WIND – Global And Local
Lesson Goals:
• Introduce a general model
of winds and atmospheric
pressure
• Relate these winds and
pressure cells to climate
conditions
• Note local exceptions and
problems when using the
model
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
Surface
High
Air Pressure
Force exerted by air molecules per unit area
(Result of compression of the air by gravity).
This pressure force is omnidirectional.
Building a Model of Global Wind
Circulation (1st Order Winds)
• 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?
N
• 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
• Air masses
diverge when
they collide
with an
obstruction,
including the
earth’s surface
and high level
temperature
inversions.
High
Low
• 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
High
300 N
Low
High
300 S
• 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.
Low High
High
Low
High
• 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
• 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.
• 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
we’ll see, the
source of most
mid-latitude
storms.
T
600 N
600 S
• 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.
• Now that we
have finished
this profile
view of earth
circulation
let’s transfer
the surface
winds onto
the diagram.
600 N
300 N
300 S
600 S
• We have a problem.
This diagram does
not accurately
depict Earth’s
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
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
• Zero Coriolis at Equator; max. at poles.
• How does this work?
• 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
• Each band of
resulting
prevailing
winds is
named.
• Winds are
labeled by
their source
direction
(“where they
came from”).
Polar Easterlies
600 N
Westerlies
NE Trade
Winds
SE Trade
Winds
Westerlies
Polar Easterlies
300 N
300 S
600 S
Equatorial Low Pressure Trough:
Clouds and Rain
• The Intertropical Convergence Zone (ITCZ)
• Doldrums
L
ITCZ
Subtropical High-Pressure Cells:
Hot Desert Air
• The Horse Latitudes
• Broad Cells of High
Pressure
H
Subtropical High-Pressure Cells:
Hot Desert Air
Subpolar Low-Pressure Cells:
Cool and Moist
• Large semi-permanent low pressure band surrounds Antarctica
• Aleutian Low
• Icelandic Low
Polar High Pressure Cells:
Frigid Deserts
• Arctic and
Antarctic Highs
• Very dry, despite
general snow
cover.
• Precipitation is
rare, but rarely
melts.
Map View of Wind Circulation
L
H
L
H
Equator
Ocean Currents
Note: Cold California Current
Ocean Currents
Note: Dramatically Warm Gulf Stream
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.
Exceptions to the modelSeasonal Variation and
Localized Winds
•
•
•
•
•
Seasonal Latitude Shift
Monsoons
Mountain-Valley Winds
Land-Sea Breezes
Santa Ana Winds
Seasonal Shift of Winds and
Pressure Cells (January)
Seasonal Shift of Winds and
Pressure Cells (July)
Monsoon
• Seasonal reversal of winds, not a storm
• Arabic origin: ‘mausim’ meaning season
Bangladesh
Summer Monsoon, India
Mountain-Valley
Breezes
Land and Sea Breezes
L
H
x
5x
Compressional Heating Winds
(Santa Ana Winds, Chinooks, Foehn Winds)
Santa Ana Winds
San Diego, October 2003