1. isobars and wind barbs
sea level pressure

2. factors affecting wind
wind is the result of horizontal differences in pressure
air flows from higher to lower pressure
wind tends to balance inequalities in air pressure
since unequal heating of Earth’s surface generates
pressure differences, solar radiation ultimately
drives winds

3. factors affecting wind (cont.)
if Earth did not rotate, and if there was no friction, air would flow directly from high to low pressure

4. pressure gradient force

5. pressure gradient force
to change velocity (accelerate) requires a force
when air is subjected to greater pressure on one side than on another this imbalance produces a force directed from high to low pressure
pressure differences cause the wind to blow
the greater these differences, the greater the wind speed

6. isobars - lines of equal pressure
change of pressure over a given distance is pressure gradient
closely spaced isobars --> high pressure gradient, strong winds
widely spaced isobars --> low pressure gradient, weak winds

7. Hurricane Andrew

8. “Storm of the Century”

9. coriolis force

10. coriolis force

11. coriolis force

12. Figure 6.6

13. coriolis force

14. planetary boundary layer

15. tubulence

16. factors affecting wind (cont.)
on a rotating Earth with friction, wind is determined by a combination of forces
pressure gradient force
coriolis force
friction

17. Figure 6.8

18. Figure 6.12

19. upper air flow is geostrophic
balance between pressure gradient force and coriolis force
==> winds blow parallel to the isobars

20. cyclonic and anticyclonic flow

21. balance between pressure gradient and coriolis force
geostrophic wind
balance between pressure gradient and coriolis force

22. Figure 6.10

23. Figure 6.11

24. force balance at upper and lower levels

25. surface high and low pressure systems

26. Newton’s second law for horizontal and vertical motions
Table 6.1
Table 6.1
Newton’s second law for horizontal and vertical motions
(PGF denotes the pressure gradient force)
Type of Motion
Newton’s Second law
Horizontal motions above the boundary layer
Acceleration = Horizontal PGF + Coriolis Force
Horizontal motions within the boundary layer
Acceleration = Horizontal PGF + Coriolis Force + Friction
Vertical motions
Acceleration = Vertical PGF + Gravity

27. Direction in which force acts
Table 6.2 part 1
Table 6.2: Summary of the properties of forces acting on air in the Earth’s atmosphere
Force
Direction in which force acts
Strength depends on
Effect on air
Balances
Vertical Pressure Gradient Force
Upward, from higher to lower pressure
Magnitude of the vertical pressure gradient
Accelerates air vertically toward lower pressure
Hydrostatic balance when equal and opposite to gravitational force
Horizontal Pressure gradient force
Horizontally, from higher to lower pressure
Magnitude of the horizontal pressure gradient
Accelerates air horizontally
Geostrophic balance when equal and opposite to Coriolis force
Coriolis Force
To the right (left) of the wind direction in the Northern (Southern) Hemisphere
Wind speed and latitude
Affects wind direction, but no effect on wind speed
Geostrophic balance when equal and opposite to horizontal pressure gradient force

28. Direction in which force acts
Table 6.2 part 2
Table 6.2 continued
Summary of the properties of forces acting on air in the Earth’s atmosphere
Force
Direction in which force acts
Strength depends on
Effect on air
Balances
Frictional force
Opposite the direction of the flow
The roughness of the underlying surface
Reduces air velocity, important primarily in boundary layer
_________
Gravitational force
Toward the center of the earth
Essentially constant in the troposphere
Accelerates air downward
Hydrostatic balance when equal and opposite to vertical pressure gradient force.

29. how winds generate vertical air motion
vertical motion associated with a cyclone
winds spiral into a surface low (horizontal convergence)
compensation occurs aloft (horizontal divergence)
net upward movement

30. vertical air motion is:
small relative to horizontal
very important weather maker
rising air associated with clouds and rain
sinking air (subsidence) -> adiabatic heating, clear skies

31. convergence and divergence