Class #14 Wednesday, September 30

Class #14: Wednesday, September 30

Chapters 6 and 7

Thermal Circulation, Scales of Motion, Global Winds

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Class #14 Wednesday, September 30

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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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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.

Class #14 Wednesday, September 30

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Class #14 Wednesday, September 30

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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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Class #14 Wednesday, September 30

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Class #14 Wednesday, September 30

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Class #14 Wednesday, September 30

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Class #14 Wednesday, September 30

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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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Class #14 Wednesday, September 30

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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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Class #14 Wednesday, September 30

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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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Class #14 Wednesday, September 30

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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.