The document discusses the key forces that cause wind: pressure gradient force (PGF), Coriolis force, and geostrophic wind. PGF is due to differences in pressure over distance and always acts from high to low pressure, perpendicular to isobars. Coriolis force is caused by the Earth's rotation and deflects motion to the right in the Northern Hemisphere. When only PGF and Coriolis force are in balance, with their magnitudes equal and directions opposite, the resulting wind flow is parallel to isobars and called geostrophic wind.

1. 1

2. Pressure
gradient
force (PGF)
2

3. Forces
3

4. What creates wind?
Wind is a result of different forces acting on a parcel of air.
air moving must follows Newton’s second law
F = ma
F = force (a push or pull)
m = mass of an object (parcel)
a = acceleration (speed up or slow down)
if there are many forces, the acceleration of wind will be
a
F1
F2
F3
m
4
...
Fn

5. What are these forces in the atmosphere?
 Pressure gradient force
 Coriolis force
 Centripetal force
 Friction
5

6. A force has two properties
 Magnitude
Direction
6

7. Pressure
gradient
force (PGF)
7

8. Pressure gradient force (PGF)
a change in pressure over a given distance, i. e.
pressure gradient =
pressure difference
distance
Plow
distance
1020m b
8
=
Phigh
1016m b
1 0 0 km
4mb
1 0 0 km

9. Pressure gradient force (PGF)
estimate the magnitude of PGF from a map
pressure gradient =
pressure difference
distance
if the isobars are far
apart, the PGF is small
if the isobars are close
together, the PGF is large
9
=
Phigh
Plow
distance

10. Pressure gradient force (PGF)
magnitude and direction
pressure gradient =
Phigh
Plow
distance
PGF magnitude is calculated by formula (or assessed by
spacing of isobars
PGF = 8mb/100km
PGF direction is always directed from high
pressure to low pressure and is always
perpendicular to the isobars
1008 mb
1012 mb
H
10
1004 mb
L
100 km

11. Pressure gradient force (PGF)
magnitude and direction
1008 mb
mb
Q: Is the magnitude of PGF larger
surrounding the High or the Low?
Q: What is the direction of PGF
surrounding an area of low pressure?
L
1008 mb
mb
Q: What is the direction of PGF
surrounding an area of high pressure?
11
H

12. Q: What direction should the winds aloft at 5 km
be blowing based on PGF?
200mb
Q:
Actually, prevailing
wind direction is
from west to
east, what does
cause this
discrepancy?
500 mb
PGF
700 mb
5 km
200mb
500 mb
700 mb
12

13. Coriolis force
(CF)
13

14. Coriolis force (CF) due to earth’s rotation
Example
On nonrotating
platform A, the thrown
ball moves in a straight
line. On platform
B, which rotates
counterclockwise, the
ball continues to move
in a straight line.
However, to anyone on
the platform B, the ball
appears to deflect to
the right of its
intended path.
Nonrotating
14
Rotating

15. CF direction
Northern hemisphere Deflection to right
15

16. CF direction
as air moves from high pressure to low pressure in the
northern (southern) hemisphere, it is deflected to the
right (left) by the Coriolis force
N
60oN
Northern hemisphere
Deflection to right
30oN
Northern
hemisphere
0o
Southern
hemisphere
Southern hemisphere
Deflection to left
30oS
60oS
S
16

17.  CF magnitude
CF=maximum, sin(90o)=1
CF
2 V sin
N
sin(60o)=0.87
Deflection
to right
60oN
30oN
sin(30o)=0.5
CF =0, sin(0o)=0
0o
30oS
CF =0 as V=0
60oS
CF is large as a parcel moves faster
CF = Coriolis force;
V = parcel speed;
Deflection
to left
S
= the rate of the earth’s rotation
= latitude
17

18. The Coriolis force can
only change a parcel's
direction, it CAN NOT
affect its speed.
18

19. CF acts on objects not rigidly attached to the earth
Q: if a high speed train travels from LA to
NY, will the CF act on the train?
Answer is No
Q: Does the CF act on the ocean currents?
Answer is Yes
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20. Geostrophic
Wind
20

21. Q: Actually, prevailing wind direction is from west to
east, what does cause this discrepancy?
200mb
500 mb
PGF
700 mb
5 km
200mb
500 mb
700 mb
21

22. Parcel motion aloft
Q: Where will the parcel be at later time? Position #1 or #2?
N
425 mb
450 mb
W
E
1
475 mb
2
PGF
500 mb
Initially at rest
22
S
Z= 5 km

23. Parcel motion aloft
Q: Where will the parcel be at later time? Position #1 or #2?
N
425 mb
1
W
2
Wind
450 mb
E
PGF
475 mb
CF
PGF
500 mb
Initially at rest
23
S
Z= 5 km

24. Parcel motion aloft
N
Q: What cause CF increase?
425 mb
Wind
PGF
450 mb
W
Wind
PGF
CF
E
475 mb
CF
PGF
500 mb
Initially at rest
24
S
Z= 5 km

25. Parcel motion aloft
the parcel now is moving from west to east, not from south to
north.
N
425 mb
PGF
Wind
PGF
the PGF is equal and
opposite to the CF
CF
W
Wind
PGF
CF
Wind
450 mb
E
475 mb
CF
PGF
500 mb
Initially at rest
25
S
Z= 5 km

26. Geostrophic Wind
when the isobars are straight, parallel lines, and the only
two forces acting on a parcel are the PGF and the CF, then
the wind is called geostrophic
wind
L
475 mb
• PGF and CF are equal in magnitude and opposite in direction
• Geostrophic winds are always parallel to the isobars
PGF
CF
Wind
500 mb
H
500 mb
Surface
26