Global Pattern of Surface Pressure and Wind

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Global Pattern of Surface Pressure and Wind
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Global Pattern of Surface Pressure

• If we averaged the sea level pressures over the
  Earth throughout the year we would see broad
  regions of high and low pressure.
• If we ignore the locations of continents and
  oceans and major mountain ranges, the broad
  regions would fall in latitudinal belts around
  the Earth.

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North Pole
Polar High Pressure
Polar Front low pressure
Subtropical High Pressure
Intertropical Convergence Zone low pressure
Subtropical High Pressure
Polar Front low pressure
Polar High Pressure
South Pole
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Pressure and Wind Patterns

• These latitudinal bands of high and low pressure
  regions help to determine the average wind
  directions that we see at the surface of the
  Earth.

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North Pole
Polar High Pressure
Polar Front low pressure
Subtropical High Pressure
Intertropical Convergence Zone low pressure
Subtropical High Pressure
Polar Front low pressure
Polar High Pressure
South Pole
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Subtropical High Pressure System
N
Northeast Trade Winds
E
CE
PGF
Intertropical
Convergence Zone (ITCZ) low pressure
PGF
CE
Southeast trade Winds
Subtropical High Pressure System
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Intertropical Convergence Zone

• The InterTropical Convergence Zone (ITCZ) is a
  zone of low pressure near the equator where the
  trade winds from the northern and southern
  hemispheres meet.

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Trade Winds

• The trade winds are the relatively steady
  northeasterly or southeasterly winds between the
  subtropical high pressure system and the ITCZ
  that were used by sailing ships to travel across
  the oceans.

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Tropopause
Height
Low
HIgh
High
North
South
Subtropical High
Subtropical High
ITCZ
Northeast Trade Winds
Southeast Trade Winds
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Circulation In the Tropics

• Most of the air converging on the ITCZ rises,
  creating an area of relatively calm winds at the
  surface called the doldrums.
• Most of the air is sinking near the centers of
  the subtropical highs, which also creates an area
  of relatively calm winds at the surface called
  the horse latitudes.

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Tropopause
Height
Low
HIgh
High
North
South
Subtropical High
Subtropical High
ITCZ
Northeast Trade Winds
Southeast Trade Winds
Doldrums
Horse latitudes
Horse latitudes
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Hadley Cells

• The circulation cell formed by the sinking air in
  the subtropical high, the trade winds blowing
  toward the ITCZ, the air rising at the ITCZ and
  the diverging poleward flow just below the
  tropopause is called a Hadley cell.
• There are two Hadley cells, one in the northern
  hemisphere and one in the southern hemisphere.

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Tropopause
Height
Hadley Cell
Hadley Cell
Low
HIgh
High
North
South
Subtropical High
Subtropical High
ITCZ
Northeast Trade Winds
Southeast Trade Winds
Doldrums
Horse latitudes
Horse latitudes
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Precipitation Patterns in the Hadley Cells

• The rising motion in the region of the ITCZ
  produces clouds and precipitation.
• The heavy rains near the ITCZ are sufficient to
  support the growth of tropical rainforests.
• The sinking of hot, dry air in the subtropical
  highs leads to the formation of deserts.

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Tropopause
Height
Hadley Cell
Hadley Cell
Low
HIgh
High
North
South
Subtropical High
Subtropical High
ITCZ
Northeast Trade Winds
Southeast Trade Winds
Doldrums
Horse latitudes
Horse latitudes
Tropical Rainforests
Deserts
Deserts
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ITCZ
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ITCZ
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The Hadley Cells and the Seasons

• The subtropical highs and the ITCZ shift toward
  the hemisphere that is having its warm seasons.
• Thus, in January the pattern shifts toward the
  southern hemisphere and the ITCZ is south of the
  equator.
• In July the pattern is shifted toward the north
  and the ITCZ is found north of the equator.

