t

ha
g
Pa y
si
ph
e, ra
eg og
ll Ge
Co f
u o
hr nt
Ne e
tm
al ar
rl ep
ha D
wa
Ja

LIEUTANANT DR K K MISHRA
Gairloch High School / Invergordon Academy
1
 ATMOSPHERIC CIRCULATION
The Three Cell Model
Global Winds
The ITCZ

t
ha
The purpose of this presentation is to introduce the topic of atmospheric circulation.

g
Pa y
si
ph
e, ra
Main concepts:

eg og
• atmospheric circulation is a response to differences in insolation between low and high

ll Ge
latitudes.

Co f
• u o
there is a transfer of energy from low to high latitudes
hr nt
• a global pattern of surface winds and pressure belts results from this circulation
Ne e
tm

• there is a seasonal shift of the surface wind and pressure belts

al ar
rl ep

• this shift has a pronounced effect on the climate of areas such as sub-saharan Africa
ha D
wa
Ja

Q 2
 THE THREE CELL MODEL
FORMATION OF THE HADLEY CELL (1)

Insolation in tropical
areas causes warm air
to rise and spread

t
polewards, carrying

ha
g
heat energy.

Pa y
si
ph
e, ra
eg og
ll Ge
Co f
SOLAR ENERGY
u o
hr nt
Ne e
tm
al ar
rl ep
ha D
wa
Ja

3
 THE THREE CELL MODEL
FORMATION OF THE HADLEY CELL (2)
Air cools and begins to fall at
about 30ºN and 30ºS of Equator.
Cooled air returns to the Equator.

t
ha
This circulation of air is

g
caused by solar heating.

Pa y
si
ph
e, ra
eg og
ll Ge
Co f
SOLAR ENERGY
u o
Heat energy is transferred
hr nt

from the Equator to sub-

Ne e

tropical latitudes.
tm
al ar
rl ep

It is called the HADLEY CELL.

ha D
wa
Ja

Q 4
 THE THREE CELL MODEL
FORMATION OF THE POLAR CELL (1)

t
ha
g
Pa y
si
ph
Intensely cold,

e, ra
dense air sinks at

eg og
the poles, then

ll Ge
blows as surface

Co f
winds towards the u o
hr nt
Equator.
Ne e
tm
al ar
rl ep
ha D
wa
Ja

5
 THE THREE CELL MODEL
FORMATION OF THE POLAR CELL (2)

At about 60ºN and 60 ºS,

the cold polar air is warmed
in contact with the earth’s

t
ha
surface.

g
Pa y
si
ph
e, ra
eg og
This warmed air rises and

ll Ge
returns polewards,

Co f
carrying heat energy. u o
hr nt
Ne e
tm
al ar

This circular motion is

rl ep

called the POLAR CELL.

ha D
wa
Ja

Q 6
 THE THREE CELL MODEL
FORMATION OF THE FERREL CELL (1)

The Hadley Cell is driven

by differences in heat
energy at the Equator.

t
ha
g
Pa y
si
ph
e, ra
eg og
ll Ge
Co f
As the air in the Hadley u o
hr nt
Cell falls at about 30ºN
Ne e

and 30ºS, it pulls the air

tm
al ar

beside it down as well,

rl ep

due to friction
ha D
wa
Ja

7
 THE THREE CELL MODEL
FORMATION OF THE FERREL CELL (2)

The Polar Cell is driven by

differences in heat energy.
Cold polar air falls and
spreads towards the Equator.

t
ha
g
Pa y
si
ph
e, ra
eg og
ll Ge
Co f
As the air in the Polar Cell u o
rises at about 60ºN and
hr nt
Ne e

60ºS, it pulls the air

tm

beside it up as well, due to

al ar

friction.
rl ep
ha D
wa
Ja

8
 THE THREE CELL MODEL
FORMATION OF THE FERREL CELL (3)
Unlike the Hadley and Polar
Cells, the Ferrel Cell is not
driven by differences in heat
energy.

t
ha
The Ferrel Cell is caused by

g
friction where air is in

Pa y
si
ph
contact with the other two

e, ra
cells.

eg og
ll Ge
Co f
u o
The Hadley Cell drags air
hr nt

down at about 30ºN and S.

