Energy Transfer Mechanisms

1
Global Circulation of the Atmosphere and the
Semi-permanent Pressure Cells(Global Winds)

• Lesson Goals
• Introduce a general model of winds and
  atmospheric pressure
• Relate these winds and pressure cells to climate
  conditions
• Note exceptions and problems when using the model

2
Wind

• What causes wind?

• Wind results from the horizontal motion of air
  from
• areas of high surface pressure to areas of low
  surface
• pressure.

High
Low
High
Surface
3
Air Pressure

• Force exerted by air molecules per unit area
• (Result of compression of the air by gravity).
• This pressure force is omnidirectional.

4
Building a Model of Global Wind Circulation (1st
Order Winds)
N

• Warm surface conditions or strongly rising air
  often produce low surface pressure
• Cold surface conditions or strongly descending
  air often produce high surface pressure

Where should we expect high pressure? Low
pressure?
5

• The Polar regions should consist of sinking cold
  air and high pressure.
• The equatorial region should exhibit rising warm
  air and low pressure.

Low
High
6

• Air masses diverge when they collide with an
  obstruction, including the earths surface and
  high level temperature inversions.

High
Low
7

• At roughly 300 north and south of the equator air
  that was warmed at the equator sinks back towards
  the surface as it cools and is forced into the
  decreasing circumference of the earth.

High
300 N
High
Low
High
300 S
8

• Again, when these air masses collide with the
  surface, they diverge.
• Notice the emerging pattern of alternate bands of
  high and low pressure.
• When surface air masses collide the effect is
  termed convergence.

High
Low
High
Low
High
9

• Also notice the cyclical pattern of air motion
  between the equator and 300 N and S. These broad
  cells are called Hadley Cells, after the man who
  discovered them.
• These convection cells are almost always present
  in the tropical regions.

Hadley Cell
Hadley Cell
10

• Convergence between the tropics creates a large
  region of generally low pressure called the
  intertropical convergence zone (ITCZ). This area
  is often wet and cloudy.

11
T

• Note that we now see warm midlatitude air
  colliding (converging) with cold polar air at
  roughly 600 N and S of the equator.
• These Polar Fronts are other areas of relatively
  low pressure, atmospheric instability, and, as
  well see, the source of most mid-latitude storms.

600 N
600 S
12

• Warm air, of course, rises above the colder air
  from the polar regions, but convection cells,
  like the equatorial Hadley Cells, are not as
  common.

13

• Now that we have finished this profile view of
  earth circulation lets transfer the surface
  winds onto the diagram.

600 N
300 N
300 S
600 S
14

• We have a problem. This diagram does not
  accurately depict Earths prevailing winds. Why?
• We have neglected to consider that the earth is
  in constant rotation. This has a dramatic effect
  on wind direction.

600 N
300 N
300 S
600 S
15
The Coriolis Force

• Apparent deflection of all free-moving objects
  from a straight path.
• Caused by the Earth rotation eastward, out from
  under the path of the object.
• Result?
• Deflection to RIGHT in Northern Hemisphere
• Deflection to LEFT in Southern Hemisphere
• How does this work?

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17

• Thus, in actuality, the Coriolis force deflects
  all winds to the right of their intended
  direction in the northern hemisphere and to the
  left in the southern hemisphere.

600 N
300 N
300 S
600 S
18

• Each band of resulting prevailing winds is named.
• Winds are labeled by their source direction
  (where they came from).

Polar Easterlies
600 N
Westerlies
300 N
NE Trade Winds
SE Trade Winds
300 S
Westerlies
600 S
Polar Easterlies
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Equatorial Low Pressure Trough Clouds and Rain

• The Intertropical Convergence Zone (ITCZ)
• Doldrums

L
ITCZ
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Subtropical High-Pressure Cells Hot Desert Air

• The Horse Latitudes
• Broad Cells of High Pressure

H
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Subtropical High-Pressure Cells Hot Desert Air
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Subpolar Low-Pressure Cells Cool and Moist

• Large semi-permanent low pressure band surrounds
  Antarctica
• Aleutian Low
• Icelandic Low

23
Polar High Pressure Cells Frigid Deserts

• Arctic and Antarctic Highs
• Very dry, despite general snow cover.
• Precipitation is rare, but rarely melts.

24
Map View of Wind Circulation
H
L
Equator
L
H
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Ocean Currents
Note Cold California Current
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Summary

• There are seven components to the global
  circulation model. From Pole to Equator they are
  polar highs, subpolar lows, westerlies,
  subtropical highs, trade winds, and the
  intertropical convergence zone.
• All of these patterns are displaced seasonally by
  earth-sun relationships.
• Local and regional winds are sometimes more
  prevalent than the broader scale global winds of
  the model.

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Exceptions to the model-Seasonal Variation and
Localized Winds

• Seasonal Latitude Shift
• Monsoons
• Mountain-Valley Winds
• Land-Sea Breezes
• Santa Ana Winds

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Seasonal Shift of Winds and Pressure Cells
(January)
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Seasonal Shift of Winds and Pressure Cells (July)
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Monsoon

• Seasonal reversal of winds, not a storm
• Arabic origin mausim meaning season

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Mountain-ValleyBreezes
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Land and Sea Breezes
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L
H
5x
x
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Compressional Heating Winds (Santa Ana Winds,
Chinooks, Foehn Winds)