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
Figure 12-11. Favorable winds near a high pressure system.
Figure 12-12. Convective turbulence avoidance.
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 12-13]
Convective currents close to the ground can affect a pilot’s
ability to control the aircraft. For example, on final approach,
the rising air from terrain devoid of vegetation sometimes
produces a ballooning effect that can cause a pilot to overshoot
the intended landing spot. On the other hand, an approach over
a large body of water or an area of thick vegetation tends to
create a sinking effect that can cause an unwary pilot to land
short of the intended landing spot. [Figure 12-14]
Effect of Obstructions on Wind
Another atmospheric hazard exists that can create problems
for pilots. Obstructions on the ground affect the flow of
wind and can be an unseen danger. Ground topography and
large buildings can break up the flow of the wind and create
wind gusts that change rapidly in direction and speed. These
obstructions range from man-made structures, like hangars,
to large natural obstructions, such as mountains, bluffs, or
canyons. It is especially important to be vigilant when flying
in or out of airports that have large buildings or natural
obstructions located near the runway. [Figure 12-15]
The intensity of the turbulence associated with ground
obstructions depends on the size of the obstacle and the
primary velocity of the wind. This can affect the takeoff and
landing performance of any aircraft and can present a very
serious hazard. During the landing phase of flight, an aircraft
Return flow
Land breeze
Return flow
Sea breeze
Warm
Cool
Cool
Warm
Figure 12-13. Sea breeze and land breeze wind circulation patterns.
Warm
air
rising
Cool
air
sinking
Intended Flight path
Figure 12-14. Currents generated by varying surface conditions.
I N D W
Figure 12-15. Turbulence caused by manmade obstructions.
WIND
Figure 12-16. Turbulence in mountainous regions.
a similar manner. As the air flows down the leeward side of
the mountain, the air follows the contour of the terrain and
is increasingly turbulent. This tends to push an aircraft into
the side of a mountain. The stronger the wind, the greater the
downward pressure and turbulence become.
Due to the effect terrain has on the wind in valleys or canyons,
downdrafts can be severe. Before conducting a flight in or
may “drop in” due to the turbulent air and be too low to clear
obstacles during the approach.
This same condition is even more noticeable when flying in
mountainous regions. [Figure 12-16] While the wind flows
smoothly up the windward side of the mountain and the
upward currents help to carry an aircraft over the peak of
the mountain, the wind on the leeward side does not act in
Intended Path
1
2
3 4
Increasing tailwind Increasing headwind
Strong downdraft
Outflow
Outflow
Figure 12-17. Effects of a microburst wind.
near mountainous terrain, it is helpful for a pilot unfamiliar
with a mountainous area to get a checkout with a mountain
qualified flight instructor.
Low-Level Wind Shear
Wind shear is a sudden, drastic change in wind speed and/or
direction over a very small area. Wind shear can subject an
aircraft to violent updrafts and downdrafts, as well as abrupt
changes to the horizontal movement of the aircraft. While
wind shear can occur at any altitude, low-level wind shear is
especially hazardous due to the proximity of an aircraft to the
ground. Low-level wind shear is commonly associated with
passing frontal systems, thunderstorms, temperature inversions,
and strong upper level winds (greater than 25 knots).
Wind shear is dangerous to an aircraft. It can rapidly change
the performance of the aircraft and disrupt the normal flight
attitude. For example, a tailwind quickly changing to a
headwind causes an increase in airspeed and performance.
Conversely, a headwind changing to a tailwind causes a
decrease in airspeed and performance. In either case, a pilot
must be prepared to react immediately to these changes to
maintain control of the aircraft.
The most severe type of low-level wind shear, a microburst,
is associated with convective precipitation into dry air at
cloud base. Microburst activity may be indicated by an
intense rain shaft at the surface but virga at cloud base
and a ring of blowing dust is often the only visible clue.
A typical microburst has a horizontal diameter of 1–2
miles and a nominal depth of 1,000 feet. The lifespan of a
microburst is about 5–15 minutes during which time it can
produce downdrafts of up to 6,000 feet per minute (fpm)
and headwind losses of 30–90 knots, seriously degrading
performance. It can also produce strong turbulence and
hazardous wind direction changes. Consider Figure 12-17:
During an inadvertent takeoff into a microburst, the plane
may first experience a performance-increasing headwind
(1), followed by performance-decreasing downdrafts (2),
followed by a rapidly increasing tailwind (3). This can result
in terrain impact or flight dangerously close to the ground (4).
An encounter during approach involves the same sequence
of wind changes and could force the plane to the ground
short of the runway.
The FAA has made a substantial investment in microburst
accident prevention. The totally redesigned LLWAS-NE, the
TDWR, and the ASR-9 WSP are skillful microburst alerting
systems installed at major airports. These three systems were
extensively evaluated over a 3-year period. Each was seen
to issue very few false alerts and to detect microbursts well
above the 90 percent detection requirement established by
Congress. Many flights involve airports that lack microburst
alert equipment, so the FAA has also prepared wind shear
training material: Advisory Circular (AC) 00-54, FAA
Pilot Wind Shear Guide. Included is information on how to
recognize the risk of a microburst encounter, how to avoid an
encounter, and the best flight strategy for successful escape
should an encounter occur.
It is important to remember that wind shear can affect any
flight and any pilot at any altitude. While wind shear may be
reported, it often remains undetected and is a silent danger
to aviation. Always be alert to the possibility of wind shear,
especially when flying in and around thunderstorms and
frontal systems.