Ne e
tm
al ar
rl ep

The Polar Cell causes an

ha D

uplift at about 60ºN and

S.
wa
Ja

9
 THE THREE CELLS TOGETHER
Polar Cell

Ferrel Cell

t
gha
Pa y
si
ph
Hadley Cell

e, ra
eg og
ll Ge
Co f
u o
hr nt

Hadley Cell
Ne e
tm
al ar
rl ep
ha D

Ferrel Cell
wa
Ja

Q 10
Polar Cell
 THE TRANSFER OF HEAT ENERGY FROM
EQUATORIAL TO POLAR AREAS
Where air carrying energy from the
Equator in the Hadley Cell comes into
contact with air in the Ferrel Cell,
there is a transfer of heat energy
into the Ferrel Cell.

t
ha
g
Pa y
si
ph
There is a similar transfer
of heat energy from the

e, ra
Ferrel Cell to the Polar Cell.

eg og
ll Ge
Co f
SOLAR ENERGY
u o
hr nt

In this way, heat energy is

Ne e
tm

transferred from the Equator,

where there is a surplus of
al ar

energy, to the poles where

rl ep

there is a deficit.
ha D
wa
Ja

11
 THE CORRESPONDING MOVEMENT OF COLDER AIR

In the Polar cell cold air from polar

regions flows to mid-latitudes as
polar easterly winds

t
ha
g
Pa y
si
ph
e, ra
In the Ferrel Cell there is a

eg og
movement of cold air at
high altitude.

ll Ge
Co f
u o
hr nt
Ne e
tm
al ar

In the Hadley Cell, cooler

rl ep

air moves from the sub-

tropics to the Equator.
ha D
wa
Ja

Q 12
 ASSOCIATED PRESSURE BELTS
Rising air at the equator causes the
equatorial belt of low pressure

Descending air at about 30ºN and Polar high pressure

30ºS causes the sub-tropical belt

t
ha
of high pressure Mid latitude low pressure

g
Pa y
si
ph
e, ra
Sub-tropical high pressure
Rising air at about 60ºN and

eg og
60ºS causes a mid-latitude

ll Ge
belt of low pressure

Co f
u o
Equatorial low pressure
hr nt
Ne e
tm

Descending air at the poles

al ar

Sub-tropical high pressure

causes the polar high
rl ep

pressure areas
ha D

Mid latitude low pressure

Polar high pressure

wa
Ja

Q 13
 ASSOCIATED SURFACE WIND PATTERNS

Winds always blow from high pressure to

low pressure.
They are deflected because of the Polar high pressure
Coriolis Force which come about because

t
ha
of the rotation of the earth. Mid latitude low pressure

g
Pa y
si
ph
e, ra
Sub-tropical high pressure

eg og
Winds in Northern Hemisphere

ll Ge
are deflected to the right.

Co f
u o
Equatorial low pressure
Winds in the southern hemisphere
hr nt
are deflected to the left.
Ne e
tm

Sub-tropical high pressure

al ar
rl ep

These wind belts shift

ha D

seasonally. (See next section) Mid latitude low pressure

Polar high pressure

wa
Ja

14
 Ja

Q
wa
ha D
rl ep
al ar
tm
Ne e
hr nt
u o
Co f
ll Ge
eg og
e, ra
ph
Pa y
si
gha
15 t
 POSITION OF THE THREE CELLS IN DECEMBER
Polar Cell
The sun is overhead at the
Tropic of Capricorn, 23ºS
of the Equator.
Ferrel
Cell

t
ha
g
Pa y
si
ph
Hadley

e, ra
Cell

eg og
ll Ge
SUN OVERHEAD 23½ºS

Co f
u o
hr nt
The cells shift Hadley
Ne e
tm

southwards as the
Cell
heat equator is in the
al ar

southern hemisphere.
rl ep
ha D

Ferrel
wa

Cell
Ja

Q 16
Polar Cell
 POSITION OF THE THREE CELLS IN JUNE

Polar Cell
The sun is overhead at the
Tropic of Cancer, 23ºN of
the Equator. Ferrel
Cell

t
ha
g
Pa y
si
ph
Hadley

e, ra
Cell

eg og
ll Ge
SUN OVERHEAD 23½ºN

Co f
u o
Hadley
hr nt
Ne e

Cell
tm

The cells shift

al ar

northwards as the heat

rl ep

equator is in the
ha D

northern hemisphere.

