1. Atmospheric
Circulation

2. Air Pressure Experiments
Lessons from Paper Cup experiment:
1. Air pressure is present everywhere
Air “tries to” move from an area of higher
pressure to an area of low pressure
Lesson from Pop Bottle experiment:
3. Warm air occupies more space than an
equal number of molecules of cold air

3. Wind Power Generation
in Southern Alberta

4. “Don’t try this at home”

5. The speed and direction of wind is determined by three forces:
Pressure Gradient Force
Inertial Coriolis Force
Friction Force

6. H L Pressure Gradient Force Definition: The difference in atmospheric
pressure per unit
distance
PGF acts at right
angles to isobars
of equal pressure H L
102.2
99.8
101.4
100.6
600 km
Pressure Gradient Force = 2.4 kPa / 600 km
= 0.4 kPa / 100 km

7. Where is the PGF forecast to be strongest today ?
Regina or
Lethbridge?
Solution:
Check the
spacing of the
isobars of
equal surface
pressure
Source:

8. The Inertial Coriolis Force
Objects moving in an “absolute” straight line
between two points on the Earth’s surface are
deflected:
To the RIGHT in the N hemisphere
To the LEFT in the S hemisphere
Why ?
The Earth rotates more quickly at the equator.

9. Visualizing the Coriolis Force
Source: NASA

10. The Friction Force H L Surface roughness decreases wind speed
Reduces impact of Inertial Coriolis Force
Winds cross isobars, spiralling out of
ANTICYCLONES (H), and into CYCLONES (L) H L

11. Can you infer wind direction and relative speed from this map ?
weather.unisys.com

12. Sea level
pressure:
Altitude
Correction
Source: Ahrens (1994)

13. Weather
symbols and
wind barbs

14. Classic Low Pressure System In Temperate Latitudes
0600h GMT
APRIL 5
2003
NORTH-
EAST
WINDS
SHARP
COLD FRONT
WARM,
MOIST SOUTHERLY FLOW

15. Cold Front

16. Arctic high pressure drives cold arctic air behind low

17. Warm Front Not as steep a division as in a cold front
                                                                                                                                                                                                                        
Not as steep a division as in a cold front
It takes longer to scour out surface air
(warm air rises)

18. The weather pattern right now
COOL
NW WIND
WARM FRONT
WARM,
SOUTH WIND
COLD FRONT
HURRICANE ISABEL

19. Main Low and High Pressure Zones
Atmospheric
Circulation
Main Low and High Pressure Zones
Equatorial Low Pressure Trough
Subtropical High Pressure Cells
Subpolar Low Pressure Cells
4. Weak Polar High Pressure Cells

20. Atmospheric Circulation Overview
POLAR CELL
FERREL CELL
HADLEY CELL

21. Equatorial low pressure trough (warm, wet)
High solar angle
Consistent daylength
Convergence
Heating
ITCZ shifts with season

22. Hadley Cells 1. Warm, moist air rises in equatorial low
Cools, condenses, and causes heavy rain
Outward flow to subtropical high at high
altitude
Air descends in subtropical high
Heats, compresses and becomes very dry
The subtropical high provides the gradient
for trade winds and westerlies
eg. Bermuda/Azores and Pacific/Hawaii highs

23. Strahler and Strahler (2002)

24. Ferrel Cells Between subtropical highs and subpolar lows
Poleward transport of excess heat through
eddies and migration of lows toward polar front
Strong low pressure develops in a belt around
Antarctica, near the Aleutians and near Iceland
Lows strongest in winter (shift and diminish
periodically, especially in the summer)
Why ? Water much warmer than land in winter
leading to lower pressure over oceans

25. H L Air tends to be unstable in low pressure (tendency to rise)
Air tends to be stable in high pressure (tendency to fall)
(more on stability in next class)

26. WINTER SUMMER
Generalized Overview of Seasonal Surface Pressure

27. Average Global Surface Pressure in January and July Can you explain
the monsoon season
of the Indian sub-
continent with this
chart ?

28. Polar High Pressure Cells
Tendency for higher pressure near poles
than at the polar front
Anticyclonic flow develops
Weak and variable polar easterlies result
(stronger in southern hemisphere)
In northern hemisphere winter, the polar
front usually lies over Canada and Russia,
(further south than in the summer)

29. Geostrophic Winds 500 mbar height map Lower heights where air is cold
Airflow parallel
to isobars in
upper troposphere
Why ?
Combination
of PGF and
Coriolis force
Source:

30. Effect of Air temperature on 500 mb heights
Source: Ahrens (1994)

31. Upper Atmospheric Circulation
Jet Streams
A band of wind in the upper troposphere
150 – 500 km wide
km thick
Speeds may exceed 300 km/h
Polar Jet Stream:
Between Polar and Ferrel cells
Subtropical Jet Stream:
Between Hadley and Ferrel Cells

32. Source: http://apollo.lsc.vsc.edu

33. Source: http://apollo.lsc.vsc.edu

34. Jet Stream Cross Section
“Rivers” of strong wind
where cold and warm meet m m
Tropopause
height
6 000 m
Discontinuity or
step in tropopause
height
See:

35. Subtropical Jet Stream
Polar Jet Stream
Meanders from 30-70° N or S
Moves more poleward in summer
Influences (and is influenced by) storm paths
Subtropical Jet Stream
Meanders from 20-50° N or S
May occur simultaneously with Polar Jet in NA

36. Rossby Waves The polar jet stream follows the Rossby Waves
Rossby Waves are undulations in the upper-air
westerlies extending from the middle to upper
troposphere
Form along the polar front
Mechanism of poleward heat transport

37. (Strahler and Strahler, 2002)

38. Smaller Scale Winds 1. The Land-Sea Breeze Land heats up
in morning sun,
causing air
molecules to rise
Air pressure
drops over land
as molecules
removed

39. High pressure
develops over
the ocean
In the evening,
the land cools
faster than the sea
and the process is
reversed

40. 2. Mountain Valley Breezes
Daytime
The sun heats the
hillslope, causing
air to move up the
slope - a convection
loop forms
Night
Night radiation cools
the slopes
Cooler, denser air
moves downslope
Source:

41. 3. Katabatic Winds Air cools on a plateau or sloping terrain, becomes
more dense and descends
Winds get faster and faster downslope
Relatively warm water at base can further increase
winds, which can be very strong as a result
Can occur on large scale (eg. Greenland, Antarctica)
Also referred to as gravity drainage winds

42. Oceanic Circualtion Water piles up around equator due to trade winds
Along western edge of oceans, water spills N and S
along shorelines of continents (also downwelling)
Upwelling occurs near east edge of oceans (west coasts)

43. Upwelling of cool waters

44. The Thermohaline Circulation
(1) Intensive cooling at the ocean surface in North Atlantic
(2) Northward transport of salty surface water from lower
latitudes (both increase the density).