Wind Continued

1
Wind Continued

• The Coriolis Effect and the Frictional Force

2
The Effect of the Earths Rotation

• Newton originally developed his Laws of Motion
  for an inertial (absolute) coordinate system in
  which the origin and the orientation of the axes
  are fixed in space.

z
y
x
3
The Effect of the Earths Rotation (Cont.)

• Our instruments on the surface of the Earth used
  to measure the wind speed and direction are
  rotating with the Earth.
• We also map weather systems and the winds they
  produce with respect to features on the surface
  of the Earth, which rotate with the Earth.

4
The Effect of the Earths Rotation (Cont.)
NP
Weather instruments on the Earths surface are
constantly moving toward the east as the Earth
rotates.
X
SP
5
The Effect of the Earths Rotation (Cont.)

• We need to adjust Newtons Laws of Motion for the
  rotation of the Earth.
• Coriolis was a French mathematician who
  determined the necessary adjustments to Newtons
  Laws of Motion.

6
The Coriolis Effect

• The Coriolis effect computes the apparent change
  of direction of the movement of air (i.e. the
  wind) because we live on the surface of a
  spherical, rotating Earth.

7
The Coriolis Effect (Cont.)

• Suppose the Earth was a flat rotating disc rather
  than a rotating sphere and that four people were
  watching air move.
• Two people are at fixed locations out in space
  looking at each other.
• The other two people are on the edge of the
  rotating disc looking at each other.

8
The Coriolis Effect (Cont.)
observer fixed in space
observer fixed in space
9
The Coriolis Effect (Cont.)

• Suppose the disc was not rotating and an air
  parcel started at one side and moved at a
  constant speed and direction

10
The Coriolis Effect (Cont.)
observer fixed in space
All four observers agree that the air parcel
moves in a straight line at a constant speed
because no force is acting on it.
No rotation of the disc.
observer fixed in space
11
The Coriolis Effect (Cont.)

• Now suppose we let the disc rotate like the Earth
  rotates and the two people on the disc rotate
  with it, while the other two people remain fixed
  in space.
• Now, watch what happens as a second air parcel
  starts to move in a constant direction at a
  constant speed.

12
The Coriolis Effect (Cont.)
observer fixed in space
observer fixed in space
13
The Coriolis Effect (Cont.)
observer fixed in space
The observers in space see that the air parcel
travels in a straight line.
As this person rotates with the disc and watches
his counterpart on the disc, the air parcel
appears to turn to the right.
The disc rotates counter-clockwise.
observer fixed in space
14
The Coriolis Effect (Cont.)

• The Coriolis effect causes the wind to appear to
  turn to the right in the northern hemisphere.
• The Coriolis effect causes the wind to appear to
  turn to the left in the southern hemisphere.

15
The Coriolis Effect (Cont.)

• Three factors determine the magnitude of the
  Coriolis effect
• latitude
• wind speed
• the rate of planetary rotation

16
The Coriolis Effect (Cont.)

• Latitude the Coriolis effect increases as the
  latitude increases.
• Wind speed the Coriolis effect increases as the
  wind speed increases
• Rate of planetary rotation the Coriolis effect
  increases as the rate of planetary rotation
  increases.

17
The Coriolis Effect (Cont.)

• The Coriolis effect is often computed as
• CE 2 O sin f V
• where
• CE is the Coriolis effect
• O is the angular velocity of the Earth
• (O 7.292 x 10-5 s-1)
• f is the latitude
• V is the wind speed

18
The Coriolis Parameter

• Meteorologists sometimes define a Coriolis
  parameter, f, such that
• f 2 O sin f
• In this case, they write the Coriolis effect as
• CE f V

19
Rotation of Air Around Pressure Systems

• The combination of the pressure gradient force
  and the Coriolis effect determines the direction
  of rotation around pressure systems.

20
Highs and Lows in the Northern Hemisphere
N
CE
PGF
E
L
H
CE
PGF
Air rotates clockwise around high pressure
systems in the northern hemisphere (anticyclonic
flow).
Air rotates counterclockwise around low pressure
systems in the northern hemisphere (cyclonic
flow).
21
Highs and Lows in the Southern Hemisphere
N
CE
PGF
E
H
L
CE
PGF
Air rotates counterclockwise around high pressure
systems in the southern hemisphere.
Air rotates clockwise around low pressure systems
in the southern hemisphere.
22
The Geostrophic Wind

• At heights greater than 1000 m above the Earths
  surface the frictional force becomes less
  important.
• Meteorologists sometimes assume that the wind
  above 1000 m is the result of a balance between
  the acceleration due to the pressure gradient
  force and the Coriolis effect.

23
The Geostrophic Wind (Cont.)

• The wind that results from the balance between
  the acceleration due to the pressure gradient
  force and the Coriolis effect is called the
  geostrophic wind.

24
The Geostrophic Wind (Cont.)
N
Lower pressure
E
p 900 mb
PGF
Geostrophic wind
CE
p 910 mb
Higher pressure
The geostrophic wind moves
parallel to the isobars.
25
The Frictional Force

• Near the surface of the Earth the effects of the
  frictional force slow the wind speed.
• Meteorologists sometimes assume that the wind
  near the Earths surface is the result of a three
  way balance between the acceleration due to the
  pressure gradient force, the Coriolis effect and
  the acceleration due to the frictional force.

26
The Wind Near the Earths Surface

• Remember, the Coriolis effect is a function of
  the wind speed. So, when the frictional force
  slows the wind speed, it also reduces the
  magnitude of the Coriolis effect.

27
The Wind Near the Earths Surface (Cont.)
N
Lower pressure
E
p 1000 mb
PGF
Fr
CE
p 1010 mb
Higher pressure
The wind near the surface moves across the
isobars at an angle from the higher toward the
lower pressure.
28
The Wind Near the Earths Surface (Cont.)

• The effect of the combination of the frictional
  force with the pressure gradient force and the
  Coriolis effect is to cause air to converge on
  surface low pressure sytems and to cause air to
  diverge from surface high pressure systems.

29
The Wind Near the Earths Surface (Cont.)
N
E
L
H
The airflow around surface high pressure systems
in the northern hemisphere is clockwise and
divergent.
The airflow around surface low pressure systems
in the northern hemisphere is counterclockwise
and convergent.
30
The Wind Near the Earths Surface (Cont.)

• The convergence and divergence near the Earths
  surface contribute to the pattern of vertical
  motion.

31
The Wind Near the Earths Surface (Cont.)
The divergence and sinking motion associated with
surface high pressure are more likely to produce
clear skies.
Since clouds and precipitation require rising
air, they are most likely to form in areas of
surface low pressure.
High
Low
Divergence from high pressure at the surface
creates sinking motion.
Convergence into low pressure at the surface
creates rising motion.
32
Wind Direction

• The official wind direction is defined as the
  compass direction from which the air is blowing.
• Thus, a west wind means that the air is moving
  from west to east.

west
east
33
Wind Direction (Cont.)
North
Northeast
Northwest
East
West
Southeast
Southwest
South