Ferrel
wa

Cell Polar Cell

Ja

Q 17
 THE INTER-TROPICAL CONVERGENCE ZONE

IN THE NORTHERN HEMISPHERE:

The winds that blow to the equatorial
low pressure belt are called the North
East Trade Winds

t
g ha
Pa y
si
ph
IN THE SOUTHERN HEMISPHERE:

e, ra
The winds that blow to the
Sub-tropical high pressure

eg og
equatorial low pressure belt are

ll Ge
called the South East Trade Winds

Co f
u o
Equatorial
Inter-Tropical low pressure
Convergence Zone
hr nt

The line along which they converge

Ne e
tm

(meet) is called the INTER- Sub-tropical high pressure

al ar

TROPICAL CONVERGENCE ZONE.

rl ep

This is often abbreviated to ITCZ

ha D
wa
Ja

Q 18
 THE INTER-TROPICAL CONVERGENCE ZONE
IN THE NORTHERN HEMISPHERE
OVER WEST AFRICA
The sub-tropical high pressure belt
develops over the Sahara so is hot and
dry.

t
ha
This is known as continental Tropical

g
Pa y
(cT) air.

si
ph
e, ra
Sub-tropical high pressure

eg og
ll Ge
Co f
IN THE SOUTHERN HEMISPHERE
u o
Equatorial low pressure
Inter-Tropical Convergence Zone
OVER WEST AFRICA
hr nt
Ne e

The sub-tropical high pressure belt

tm

Sub-tropical high pressure

develops over the Atlantic so is warm
al ar

and moist.
rl ep
ha D

This is known at maritime Tropical (mT)

air.
wa
Ja

Q 19
 THE ITCZ IN DECEMBER

In December, the zone of maximum

insolation (solar energy) is south of
the Equator. This means that the wind
belts shift southwards.

t
ha
g
Pa y
si
ph
e, ra
eg og
SOLAR ENERGY

ll Ge
This means that winds blow out of the
sub-tropical high pressure area over the

Co f
Sahara, and take dry air from the
u o
continental Tropical (cT) air mass across
hr nt
most of West Africa. This causes a dry
season.
Ne e
tm
al ar
rl ep

Moist air from the maritimeTropical

ha D

(mT) air mass from the Atlantic cannot

reach far inland, where there is a dry
season.
wa
Ja

Q 20
 THE ITCZ IN JUNE
By contrast, in June, the zone of
maximum insolation is well to the north
of the Equator. This means that the
wind belts shift northwards.

t
g ha
Pa y
si
ph
e, ra
eg og
ll Ge
SOLAR ENERGY

Co f
u o
Moist maritime Tropical air from the
hr nt
Atlantic now reaches far inland, where
Ne e

there is a rainy season. These winds flow

tm

northwards to the ITCZ to replace air

al ar

that has become unstable and risen.

rl ep
ha D

The winds blow out of the sub-tropical high

wa

pressure area over the Sahara, now only affect

Ja

the northern part of sub-Saharan Africa. 21

 Ja
wa
ha D
rl ep
al ar
tm
Ne e
hr nt
u o
Co f
ll Ge
eg og
e, ra
ph
Pa y
si
gha
22 t
 THE EFFECT OF THE SUN’S MIGRATION ON
SEASONAL TEMPERATURES AFRICA

