1. 1

2. 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
2

3. Strength of Geostrophic Wind
geostrophic conditions are that PGF and CF are
equal in magnitude and opposite in direction
P G F  C F  2  V g sin 
Vg 
PGF
L
475 mb
PGF
CF
2  sin 
Wind
500 mb
H
strength of geostrophic wind is determined by PGF not by CF, why?
3

4. Vg 
PGF
2  sin 
The Coriolis force can only
change a parcel's direction,
it CAN NOT affect its speed.
4

5. Centripetal force
and
gradient wind
5

6. Gradient Wind
counter lines are
relatively straight
Counter lines are curvature
Geostrophic Wind
6

7. Gradient Wind
Centripetal force
if you attach a string to a ball and swing it in a circular manner, then
the force required to keep the ball moving in a circular path is called
centripetal force
magnitude of the centripetal
force is
centripital force 
V
2
r
V=speed of a parcel (object)
r= distance from the center
direction of the centripetal force is
directed inward, towards the axis of
7
rotation
V

8. recall geostrophic wind
L
475 mb
PGF
Wind
CF
500 mb
H
8

9. Gradient Wind Centripetal force
Gradient wind is produced by the balance between PGF, CF and
centripetal force
PGF
PGF
PGF
PGF
9

10. Free troposphere
and
Boundary layer
10

11. above 850 mb level, the flow is either geostrophic wind
or gradient wind(free troposphere)
near the surface about 1-2 km (boundary layer), we
must include the effect of friction and therefore, the flow
is no longer geostrophic or gradient balance
11

12. Q: As a result, which
force becomes smaller,
the PGF or the CF?
the atmosphere above
the boundary layer is called
free troposphere
friction has a significant
impact on wind in boundary
layer (1-2 km)
12

13. Four forces in the atmosphere
1. Pressure gradient force
2. Coriolis force

Phigh  Plow
distance
 2  V sin( )
3. Centripetal force

V
2
r
4. Friction
  kV
which force is not caused by wind?
which two forces do not affect wind speed?
Coriolis force and centripetal force
13
PGF

14. at the center of a surface low, the air converges, and then must rise
H
L
at the center of a surface high, the air diverges, and must come
from aloft due to sinking motion 14

15. Why temperature is the
most important to
understand weather?

16. What is temperature
definition?
Why earth’s radiation is infrared
while solar radiation is short-wave?
Wien’s Law
 m ax 
3000
T (K )
( m)

17. The earth’s radiation energy is
determined by what?
Stefan-Boltzmann Law
E  T
4

18. The capacity of air for
holding water vapor is
determined by what?

19. Saturation vapor pressure (mb)
Temperature determines
the capacity of holding water vapor in the air
120
110
100
90
80
70
60
50
40
30
20
10
0
*
H 2O
P
This curve indicates how
much water vapor can be
held in the air at a given
temperature
17.27 T
PH 2 O (T )  0.6108 e T  273.3
*
T=44 oC
T=10 oC
T=30 oC
0
10
20
30
40
Temperature (oC)
50

20. Thermal circulation is
generated by pressure gradient
Pressure gradient is produced
by different heating

21. Rule
Cool surface  air sinking  high surface pressure
Warm surface air rising  low surface pressure
Wind is blowing from high pressure to low pressure
Where is
clear sky?
Cool
High P
Warm
Low P

23. Scales of Atmospheric Motions
Time and space scale of atmospheric motions
Typical size
Global scale 5000 km
Synoptic scale 2000 km
Mesoscale 20 km
Microscale 2m
Typical life span
23

24. Sea breeze

25. isobars
7 AM
820 mb
860 mb
900 mb
65oF
65oF

26. Sea breeze
isobars
820 mb
860 mb
Cold
Warm
780 mb
900 mb
90oF
70oF

27. isobars
820 mb
860 mb
900 mb
90oF
70oF
Cold
Warm
780 mb