1. Outline for Lecture 13 Factors Affecting Wind
Winds Aloft and Geostrophic Flow
Curved Flow and the Gradient Wind
Surface winds
How Winds Generate Vertical Air Motion
3/13/03

2. Formation of A Sea Breeze
No pressure gradient,
no wind.
(b) Unequal heating
creates pressure
differences aloft which
causes air to flow.
The transfer of air aloft
(from the land to the sea)
creates a surface high over
the sea which results in a
flow of air from sea to land
(a.k.a. a sea breeze).
Returning air
Sea Breeze

3. Pressure Gradient Force
steep pressure gradient
(fast winds)
shallow pressure gradient
(slow winds)
pressure p n n
distance

4. The Coriolis Force
Named after the French Scientist Gaspard Gustave Coriolis
free moving objects
are deflected to the right
of their path in the Northern
Hemisphere (to the left of
their path in the Southern
Hemisphere) because of the
Earth’s rotation.
It depends on an object’s
speed—higher speed means
stronger Coriolis Force.

5. The Coriolis Force
Coriolis deflection of winds blowing eastward at different latitudes
Coriolis force also
increases with increasing
Latitude…
strong over poles,
no effect over equator.
Strong
Middling
Weak
None

6. Friction Altitude Friction acts at the surface.
winds at the surface aren’t as strong as those at higher altitudes
Air is a little viscous, so the layer next to the surface is also affected, but not as much.
Altitude

7. Winds Aloft and Geostrophic Flow
Balance pressure gradient force Coriolis Force: Geostrophic Flow.
Geostrophic winds:
go in a straight line
go parallel to the isobars
have speeds proportional to the pressure gradient force.

8. Geostrophic Flow WHAT!?!? pressure difference starts wind
wind gets going a little, starts being deflected by Coriolis force
wind goes faster in response to pressure difference, gets deflected more by Coriolis Force
Eventually, the two balance

9. Newton’s Laws of Motion (condensed version)
An object in motion tends to stay in motion
(unless acted upon by an external force)
II. F = ma
(that’s pretty much it.)

10. Put down the pencils
Let’s break down the forces

11. Just a Pressure Gradient
Start with just a pressure gradient
Then wind blows straight from High P to Low P
And accelerates as it goes
View from top H L
Pressure
return

12. Just a Pressure Gradient
Wind blows straight from High to Low, eventually evening out the pressure and stopping the wind. H L
Pressure

13. Just a Pressure Gradient
The wind would blow at the same speed regardless of altitude. H L
Pressure
return

14. Now add Friction
Then wind still blows straight from High P to Low P, but it doesn’t get moving as fast as soon, especially near the ground
View from top H L
Pressure
back up

15. Friction and Pressure Gradient
The pressure difference also evens out eventually, though it might take a bit longer. H L
Pressure

16. Friction and Pressure Gradient
Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere. H L
Pressure

17. Friction and Pressure Gradient
Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere. H L
Pressure
back up

18. Pressure Gradient and Coriolis
Forget friction.
The wind starts out straight, but as soon as it starts building up speed, the Coriolis force turns it a bit to the right. H L
The wind can’t accelerate any more over here
because it’s going parallel to the isobars
This is the Geostrophic Flow

19. Pressure Gradient and Coriolis
The Coriolis force limits the wind speed by redirecting it AND it prevents wind from blowing straight from H to L H L

20. Pressure Gradient and Coriolis
Since the wind never really reaches the low, the pressure difference is maintained, and the low never fills! H L

21. Reality: Pressure Gradient, Coriolis Force, and Friction
Coriolis Force turns the wind some, friction slows the wind some, and the result is roughly a 30º angle between isobars and wind. H L
30º

22. Reality: Pressure Gradient, Coriolis Force, and Friction
The wind doesn’t blow straight from High to Low, but it does eventually get in there and even out the pressure difference, so H and L don’t last forever without a source of energy H L
30º

23. Reality: Pressure Gradient, Coriolis Force, and Friction
Friction slows the wind at the ground—its effects decrease as you go up in the atmosphere. H L
Pressure

24. Reality: Pressure Gradient, Coriolis Force, and Friction
Coriolis Force is turning the wind toward us in the right part of the picture. H L
Pressure

25. Reality: Pressure Gradient, Coriolis Force, and Friction
Since the Coriolis Force depends on wind speed, its effect decreases toward the ground where the wind speed is slower.
Coriolis H L
Friction
Pressure

26. Pencils are now allowed

27. Winds Aloft and Geostrophic Flow
Geostrophic winds are up high and go straight:
only Coriolis and Pressure Gradient Forces are important.
Friction is important down low:
below about 1500 meters.
How do the Coriolis and Pressure Gradient forces change?
Coriolis Force
Wind speed
Latitude
Pressure Gradient
Wind flows from
high to low pressure.
“Isobaric packing”
P.G.F.

28. Winds Aloft and Geostrophic Flow
Wind direction is directly linked to the prevailing
pressure pattern.
Dutch meteorologist Buys Ballott, 1857
Buys Ballott’s Law states: In the Northern Hemisphere if you
stand with your back to the wind, lower pressure will be found to
your left and higher pressure will be found to the right.
Best when there are no frictional forces
or topography involved .

29. Curved Flow and the Gradient Wind
Cyclonic Flow
Anticyclonic Flow
Actual flow around pressure systems are never this regular (because of small changes in the pressure field).

30. Surface winds—high pressure (anticyclone)
COLD
DRY H
WARM
MOIST

31. Surface winds—low pressure (cyclone)
COLD
DRY L
WARM
MOIST

32. Upper Level Weather Chart
500mb chart
Variations in height are analogous
to variations in pressure. High
height fields correspond to high
pressure fields.

33. Troughs and Ridges
RIDGE
RIDGE
TROUGH

34. Troughs and Ridges
An elongated region of low pressure (trough) or high pressure (ridge)
Tend to be quite common at higher altitudes
At the surface, a trough is usually a fairly weak feature

35. How Winds Generate Vertical Air Motion
Around a surface low pressure center, a net inward transport of air
causes a shrinking of the area occupied by the mass. This is known
as horizontal convergence.

36. Airflow Associated with Cyclones and Anticyclones
“Upper level support” is important in cyclone development

37. Wind speeds and isobars
STRONG WINDS
Slack winds
Slack winds
The tighter they’re packed, the stronger the wind

38. Factors that promote vertical airflow
Friction:
air flow from ocean to land (upward motion)
air flow from land to ocean (downward motion)
Mountain ranges

39. Wind Measurement
Wind roses provide a method of representing prevailing winds by
indicating the percentage of time the wind blows from various
directions

40. Go Terps! ACC Tournament this Weekend