TROPIC OF CANCER 23½ºN TROPIC OF CANCER 23½ºN

t
ha
g
Pa y
si
ph
e, ra
eg og
EQUATOR 0º EQUATOR 0º

ll Ge
Co f
TROPIC OF CAPRICORN 23½ºS u o TROPIC OF CAPRICORN 23½ºS
hr nt
Ne e
tm

AFRICA – TEMPERATURES IN JANUARY AFRICA – TEMPERATURES IN JULY

al ar

Because the sun is overhead in the In July, with the sun

rl ep

southern hemisphere, it is the south overhead north of the

that is hottest, (shown by the red Equator, the Sahara is
ha D

areas). The Sahara stands out as a clearly much hotter than

cooler, (lighter coloured), area. the rest of the continent.
wa
Ja

23
 POSITION OF THE ITCZ IN DECEMBER
In December the sun is overhead in the southern hemisphere.

The ITCZ is found to the south, where there is maximum

insolation.

The sea stays a fairly constant

temperature, so the ITCZ runs

t
ha
just along the coast in WAfrica. Sub-tropical high pressure

g
Pa y
si
ph
e, ra
NE Trade Winds – locally called the
Only the coastal fringe

eg og
Harmattan.They are stable and dry
receives rain from the unstable as they originate from over the

ll Ge
mT air at this time of year Sahara Desert.

Co f
Further north, the area is under u o
hr nt
the influence of the Harmattan,
Ne e
tm

(stable, dry cT air blowing out of

ITCZ
al ar

the Saharan high pressure area).

rl ep

SE Trade Winds They are

ha D

Click here to Click here to unstable and moist as they

find out about find out about originate from over the Atlantic EQUATOR 0º
stable air unstable air Ocean.
wa
Ja

24
 MIGRATION OF THE ITCZ FROM DECEMBER TO JUNE

Between December and

June, progress through the
Earth’s orbit causes the sun
to migrate northwards.

t
gha
Pa y
Sub-tropical high pressure

si
ph
e, ra
NE Trade Winds – locally
called the Harmattan.They

eg og
are stable and dry as they

ll Ge
As it does so, the ITCZ originate from over the
Sahara Desert.
also moves further north,

Co f
allowing moist mT air to u o
hr nt
reach progressively further
Ne e

inland, brining the rainy

tm

season to West Africa.

al ar
rl ep

SE Trade Winds They are

unstable and moist as they
ha D

originate from over the

Atlantic Ocean.
EQUATOR 0º
wa
Ja

25
 MIGRATION OF THE ITCZ FROM JUNE TO DECEMBER

By late June, the sun begins

to migrate southwards, and so
does the ITCZ, following the
zone of maximum insolation.

t
g ha
Pa y
si
ph
As the ITCZ moves further Sub-tropical high pressure

e, ra
south, the Harmattan NE Trade Winds – locally
carries dry, stable cT air called the Harmattan.They

eg og
further south, bringing the are stable and dry as they

ll Ge
dry season across more and originate from over the
more of West Africa. Sahara Desert.

Co f
u o
hr nt
Ne e
tm
al ar
rl ep

SE Trade Winds They are

unstable and moist as they
ha D

originate from over the

Atlantic Ocean.
EQUATOR 0º
wa
Ja

26
 THE ITCZ AND PRECIPITATION IN WINTER
North South
Stable, dry, cT air from the Sahara moves
southwards. It causes the dry season across this
area

t
mT air right at the ITCZ

ha
is prevented from rising by

g
Pa y
si
stable cT air

ph
e, ra
eg og
ITCZ

ll Ge
Co f
u o
hr nt

Warm land surface makes the mT

Ne e
tm

air unstable. It rises and rain falls

GULF OF
al ar

GUINEA
a short way behind the ITCZ
rl ep
ha D

20ºN 15ºN 10ºN 5ºN

wa
Ja

Based on a diagram from “Core Themes in Geography”, by Broadley & Cunningham

27
 THE ITCZ AND PRECIPITATION IN SUMMER

North South
Stable, dry, cT air from
the Sahara moves
southwards. It causes the Occasional Heaviest

