Download AIR PRESSURE AND WINDS

AIR PRESSURE AND
WINDS
4/30/2017
(c) Vicki Drake
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What is Air Pressure?
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Air Pressure is a
measure of the
weight of the air
above a point of
observation
It is measured as a
force/area
• The amount of
force of a substance
over a given area
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Measurements of Air Pressure
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Baseline for Air
Pressure is mean
sea level
In force/area,
mean sea level
equals 14.7 lbs/in2
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(c) Vicki Drake
Pressure changes
more quickly with
vertical distance
changes than
horizontal distance
changes
Pressure decreases
at a constant rate
with increased
elevation
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First Measures of Air Pressure
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Evangelista Torricelli, 1643, invented
the first instrument to measure air
pressure
Using a calibrated glass tube,
inserted open end down, into a
shallow dish of mercury (Hg),
Torricelli noticed that the mercury
would rise up into the tube
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TORRICELLI’S CONCLUSIONS
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Torricelli concluded correctly that
pressure of the air on the mercury
(Hg) forced it into the glass tube.
• Average height: 760 millimeters (mm)
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The height of the mercury was a
measure of atmospheric pressure
• Inches of mercury still used to day for
measuring air pressure.
• 29.92 inches of mercury is air pressure
at mean sea level.
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Other measures of Air Pressure
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Another measure of air pressure is the
“Bar” – used mostly in meteorology.
The Bar is based on the force of 1000
dynes/cm2
• a dyne is the force of acceleration of
1m/sec/sec
• One bar equals approximately 14.5 psi
(pounds per square inch)
• One bar equals 100,000 Newtons/m2

A Newton is the force required to accelerate
1 Kilo@ 1meter/sec2
• Force of a small red apple falling under gravity
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Using the Bar in Measuring Air
Pressure
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A Bar can be divided into 1000
smaller sections called ‘millibars’
Mean sea level pressure in millibars
is 1013.25 mb
• Equivalents: 760 mm of mercury;
29.92 inches of mercury; 14.7 lbs/in2
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Air Pressure Maps
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Connecting points of equal air
pressure produces ‘isobars’ (similar
to contour lines on a topographic
map)
Pressure maps are used to identify
different air pressure cells – High or
Low
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Constant Pressure Charts
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Constant pressure (isobaric) chart are
constructed to show height variations along an
equal pressure surface.
Any change in air temperature changes air
density and air pressure.
Using the USA as an example…
• Air closer to the equator is generally warm, while air
closer to the poles is generally cooler
• On an isobaric chart – higher elevations correspond to
higher pressures at any elevation
• Lower elevations on an isobaric chart correspond to
lower pressures at any elevation
• Elongated highs bend into ridges
• Elongated lows bend into troughs
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Constant Pressure Chart
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Constant Pressure Chart –
Ridges and Troughs
High Pressure
Ridge
Low Pressure
Trough
Ridges
Upper Atmosphere
air flow over high
pressure ridges and
under low pressure
troughs
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Ridges
Northern
Hemisphere
Troughs
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Constant Pressure (Upper Level)
Charts – What do they tell us?
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Show wind-flow patterns of importance to
weather forecasting
Tracks movement of weather systems
Predict behaviors of surface pressure area
A constant pressure chart helps pilot
determine they are flying at correct
altitude – using an altimeter
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What is Low or High Air
Pressure?
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Low Air Pressure develops when there are
fewer air molecules exerting a force.
• Pressure may be less than average sea
level air pressure
High Air Pressure develops when there are
more air molecules exerting a force.
• Pressure may be more than average sea
level air pressure
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TYPES OF AIR PRESSURE
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There are two ways that ‘high’ and
‘low’ air pressure can develop in the
atmosphere.
Thermal Air Pressure
• Due to unequal heating of land and
water; conduction and convection
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Dynamic Air Pressure
• Upper atmospheric winds, earth’s
rotation
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How does Thermal Low Air Pressure
Develop?
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Thermal Low Air Pressure develops over
warm to hot surfaces through the process
of conduction and convection.
Air over the warm to hot surface becomes
warmer, more buoyant and less dense
than surrounding air – it rises.
The convection process reduces the
number of air molecules close to the
surface – fewer air molecules exert a
weaker force = “Low Air Pressure”
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Low Air Pressure
warm to hot surface heats air above it –
conduction and convection – warm air is
less dense, more buoyant than surrounding
air and the warm air starts to rise
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How does Thermal High Pressure
Develop?
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Thermal High Pressure develops over cool
to cold surfaces
Cooler air is less buoyant and more dense
than surrounding air – cool air sinks
As more air sinks to the surface, it adds
more and more air molecules, which
creates a stronger force = High air
pressure
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High Air Pressure
Air over a cool to cold surface slowly
sinks toward the ground; cool air is
more dense, less buoyant than
surrounding air
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High and Low Air Pressure Air Flow
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As warm air is lifted away from the surface in a
Thermal Low Air Pressure, ‘fresh air’ is pulled into
the center of the Low to replace the lifted air
(surface air convergence).