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ITCZ
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The Pattern Outside the Tropics

• The average wind directions outside the tropics
  are determined by the polar high pressure system,
  the polar front, and the subtropical high
  pressure system.

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North Pole
Polar High Pressure
Polar Front low pressure
Subtropical High Pressure
Intertropical Convergence Zone low pressure
Subtropical High Pressure
Polar Front low pressure
Polar High Pressure
South Pole
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Polar Pressure System
N
E
Polar easterlies
CE
PGF
Polar
Front low pressure
PGF
CE
Surface westerlies (southwest winds)
Subtropical High Pressure System
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The Polar Front

• The polar front moves north and south as it is
  pushed by the polar and subtropical high pressure
  systems.
• Many of the low pressure systems that we find on
  the surface weather map form along the polar
  front.

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North Pole
Polar High Pressure
Polar easterlies
Polar Front low pressure
Surface westerlies
Subtropical High Pressure
Northeast trade winds
Intertropical Convergence Zone low pressure
Southeast trade winds
Subtropical High Pressure
Surface westerlies
Polar Front low pressure
Polar easterlies
Polar High Pressure
South Pole
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Effects of Continents and Oceans

• Continents, oceans, major mountain ranges, and
  the formation of transient weather systems all
  disrupt this nice neat and tidy pattern of winds
  to produce the actual winds we see at the Earths
  surface.

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Air Masses

• An air mass is a large body of air that has
  similar thermal and moisture characteristics in
  the horizontal directions.

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Air Masses

• Air masses form over source regions, which are
  large, relatively flat areas over the Earths
  surface with relatively uniform characteristics.
• Air masses typically have a surface high pressure
  system at their center.

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Source Regions
Air sinks and spreads out from the same
geographic region.
H
The air takes on the characteristics of the
surface.
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Source Regions (Cont.)
N
E
H
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Typical Air Mass Types

• Arctic very cold and very dry
• Continental polar cold and dry
• Maritime polar cool and moist
• Maritime tropical warm and moist
• Continental tropical hot and dry

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Winter Pressure Pattern

• The average sea level pressures are generally
  higher over the continents (Asia and North
  America) and lower over the oceans (the Pacific
  and Atlantic) during the winter.
• This pattern is less evident in the southern
  hemisphere because there is so much less land in
  the southern hemisphere.

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Summer Pattern

• The average sea level pressures are generally
  higher over the ocean (Pacific and Atlantic) and
  lower over the continents (Asia and North
  America) during the summer.
• Again, this pattern is also less evident in the
  southern hemisphere.

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The Monsoon

• Since Asia is the largest landmass on the surface
  of the Earth, it has the largest impact on the
  pattern of pressure and winds at the surface.
• The most well known manifestation of the impact
  of Asia is known as the monsoon.
• The word monsoon means a large-scale shift of
  wind directions with the seasons.

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January Pattern

• The surface pattern over Asia in January is
  dominated by a large high pressure system
  sometimes called the Siberian high.

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H
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The Siberian High

• This large high pressure system produces
  diverging clockwise flow that spreads cold, dry
  air over large portions of Asia.
• Thus, winter is the dry season over Asia

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July Pattern

• The surface pattern over Asia in July is
  dominated by low pressure over southern and
  eastern Asia.

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L
L
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July Pattern (Cont.)

• Warm moist air flows into Asia from the Indian
  and Pacific oceans.
• The warm moist air rises in low pressure systems
  and as it flows up mountain ranges like the
  Himalayas, producing clouds and precipitation.

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Rainy Season

• Thus, summer is the rainy season over much of
  Asia.

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North American Monsoon

• Although North America is not as large as Asia,
  it is large enough to produce a monsoon over the
  southwest.

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January

• The average sea level pressure over the
  southwestern portion of the U.S. is generally
  dominated by high pressure in January.

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L
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L
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July

• Low pressure develops at the surface in July.
• If the low pressure is strong enough, it is able
  to pull in moist air from the Gulf of California
  into Arizona and summertime thunderstorms can
  develop.