1. Chapter 10
Wind: Global Systems

2. General Circulation of the Atmosphere
General refers to the average air flow, actual winds will vary considerably.
Average conditions help identify driving forces.
The basic cause of the general circulation is unequal heating of the Earth’s surface
Warm air is transferred from the Tropics to the Poles
Cool air is transferred from the Poles to the Tropics

3. General Circulation of the Atmosphere
Single Cell Model
Assume
uniform water surface
Sun always directly overhead the Equator
Earth does not rotate
Result: huge thermally direct convection cell rising at equator and sinking at poles(Hadley)
Three Cell Model
Allow earth to spin = three cells (Hadley, Ferrell, Polar)
Alternating belts of pressure starting with L at Equator (L H L H) (Eq, Sub Trop, Sub Polar, Polar)
Alternating belts of wind with NE just North of Equator

4. FIGURE 10.1 Diagram (a) shows the general circulation of air on a nonrotating earth uniformly covered with water and with the sun directly above the equator. (Vertical air motions are highly exaggerated in the vertical.) Diagram (b) shows the names that apply to the different regions of the world and their approximate latitudes.

5. ACTIVE FIGURE 10.2 The idealized wind and surface-pressure distribution over a uniformly water-covered rotating earth. Visit the Meteorology Resource Center to view this and other active figures at academic.cengage.
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6. General Circulation of the Atmosphere
Average Surface Wind and Pressure: The Real World
Semi-permanent high and lows
Northern vs. Southern Hemisphere
Major features shift seasonally with the high sun
North in July
South in December

7. FIGURE 10.3 Average sea-level pressure distribution and surface wind-flow patterns for January (a) and for July (b). The solid red line represents the position of the ITCZ.

8. FIGURE 10.3 Average sea-level pressure distribution and surface wind-flow patterns for January (a) and for July (b). The solid red line represents the position of the ITCZ.

9. FIGURE 10.4 A winter weather map depicting the main features of the general circulation over North America. Notice that the Canadian high, polar front, and subpolar lows have all moved southward into the United States, and that the prevailing westerlies exist south of the polar front. The arrows on the map illustrate wind direction.

10. General Circulation of the Atmosphere
General Circulation and Precipitation Patterns
Rain where air rises (low pressure)
Less rain where air sinks (high pressure)
Average Wind Flow and Pressure Patterns Aloft
North-South temperature and pressure gradient at high altitudes creates West-East winds, particularly at mid to high latitudes.

11. FIGURE 10.5 Rising and sinking air associated with the major pressure systems of the earth’s general circulation. Where the air rises, precipitation tends to be abundant (blue shade); where the air sinks, drier regions prevail (tan shade). Note that the sinking air of the subtropical highs produces the major desert regions of the world.

12. FIGURE 10. 6 During the summer, the Pacific high moves northward
FIGURE 10.6 During the summer, the Pacific high moves northward. Sinking air along its eastern margin (over California) produces a strong subsidence inversion, which causes relatively dry weather to prevail. Along the western margin of the Bermuda high, southerly winds bring in humid air, which rises, condenses, and produces abundant rainfall.

13. FIGURE 10.7 Average annual precipitation for Los Angeles, California, and Atlanta, Georgia. Wet winters and dry summers are characteristic of everywhere in the western US (west of Rockies).

14. FIGURE 10.8Average 500-mb chart for the month of January (a). Solid
lines are contour lines in meters above sea level. Dashed red lines are isotherms in oC. Arrowheads illustrate wind
direction. Lines are closer together during winter months for each hemisphere supporting the idea that large temperature differences drive stronger winds.

15. FIGURE 10.8Average 500-mb chart for the month of July (b). Solid
lines are contour lines in meters above sea level. Dashed red lines are isotherms in oC. Arrowheads illustrate wind
direction. Lines are closer together during winter months for each hemisphere supporting the idea that large temperature differences drive stronger winds.

16. Jet Streams
kt winds at 10-15km, thousands of km long, several 100 km wide and a few km thick (polar and subtropical)
Observations: Dishpan Experiment
Illustrates waves, with trough and ridge, develops in a rotating pan with heat on the exterior and cold at the center.

17. FIGURE 10.9 Average position of the polar jet stream and the subtropical jet stream, with respect to a model of the general circulation in winter. Both jet streams are flowing from west to east.

18. ACTIVE FIGURE A jet stream is a swiftly fl owing current of air that moves in a wavy west-to-east direction. The figure shows the position of the polar jet stream and subtropical jet stream in winter. Although jet streams are shown as one continuous river of air, in reality they are discontinuous, with their position varying from one day to the next. Visit the Meteorology Resource Center to view this and other active figures at academic.cengage.com/login

19. FIGURE (a) Position of the polar jet stream (blue arrows) and the subtropical jet stream (orange arrows) at the 300-mb level (about 9 km or 30,000 ft above sea level) on March 9, Solid lines are lines of equal wind speed (isotachs) in knots. (b) Satellite image showing clouds and positions of the jet streams for the same day.

20. Jet Streams Polar and Subtropical Jet Topic: Momentum
Established by steep temperature and pressure gradients between circulation cells.
Between tropical-mid-latitude cell (subtropical) and mid-latitude-polar cell (polar)
Gradients greatest at polar jet
Topic: Momentum
Low-latitudes: atmosphere gains momentum
High-latitudes: atmosphere losses momentum
Conservation of Momentum

21. Jet Streams Other Jet Streams Tropical easterly jet stream
Low-level jet (nocturnal)
Polar night jet streams