• Warm rising air cools as it rises – cloud
formation possible
As cooler air sinks toward the surface in a
Thermal High Air Pressure, the sinking air is
pushed out from the center of the low at the
surface to make room for new falling air (surface
air divergence).
• Cool sinking air warms slightly as it sinks – no
cloud formation possible
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Air Flow in Surface Low and
High Air Pressures
Cool
sinking
air
Surface
low
pressure
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Surface
high
pressure
Warm
rising air
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DYNAMIC AIR PRESSURE
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Air pressure systems created by upper
level winds and the earth’s rotation are
called “Dynamic” Air pressure.
Dynamic Highs have a core of warm
descending air
• Air is still sinking, but under a dynamic high,
the air warms considerably as it descends
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Dynamic Lows have a core of cool rising
air
• Air is still rising, but under a dynamic low,
even cool air is pulled up
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Wind: Horizontal Air Movement Due to a
Difference in Surface Pressures
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Air Movement based on two of
Newton’s Laws of Motions
• (1) An object in motion or at rest will
tend to stay in motion or at rest until a
force is exerted on it (INERTIA)
• (2) The force on an object is equal to
the mass of the object times the
acceleration produced by the force
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F = ma
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What are the forces involved
with Air Movement?
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Pressure Gradient Force
• Controls both Wind Direction and Wind
Velocity
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Coriolis Force/Effect
• Controls Wind Direction, only
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Friction
• Controls Wind Velocity, only
• Acts to slow wind down close to surface
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Pressure Gradient Force
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Pressure Gradient is the rate of
pressure change that occurs over a
given distance
Pressure Gradient Force (PGF) is the
net force produced when differences
in horizontal air pressure exist
• PGF is always directed from High
Pressure to Low Pressure and moves at
right angles to the isobars
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Pressure Gradient Force
Green arrows represent the same
horizontal distance between two points
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Isobars close together
indicate a rapid change
in air pressure producing
a steep Pressure
Gradient Force
• Result: Strong, high speed
winds
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Isobars far apart
represent a slow change
in air pressure producing
a gentle Pressure
Gradient Force
• Result: Weak, low speed
winds
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Coriolis Force
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Coriolis Force is an apparent force
due to the rotation of Earth on its
axis.
This force appears to deflect any
free-moving object (plane, ships,
rockets, bullets, air, currents) from
its original straight-line path.
The deflection is to the right in the
Northern Hemisphere and to the left
in the Southern Hemisphere
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Coriolis Force
Blue arrows indicate the direction of
deflection:
To the Left of original path in southern
hemisphere
To the Right of the original path in northern
hemisphere
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Coriolis Force
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Coriolis Force varies with speed,
altitude and latitude of a moving
object.
Coriolis Force is almost “zero” at
equator and greatest near the poles
The higher the velocity of the moving
object, the stronger Coriolis affects
the object.
Coriolis affects only wind direction
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Friction
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The effect of friction is observed
closest to Earth’s surface –
“Boundary Layers”
Friction slows down wind speed
The friction layer varies in height
across the Earth, but for the most
part lies within about a kilometer of
the surface.
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Friction
Wind speeds slow
the closer to the
surface.
No friction in upper air
High altitude
winds do not
experience
friction and are
much faster than
surface winds
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Forces and Wind Direction
Pressure Gradient Force, Coriolis
Force, and Friction affect the
movement of air into and out of Air
Pressure systems.
Air always moves into the center of a
Low – cyclonic air flow.
Air always moves out of the center of
a High – anticyclonic air flow
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Forces and Air Flows (Northern
Hemisphere)
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Cyclonic Air Flow (Surface
Lows)
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Northern Hemisphere:
• Counterclockwise and into the center of
a Low
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Southern Hemisphere:
• Clockwise and into the center of a Low
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Cyclonic Air Flow: Northern
Hemisphere
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Anticyclonic Air Flow (Surface
Highs)
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Northern Hemisphere Anticyclonic Air
Flow
• Clockwise and out of the center of a
High
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Southern Hemisphere Anticyclonic
Air Flow
• Counterclockwise and out of the center
of a High
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Anticyclonic Air Flow – Northern
Hemisphere
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Geostrophic Winds
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A theoretical horizontal wind that blows in
a straight path at a constant speed,
parallel to the isobars.
• Jet Streams are a close approximation
to a Geostrophic Wind
Wind exists at approximately 1000 meters
above the ground – above the boundary
layer (friction layer)
It develops when the Pressure Gradient
Force and Coriolis Force are in a dynamic
balance.
