Class #13 Monday, September 27, 2010 Class #13: Monday, September 27 Chapter 7 Global Winds 1.

Class #13 Monday, September 27, 2010

Class #13: Monday, September 27

Chapter 7

Global Winds

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What a description of global winds should explain

• Seasonal patterns of precipitation around the world

• Seasonal patterns of cloudiness around the world

• The relationships between average wind patterns and pressure patterns and upward and downward air motions


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Fig. 7-4a, p. 191


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Fig. 7-4b, p. 191


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Fig. 7-5a, p. 192


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Fig. 7-5b, p. 192


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The surface winds over Earth

• Are very complicated because of the changing seasons, differences between land and water, and differences in latitude.

• Can be simplified using a conceptual model.

• Have been described using a 3-cell model with no land and no seasons. Only temperature differences from equator to pole are included.


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The 3-cell conceptual model of the general circulation

• Has 3 wind belts in each hemisphere• NH and SH

– Polar easterlies– Prevailing or mid-latitude westerlies– Trade winds

• Trade winds blow equatorward– Northeasterly trade winds in NH– Southeasterly trade winds in SH


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Surface pressure in the 3-cell model

• High at both poles, called Polar Highs• High in the subtropics, about 30ºN and

30ºS, called Subtropical Highs• Low near the equator, called the Equatorial

Low, or the Intertropical Convergence Zone (ITCZ)

• Generally light winds at the Polar and Subtropical Highs, and in the ITCZ


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Average vertical motions in the 3-cell model

• Downward at the poles where surface pressure is high and the troposphere has low temperatures over ice

• Downward at the subtropical highs

• Upward in the ITCZ

• Upward at about 60°N and S near the polar front


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Thermal circulations in the 3-cell model

• The Hadley cells have their rising branch in the ITCZ and their sinking branch in the subtropics.

• The Hadley cells cover half of the surface area of Earth.

• The polar cells have a rising branch near the polar front and sinking at the pole.


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The 3-cell model’s circulation in middle latitudes

• Is thermally indirect, because the air nearer the pole is rising, and the air nearer the equator is sinking.

• Is an average based on smaller wind patterns in extratropical cyclones, in which the warmer air does rise, and the colder air sinks.

• Has the motions required by the polar and Hadley cells.


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More on the thermal circulation

• The thermal circulation begins aloft.• In diagrams of the thermal circulation, “H”

and “L” refer to the horizontal pressure gradient, not to the vertical pressure gradient.

• The thermal circulation comes about because hydrostatic balance requires that the warmer air column expands compared to the cooler air column.


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Consequences of Earth’s rotation from west to east

• The trade winds in the NH do not blow from the north, but are deflected to the right in the NH, so blow from the northeast.

• If Earth rotated much more slowly, there would be only the Hadley cell.

• If Earth rotated much more quickly, there would be more wind belts (like on Jupiter).


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More consequences of Earth’s rotation

• If it were not for the Midlatitude westerlies, Earth’s speed of rotation would slow. Easterlies alone would everywhere act to slow the rotation.

• The polar easterlies blow from the pole and curve, blowing from the northeast in the NH and from the southeast in the SH.

• The westerlies blow away from the equator and curve in both hemispheres, that is, they blow from the southwest in the NH, and from the northwest in the NH.


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Complications of the real Earth

• Earth has seasons– The ITCZ (sometimes called the thermal

equator) shifts latitude with the seasons.– The ITCZ shifts north of the equator in NH

summer, and south of the equator in SH summer (NH winter)

• Earth has large land masses– Continents and oceans set up thermal

circulations

Fig. 7-12a, p. 197

Fig. 7-12a (1), p. 197

Fig. 7-12a (2), p. 197

Fig. 7-12b, p. 198

Fig. 7-12b (1), p. 198

Fig. 7-12b (2), p. 198

Fig. 7-12b (2), p. 198


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Observed surface pressures

• Vary with the seasons, requiring both a January and a July depiction

• Are on average high in the sub-tropics (near 30°) and near the pole

• Are on average low in the ITCZ and along the polar front (near 60°)

• In summer are high over the oceans and low over the continents (thermal lows).

• In winter are high over the continents and low over the oceans.


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The thermal circulation

• The sea breeze is a thermal circulation.

• A thermal circulation has both horizontal and vertical air motions.

• The horizontal pressure gradient force is most important in a thermal circulation.

• Upward air motions occur in the warmer air column of the circulation; downward air motions occur in the cooler air column.


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The sea breeze

• Is a daytime circulation.

• Depends on differential heating at the surface between land and water.

• Has the warmer air column over the land, which absorbs more incoming solar radiation.

• Has the cooler air column over the water, which absorbs less radiation.


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The sea breeze and the land breeze

• As solar heating diminishes in the late afternoon, the sea breeze weakens.

• At night, differential cooling occurs.

• The cooler air column is over land, where radiational cooling is more rapid than over the water.

• The warmer air column is over the water.

• The land breeze develops at night.


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Scales of motion in the atmosphere

• Describe the size and lifetime of wind patterns in the atmosphere.

• Determine which forces are most important to forming the wind patterns.

• Are largest when the lifetimes are longest.

• Are smaller when the lifetime is shorter.

• Have a variety of names and definitions.


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More on scales of motion

• The horizontal pressure gradient force is important for all scales of motion.

• The Coriolis Force is important for the planetary scale, the synoptic scale, and for the larger mesoscale wind patterns.

• The vertical pressure gradient force is important for small mesoscale winds.


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Complications of the real Earth

• Earth has seasons– The ITCZ (sometimes called the thermal

equator) shifts latitude with the seasons.– The ITCZ shifts north of the equator in NH

summer, and south of the equator in SH summer (NH winter)

• Earth has large land masses– Continents and oceans set up thermal

circulations


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Observed surface pressures

• Vary with the seasons, requiring both a January and a July depiction

• Are on average high in the sub-tropics (near 30°) and near the pole

• Are on average low in the ITCZ and along the polar front (near 60°)

• In summer are high over the oceans and low over the continents (thermal lows).

• In winter are high over the continents and low over the oceans.


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Seasonal shifts

• The ITCZ, the subtropical highs, and the polar front all shift southward in NH winter and northward in NH summer.

• Seasonal shifts are most intense over Asia, which has the largest continental air mass.

• The summer monsoon is wet, with low pressure over land; the winter monsoon is dry, with high pressure over land.


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Other monsoons

• Africa, North America, and Australia have monsoon-like wind patterns, particularly in the warm season.


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Winds and pressures (heights) well above the surface

• Pressures and heights are on average high in the tropics and decrease to lows close to the poles.

• Upper-level (500mb and above) winds are generally easterlies (blowing east to west) in the tropics and westerlies (blowing west to east) in higher latitudes.


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Jet Streams

• Jet streams are regions of especially high wind speed in the atmosphere.

• In the upper-level westerlies, there can be two jet streams, the Polar front jet stream, above the polar front, and the Subtropical jet stream above the subtropical highs.

• Sometimes these jet streams merge into one.


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Class #13 Monday, September 27, 2010 Class #13: Monday, September 27 Chapter 7 Global Winds 1.

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