1. Chapter 7: Atmospheric Circulations
Scales of atmospheric motions
Eddies - big and small
Local wind systems
Global winds
Global wind patterns and the oceans

2. Scales of Atmospheric Motions
scales of motion
microscale
mesoscale
synoptic scale
planetary scale
Fig 7.1
Lots of important weather events occur on microscales, like evaporation of liquid water molecules from the earth’s surface.

3. Figure 7.2: The scales of atmospheric motion with the phenomena’s average size and life span. (Because the actual size of certain features may vary, some of the features fall into more than one category.)
Fig. 7-2, p. 171

4. Eddies - Big and Small eddy rotor wind shear turbulence
Wind shear can sometimes be observed by watching the movement of clouds at different altitudes.

5. Kelvin Helmholtz waves Clear-air turbulence Billow clouds
Figure 1: The formation of clear air turbulence (CAT) along a boundary of increasing wind speed shear. The view of wind is from the side, with a top layer of air moving over a layer below.
Figure 1, p. 173

6. Local Wind Systems Thermal Circulations isobars and density
differences
thermal circulations

7. Sea and Land Breezes sea breeze land breeze sea breeze front
Florida sea breezes
Sea and land breezes also occur near the shores of large lakes, such as the Great Lakes.

8. Figure 7.6: Typically, during the summer over Florida, converging sea breezes in the afternoon produce uplift that enhances thunderstorm development and rainfall. However, when westerly surface winds dominate and a ridge of high pressure forms over the area, thunderstorm activity diminishes, and dry conditions prevail.
Fig. 7-6, p. 175

9. Seasonally Changing Winds - the Monsoon
Monsoon wind system
India and eastern Asian monsoon
other monsoons

10. Figure 7.10: Enhanced infrared satellite image with heavy arrow showing strong monsoonal circulation. Moist, southernly winds are causing showers and thunderstorms (yellow and red areas) to form over the southwestern section of the United States during July, 2001.
Fig. 7-10, p. 178

11. Mountain and Valley Breezes
mountain breeze
The nighttime mountain breeze is sometimes called gravity winds or drainage winds, because gravity causes the cold air to ‘drain’ downhill.

12. Katabatic Winds Strong drainage winds: steep slope
Katabatic winds are quite fierce in parts of Antarctica, with hurricane-force wind speeds.
Bora: a cold, gusty northeasterly wind along the Adriatic
coast in the former Yugoslavia

13. Chinook (Foehn) Winds Chinook winds—warm and dry compressional heating
chinook wall cloud
In Boulder, Colorado, along the eastern flank of the Rocky Mountains, chinook winds are so common that many houses have sliding wooden shutters to protect their windows from windblown debris.
It is called a Foehn along the leeward slopes of Alps.

14. Figure 7.14: A chinook wind can be enhanced when clouds form on the mountain’s windward side. Heat added and moisture lost on the upwind side produce warmer and drier air on the downwind side.
Fig. 7-14, p. 180

15. Santa Ana Winds Santa Ana wind compressional heating wildfires
Many Southern California residents regularly hose down their roofs to prevent fires during Santa Ana wind season.
Difference between this and Chinook wind
Santa Ana: from elevated desert plateau;
Chinook: from cold plateau

16. Desert Winds dust and sand storms dust devils – from surface
usually with a diameter of a few
meters and a height of <100 m

17. General Circulation of the Atmosphere
cause: unequal heating of the earth’s surface
effect: atmospheric heat transport
Ocean currents also transport heat from the equator to the poles and back.

18. Single-cell Model basic assumptions: no rotation Hadley cell
why is the single-cell model wrong?
One of the world’s premier atmospheric
science research facilities,the Hadley Centre for Climate
Research, is named after George Hadley.

19. Three-cell Model model for a rotating earth Hadley cell doldrums
subtropical highs
trade winds
intertropical
convergence zone
westerlies
polar front
polar easterlies
Many global circulation terms, including ‘trade winds’ and ‘doldrums’, were named by mariners who were well acquainted with wind patterns.
Upper troposphere easterly is
inconsistent with the observed
westerly

20. Figure 7.21: The idealized wind and surface-pressure distribution over a uniformly water-covered rotating earth.
Watch this Active Figure on ThomsonNow website at
Fig. 7-21, p. 185

21. Average Surface Winds and Pressure: The Real World
semipermanent highs and lows
Bermuda high & Pacific high
Icelandic low & Aleutian low
Siberian high
The Bermuda High frequently brings hot, muggy weather to the eastern US. in summer
The ITCZ shifts toward the north in July (from January)

22. Figure 7.22: Average sea-level pressure distribution and surface wind-flow patterns for January (a) and for July (b). The heavy dashed line represents the position of the ITCZ.
Fig. 7-22a, p. 188

23. Fig. 7-22b, p. 189

24. The General Circulation and Precipitation Patterns
ITCZ, midlatitude
storms, polar front
Most of the world’s thunderstorms are found along the ITCZ.
Low rainfall over the subtropical regions

25. Westerly Winds and the Jet Stream
jet streams
subtropical jet stream
polar front jet stream
Low-level jet stream over
the Central plains of the
U.S. (within 2 km above
surface), bringing moist
and warm air to form
nighttime thunderstorms

26. Global wind-driven ocean current
Figure 7.29: Average position and extent of the major surface ocean currents. Cold currents are shown in blue; warm currents are shown in red.
Fig. 7-29, p. 193

27. Winds and Upwelling
Upwelling is strongest when wind is parallel to the coastline

28. El Niño and the Southern Oscillation
El Niño events
Southern Oscillation
La Niña
teleconnections
ENSO is an example of a global-scale weather phenomenon.
Often Arizona has a wetter winter during El Nino and a drier winter during La Nina;
Usually northwestern U.S. has a drier winter during El Nino and a wetter winter during La Nina.

29. Figure 7.32: In diagram (a), under ordinary conditions higher pressure over the southeastern Pacific and lower pressure near Indonesia produce easterly trade winds along the equator. These winds promote upwelling and cooler ocean water in the eastern Pacific, while warmer water prevails in the western Pacific. The trades are part of a circulation (called the Walker circulation) that typically finds rising air and heavy rain over the western Pacific and sinking air and generally dry weather over the eastern Pacific. When the trades are exceptionally strong, water along the equator in the eastern Pacific becomes quite cool. This cool event is called La Niña. During El Niño conditions—diagram (b)—atmospheric pressure decreases over the eastern Pacific and rises over the western Pacific. This change in pressure causes the trades to weaken or reverse direction. This situation enhances the countercurrent that carries warm water from the west over a vast region of the eastern tropical Pacific. The thermocline, which separates the warm water of the upper ocean from the cold water below, changes as the ocean conditions change from non-El Niño to El Niño.
Fig. 7-32, p. 196

31. Other Atmosphere-Ocean Interactions
North Atlantic Oscillation
Arctic Oscillation: pressure difference between Arctic and regions to its south
Pacific Decadal Oscillation

32. Figure 7.36: Typical winter sea surface temperature departure from normal in °C during the Pacific Decadal Oscillation’s warm phase (a) and cool phase (b).
(Source: JISAO, University of Washington, obtained via Used with permission of N. Mantua.)
Fig. 7-36, p. 199