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Geostrophic Wind
Northern Hemisphere
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Air Flow Across Isobars
Upper Air Flow
Surface Air Flow
NORTHERN HEMISPHERE AIR FLOWS
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Jet Streams
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A jet stream is a swift river of air
found in the upper troposphere
Two are usually found in each
hemisphere:
• Polar jet stream
• Subtropical jet stream
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Each jet stream is formed by
different processes
Polar Front
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Air sinks at the Poles, creating the Polar
highs
Air flows from the Poles down towards the
Equator
Coriolis force deflects the air to the right,
resulting in the polar easterlies
The boundary between the polar easterlies
and the westerly winds of the midlatitudes
is called the polar front
The polar front separates cold polar air
from more temperate air to the South
Polar Front
Polar Jet Stream
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It resembles a stream of water moving west to
east and has an altitude of about 10 kilometers.
Its air flow is intensified by the strong
temperature and pressure gradient that develops
when cold air from the poles meets warm air
from the tropics.
• Strong winds exist above regions where the
temperature gradient is large
• The polar jet stream forms because of this
temperature gradient
• The polar jet stream is found above the polar
front at approximately 600 N and 600 S
Subtropical Jet Stream
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The subtropical jet stream is
located approximately 13 kilometers
above the subtropical high
pressure zone.
The reason for its formation is similar
to the polar jet stream.
However, the subtropical jet stream
is weaker. Its slower wind speeds are
the result of a weaker latitudinal
temperature and pressure gradient.
Jet Streams
Polar Jet Stream Seasonal Shifts
Global Semi-Permanent Air
Pressure Systems
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There are a number of air pressure
systems that are considered ‘semipermanent’ due to their consistency
in location.
Most of these pressure systems are
found over the world’s oceans – both
in the northern and southern
hemisphere.
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Pacific Ocean Semi-Permanent
Air Pressure Systems
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Pacific High
• Located at approximately 300 N, off the coast of
California
• Seasonally shifting
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Shifts to the South (closer to Baja California) during the
winter (winter storms to southern California)
Shifts to the North during the summer (no precipitation in
southern California)
Aleutian Low
• Located at approximately 600 N, in the Gulf of Alaska
• Seasonally shifting
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to the south in winter (sending winter storms to southern
California)
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Atlantic Ocean Semi-Permanent
Air Pressure Systems
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Bermuda-Azores High
• Located approximately 300N
• Shifts seasonally: south in winter, north
in summer
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Icelandic Low
• Located approximately 600 N, near
Iceland
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Nor’easter
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Pacific and Atlantic Ocean Air
Pressure Systems
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Ocean Currents
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Ocean currents are generated by
winds blowing across the surface of
the waters
Ocean currents in both the Atlantic
and Pacific Oceans flow in a
clockwise gyre (a semi-circular flow),
responding to air flow out of the
Pacific High and the Bermuda-Azores
High
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Pacific Ocean Currents
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California Current
• A south-flowing cold current, flowing parallel to the west
coast of North America
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Equatorial Currents
• A series of westerly-flowing warm currents, flowing from
eastern to western tropical Pacific Ocean basin
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Kuroshio Current
• Northerly-flowing warm current, flowing along the east
coast of Asia
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North Pacific Drift
• An easterly-flowing, somewhat warm current, flowing
towards North America
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Atlantic Ocean Currents
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Gulf Stream
• A northerly-flowing warm current, flowing somewhat
parallel to east coast of North America
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Labrador Current
• A southerly-flowing cold current
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North Atlantic Drift
• An easterly-flowing somewhat warm current, flowing
from western to eastern Atlantic Ocean basin
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Canary Current
• A southerly-flowing cool current, flowing almost parallel
to west coast of Europe and Africa
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Equatorial Currents
• A series of westerly-flowing warm currents, flowing from
eastern to western tropical Atlantic Ocean basin
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Atlantic and Pacific Ocean Currents
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GLOBAL CIRCULATION
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Energy from the Sun heats the entire Earth, but this
heat is unevenly distributed across the Earth's
surface.
Equatorial and tropical regions receive far more solar
energy than the midlatitudes and the polar regions.
• The tropics receive more heat radiation than they emit,
while the polar regions emit more heat radiation than
they receive.
• If no heat was transferred from the tropics to the polar
regions, the tropics would get hotter and hotter while
the poles would get colder and colder.
• This latitudinal heat imbalance drives the circulation of
the atmosphere and oceans.
• Around 60% of the heat energy is redistributed around
the planet by the atmospheric circulation and around
40% is redistributed by the ocean currents.
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ATMOSPHERIC CIRCULATION
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One way to transfer heat from the
equator to the poles would be to
have a single circulation cell where
air moved from the tropics to the
poles and back. This single-cell
circulation model was first proposed
by Hadley in the 1700’s.
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HADLEY CELL CIRCULATION
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ATMOSPHERIC CIRCULATION
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Since the Earth rotates, its axis is tilted and there
is more land in the Northern Hemisphere than in
the Southern Hemisphere, the actual global air
circulation pattern is much more complicated.
Instead of a single-cell circulation, the
global model consists of three circulation cells
in each hemisphere.
These three cells are known as the tropical cell
(also called the Hadley cell), the midlatitude cell
and the polar cell.
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Three-cell Circulation: Ferrel
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GLOBAL WINDS AND AIR
PRESSURE SYSTEMS
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