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Global Pattern of Surface Pressure and Wind

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If we averaged the sea level pressures over the Earth throughout the year we ... equator where the trade winds from the northern and southern hemispheres meet. ... – PowerPoint PPT presentation

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Title: Global Pattern of Surface Pressure and Wind


1
Global Pattern of Surface Pressure and Wind
2
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.

3
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
4
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.

5
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
6
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
7
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.

8
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.

9
Tropopause
Height
Low
HIgh
High
North
South
Subtropical High
Subtropical High
ITCZ
Northeast Trade Winds
Southeast Trade Winds
10
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.

11
Tropopause
Height
Low
HIgh
High
North
South
Subtropical High
Subtropical High
ITCZ
Northeast Trade Winds
Southeast Trade Winds
Doldrums
Horse latitudes
Horse latitudes
12
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.

13
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
14
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.

15
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
16
ITCZ
17
ITCZ
18
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.

19
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20
ITCZ
21
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22
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.

23
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
24
Polar Pressure System
N
E
Polar easterlies
CE
PGF
Polar
Front low pressure
PGF
CE
Surface westerlies (southwest winds)
Subtropical High Pressure System
25
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.

26
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
27
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.

28
Air Masses
  • An air mass is a large body of air that has
    similar thermal and moisture characteristics in
    the horizontal directions.

29
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.

30
Source Regions
Air sinks and spreads out from the same
geographic region.
H
The air takes on the characteristics of the
surface.
31
Source Regions (Cont.)
N
E
H
32
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

33
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34
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.

35
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36
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.

37
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.

38
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39
January Pattern
  • The surface pattern over Asia in January is
    dominated by a large high pressure system
    sometimes called the Siberian high.

40
H
41
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

42
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43
July Pattern
  • The surface pattern over Asia in July is
    dominated by low pressure over southern and
    eastern Asia.

44
L
L
45
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.

46
Rainy Season
  • Thus, summer is the rainy season over much of
    Asia.

47
North American Monsoon
  • Although North America is not as large as Asia,
    it is large enough to produce a monsoon over the
    southwest.

48
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49
January
  • The average sea level pressure over the
    southwestern portion of the U.S. is generally
    dominated by high pressure in January.

50
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51
L
52
L
53
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.
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