1. Chapter 7 “Circulation of the Atmosphere”
Enhanced Water Vapor Image of the South Pole
3/1803

9. Chapter 6 “Air Pressure and Winds”
Understanding Air Pressure
Measuring Air Pressure
Factors Affecting Air Pressure/Wind
Geostrophic Flow
Curved Flow and the Gradient Wind
Surface Winds

10. Chapter 7 “Circulation of the Atmosphere”
Scales of Atmospheric Motion
Local Winds
Global Circulation
Observed Distribution of Pressure and Winds
Monsoons
The Westerlies

11. Scales of Motion in the Atmosphere
Although we separate atmospheric motions according to size, the
large scale global motions are comprised of all the smaller scales
of motions.

12. Hurricanes are an example of Macroscale winds

13. Large winds patterns like the jet stream are also
Macroscale winds

15. Thunderstorms are good examples of Mesoscale winds
Strong vertical winds present with thunderstorms. There
movement is determined in part by Macroscale winds.

16. A Seabreeze is also considered Mesoscale Wind
cimss.ssec.wisc.edu/wxwise/

17. Closeup image of a seabreeze

18. Microscale winds are the smallest scale of air motion
May last for just a few seconds,
chaotic in nature.

19. Dustdevils are another form of microscale winds.

20. Local Winds (These are mesoscale winds)
Land and Sea Breezes
Mountain and Valley Breezes
Chinook (Foehn) Winds
Katabatic (Fall) Winds
Country Breezes

21. Land and Sea Breezes
Temperature contrasts (the result of the differential heating
properties of land and water) are responsible for the formation
of land and sea breezes.

22. Mountain and Valley Breezes
Similar to the land and sea breeze in its diurnal cycle are the valley
and mountain breezes. Valley breezes occur in the day because air
along mountain slopes is heated more intensely than air at the same
elevation over a valley floor. Rapid radiational heat loss in the
evening reverses the process to produce a mountain breeze.

23. Local winds can act together to create very strong winds
Table Mountain in Cape Town, South Africa is an example of strong daily wind patterns because of the combination of afternoon mountain and sea breeze.

24. Chinook (Foehn) Winds
These winds are often caused by pressure systems on the leeward side
of mountains which pull air over the mountains. As the air descends
the leeward slopes of the mountain it is heated adiabatically.
Warm, dry winds sometimes move down the slopes of the Rockies,
where they are called Chinooks, and the Alps they are called foehns.
These naturally occurring winds can be very harmful to human
activities.

25. Katabatic (Fall)Winds
Cold air over highland areas is set in motion, gravity causes the air to rush over the edge of the highland like a waterfall. Katabatic winds are generally much stronger than a mountain breeze. There must be a strong temperature gradient with the colder air aloft.
Diagram of Katabatic Winds
Some Katabatic Winds
mistral
bora
Antarctica is the
windiest place on earth. Wind speeds of 300 kilometres

26. Single-Cell Circulation Model
George Hadley, in 1735, proposed that temperature contrast
between the poles and the equator creates a large convection cell in
each hemisphere.
Global circulation on a
nonrotating Earth. A
simple convection system
is produced by unequal
heating of the atmosphere
on a nonrotating Earth.

27. Three-Cell Circulation Model
The zones between the equator and about 30 ° north and 30 ° south very much resemble the Hadley cell. Intense heating (high solar angle most of the year) results in upward motion. As the flow moves northward it begins to cool and subside.
Recall the Coriolis force increases with increasing latitude. Thus, the area between ~20-35 ° is characterized by subsidence.

28. Three-Cell Circulation Model
In the 1920’s a three-cell circulation model (for each hemisphere)
was proposed.
Features of the
circulation pattern:
horse latitude
trade winds
doldrums
prevailing westerlies
polar easterlies
polar front

29. Observed Distribution of Pressure and Winds
An imaginary uniform Earth with idealized zonal (continuous) pressure belts

30. Idealized pressure Belts
Equatorial Low- warm air rising creates cell of low pressure.
Intertropical Convergence Zone (ITCZ)- referred to as the
convergence zone because this region is where the trade winds
converge. Ascending air leads to cloud formation which makes this
region clearly visible on satellite imagery.
Subtropical Highs- These zones are caused primarily by Coriolis
deflection which restricts upper-level winds from moving poleward.
Subsiding air and divergent winds at the surface cause warm, cloud-
free weather (many large desert areas are located along this
latitudinal belt). Subtropical Highs tend to persist throughout the
year, with the center of the high migrating, and are regarded as
semi-permanent pressure systems.

31. ITCZ

32. Idealized pressure Belts (cont.)
Subpolar Low – located around 50 to 60  latitude. Associated
with the polar front. The belt of low pressure is formed by the
interaction (convergence) of the polar easterlies and the westerlies
Polar Highs – located over the poles! The process which produces
the polar highs is different than the process which produces the
subtropical highs. Surface cooling is the principle reason the
polar high.

33. Semipermanent Pressure Systems:
Land Sea interactions and Topography complicate
the circulation patterns.
We have viewed these pressure belts as continuous systems around
the earth up to this point. However, because the Earth is not
uniform at most latitudes (more so in the northern hemisphere),
the zonal belts are replaced by semipermanent cells of high and
low pressure.

34. Semipermanent Pressure Systems
The real Earth with disruptions of the zonal pattern caused by large landmasses. These disruptions break up pressure zones into semi-permanent high and low pressure cells.

35. January Average Surface Pressure Systems and Associated Circulation
Siberian High, Azores High
Aleutian Low, Icelandic Low

36. July Average Surface Pressure Systems and Associated Circulation
Bermuda High

37. Monsoons The seasonal reversal of wind direction associated with large
continents, especially Asia. In the winter, the wind blows from
the land to the sea; in the summer, it blows from the sea to the
land.
The Asian Monsoon
The Asian Monsoon is the result of a complex interaction between
the Siberian High (which is strongest in the wintertime), the
migration of the ITCZ, and the topography of the region (i.e. the
Himalayan mountain range and the Tibetan Plateau).

38. The Asian monsoon circulation occurs in conjunction with the
seasonal shift of the ITCZ and development of the Siberian High.
(dry, cool, continental air)

39. In the summer ITCZ migrates north and the Siberian High
weakens which allows results in a reversal in wind direction.
(moist, warm, maritime air)

40. Monsoons The North American Monsoon
Extreme summertime heating over the desert Southwest, creates
a low pressure system centered over Arizona that draws in
warm, moist air from the Gulf of California.