Chapter 10, Wind 10-1
10 Wind
10.1 Introduction
Wind is the air in motion relative to the surface of the Earth. Although people cannot actually see the air
moving, it can be measured by its motion of force that it applies on objects. For example, leaves rustling or
trees swaying on a windy day indicate that the wind is blowing. Winds are a major factor to both weather
and aircraft. Winds cause the formation, dissipation, and redistribution of weather. Winds also affect aircraft
during all phases of flight.
Adverse wind is a category of hazardous aviation weather that is responsible for many weather -related
accidents. Adverse winds include crosswinds, gusts, tailwind, variable wind, sudden wind shift, wind shear,
and mountain wind hazards . Takeoff and landing are the most critical periods of any flight and are most
susceptible to the effects of adverse wind. The most at -risk group is pilots flying aircraft with lower
crosswind and tailwind threshold values.
This chapter discusses the origin of wind as well as adverse winds.
Chapter 10, Wind 10-2
10.2 Naming of the Wind
Wind is named according to the direction from which it is blowing. For example, a west wind indicates the
wind is blowing from the west to the east. There are 36 specific azimuth degrees expressed in intervals of
10 degrees. In aviation, the points of the compass are normally used to represent the direction from which
the wind is blowing. For example, north winds come from 360°, east from 90°, south fr om 180°, and west
from 270°.
There are also 16 cardinal compass directions relative to wind. The four primary cardinal directions are
north (N), south (S), east (E), and west (W). There are also four intermediate directions, such as northeast
(NE), northwest (NW), southeast (SE), and southwest (SW). Additionally, there are eight subdivision s,
including north-northeast (NNE), north-northwest (NNW), south-southeast (SSE), south-southwest (SSW),
east-northeast (ENE), east-southeast (ESE), west-southwest (WSW), and west-northwest (WNW).
10.3 Forces That Affect the Wind
Three primary forces affect the flow of wind: Pressure Gradient Force (PGF), Coriolis force, and friction.
10.3.1 Pressure Gradient Force (PGF)
Wind is driven by pressure differences, which create a force called the PGF. Whenever a pressure difference
develops over an area, the PGF makes the wind blow in an attempt to equalize pressure differences. This
force is identified by height contour gradients on constant -pressure charts and by isobar gradients on
surface charts.
PGF is directed from higher height/pressure to lower height/pressure and is perpendicular to
contours/isobars. Whenever a pressure difference develops over an area, the PGF begins moving the air
directly across the contours/isobars. See Figure 10-1.
PGF is directed across contours/isobars towards lower height/pressure.
Figure 10-1. Direction of Pressure Gradient Force
Wind speed is directly proportional to the PGF, which itself is directly proportional to the contour/isobar
gradient. Closely spaced contours/isobars indicate strong winds, while widely spaced contours/isobars
mean lighter wind. From a pressure analysis, users can get a general idea of wind speed from contour/isobar
spacing.
Chapter 10, Wind 10-3
Figure 10-2. Magnitude of Pressure Gradient Force
In Figure 10-2, on the left panel, the contours/isobars are widely spaced apart, PGF is weak, and the wind
speed is weak. On the right panel, the contours/isobars are more closely spaced, the PGF is stronger, and
the wind speed is stronger.
The wind would flow from high to low pressure if the P GF was the only force acting on it. However,
because of the Earth’s rotation, there is a second force called the Coriolis force that affects the direction of
wind flow.
10.3.2 Coriolis Force
A moving mass travels in a straight line until acted on by some outside force. However, if one views the
moving mass from a rotating platform, the path of the moving mass relative to their platform appears to be
deflected or curved. To illustrate, consider a turntable. If one used a pencil and a ruler to draw a straight
line from the center to the outer edge of the turntable, the pencil would have traveled in a straight line.
However, stopping the turntable, it is evident that the line spirals outward from the center (see Figure 10-3).
To a viewer on the turntable, some apparent force deflected the pencil to the right.
Figure 10-3. Illustration of Coriolis Force
A similar apparent force deflects moving particles on the Earth. Because the Earth is spherical, the
deflective force is much more complex than the simple turntable example. The principle was first explained
by Gaspard-Gustave de Coriolis, and now carries his name—the Coriolis force.
Coriolis force is an apparent force that affects all moving objects. The force deflects air to the right in the
Northern Hemisphere and to the left in the Southern Hemisphere.
Coriolis force is at a right angle to wind direction and directly proportional to wind speed; that is, as wind
speed increases, Coriolis force increases. At a given latitude, double the wind speed and the Coriolis force
is doubled. Why at a given latitude?
Chapter 10, Wind 10-4
Coriolis force varies with latitude from zero at the Equator to a maximum at the poles. It influences wind
direction everywhere except immediately at the Equator, but the effects are more pronounced in middle and
high latitudes.
Figure 10-4. Coriolis Force Variations Across the Earth
Coriolis force deflects moving objects to the right of their path in the Northern Hemisphere and to the left
of their path in the Southern Hemisphere. Coriolis deflection is maximized at the poles and zero at
the Equator.
Chapter 10, Wind 10-5
Figure 10-5. Coriolis Force Magnitude Variations with Wind Speed
Coriolis force magnitude is directly proportional to wind speed. In Figure 10-5, wind speed is twice as
strong in the right panel; thus, the Coriolis force is doubled.
10.3.3 Friction Force
Friction between the wind and the terrain surface slows the wind. The rougher the terrain, the greater the
frictional effect. Also, the stronger the wind speed, the greater the friction. One may not think of friction as
a force, but it is a very real and effective force always acting opposite to wind direction.
Figure 10-6. Friction Force Magnitude Variations with Terrain Roughness
Friction force magnitude is directly proportional to terrain roughness. Even though the wind speed is the
same in both panels in Figure 10-6, the terrain is rougher in the right panel; thus, the friction force is
stronger.
Figure 10-7. Friction Force Magnitude Variations with Wind Speed
Friction force magnitude is directly proportional to wind speed. In Figure 10-7, wind speed is twice as
strong in the right panel; thus, the friction force is doubled.
Chapter 10, Wind 10-6
The frictional drag of the ground normally decreases with height and becomes insignificant above the
lowest few thousand feet. However, this may vary somewhat , since both strong winds and rough terrain
extend the friction layer to higher altitudes.
10.4 Upper Air Wind
In the atmosphere above the friction layer (lowest few thousand feet), only PGF and Coriolis force affect
the horizontal motion of air. Remember that the PGF drives the wind and is oriented perpendicular to height
contours. When a PGF is first established, wind begins to blow from higher to lower heights directly across
the height contours. However, the instant air begins moving, Coriolis force deflects it to the right. Soon the
wind is deflected a full 90° and is parallel to the height contours. At this time, Coriolis force exactly balances
PGF, as shown in Figure 10-8 on a 500 mb constant -pressure chart (see Section 25.3 for information on
constant-pressure charts). With the forces in balance, wind will remain parallel to height contours as shown
in Figure 10-9. This is called the geostrophic wind.
Figure 10-8. Geostrophic Wind