t
dry season across this

ha
area rainfall rainfall

g
Pa y
si
ph
e, ra
eg og
ITCZ

ll Ge
Co f
u o
hr nt

Moist unstable mT air now reaches far inland,

Ne e
tm

bringing rain to about 17ºN GULF OF

al ar

GUINEA
rl ep
ha D

20ºN 15ºN 10ºN 5ºN

wa

Based on a diagram from “Core Themes in Geography”, by Broadley & Cunningham

Ja

28
 THE ITCZ AND PRECIPITATION IN A SUMMER DROUGHT

North South
Strong high pressure over Sahara
causes strong subsidence. This
prevents advance of ITCZ to Heaviest

t
northernmost areas. Occasional rainfall

ha
rainfall

g
Pa y
si
ph
e, ra
eg og
ITCZ

ll Ge
Co f
u o
hr nt
Moist unstable mT air is prevented from reaching far inland.
Ne e
tm

GULF OF
al ar

GUINEA
rl ep
ha D

20ºN 15ºN 10ºN 5ºN

wa

Based on a diagram from “Core Themes in Geography”, by Broadley & Cunningham

Ja

29
 RAINFALL IN WEST AFRICA

t
g ha
Pa y
si
ph
20ºN
3

e, ra
250

eg og
15 15ºN
500

ll Ge
25
36 250
1000

Co f
60

u o
500
2000 10ºN
110
hr nt
3000
75
Ne e

1000
tm

123
3000 111
al ar

1000 5º
2000 2000
3000 3000
rl ep

2000
ha D

KEY 3000
250 EQUATOR 0º
Isohyet, with rainfall in mm
75 Average number of days with rain
wa
Ja

30
 THE FORMATION OF A
STABLE AIRMASS
Because no air rises,

t
precipitation is not

ha
likely.

g
Pa y
si
ph
e, ra
eg og
ll Ge
Co f
WARM SURFACE u o COLD SURFACE
hr nt
Ne e

A large volume of air develops uniform characteristics of The lower layers of the air mass are chilled. They are
tm

high temperature and humidity. dense and are not likely to rise.
al ar
rl ep

This creates a warm air mass This air is stable.

ha D
wa
Ja

31
 THE FORMATION OF AN
UNSTABLE AIRMASS
Unstable air rises and
condenses. Precipitation is
the likely result.

t
g ha
Pa y
si
ph
COLD AIR COLD AIR COLD AIR AIR MASS

e, ra
COLD MASS MASS
AIR MASS WARMER

eg og
MASS

ll Ge
Co f
u o
hr nt
COLD SURFACE WARM SURFACE
Ne e
tm

A large volume of air develops uniform The lower layers of the air mass are heated and begin to
al ar

characteristics of low temperature and humidity rise. As the air rises, it cools, and any water vapour
rl ep

over a cold surface. condenses, giving the potential for precipitation.

ha D

This creates a cold air mass. This air is unstable.

wa
Ja

32
 THE THREE CELL MODEL

Describe, in detail, what

happens as a result of

t
ha
insolation at the Equator.

g
Pa y
si
ph
e, ra
eg og
ll Ge
Co f
SOLAR ENERGY

What energy is being

u o
hr nt

transferred from where

Ne e
tm

to where?
al ar
rl ep

What is the name of

ha D

this cell?
wa
Ja

A 33
 THE THREE CELL MODEL

Describe, in detail, what

happens at the poles.

t
ha
g
Pa y
si
ph
e, ra
What is being transferred

eg og
from where to where?

ll Ge
Co f
u o
What is the name of
hr nt
Ne e

this cell?
tm
al ar
rl ep
ha D
wa
Ja

A 34
 THE THREE CELL MODEL

Explain how energy is

transferred between

t
30ºN or S and 60ºN or

ha
S.

g
Pa y
si
ph
e, ra
eg og
ll Ge
What is the name of

Co f
this cell? u o
hr nt
Ne e
tm
al ar
rl ep
ha D
wa
Ja

A 35
 THE TRANSFER OF HEAT ENERGY

Explain how heat energy

t
is transferred from the

ha
g
zones of surplus to zones

Pa y
si
ph
of deficit.

e, ra
eg og
ll Ge
Co f
SOLAR ENERGY
u o
hr nt
Ne e
tm
al ar
rl ep
ha D
wa
Ja

A 36
 ASSOCIATED PRESSURE BELTS

Describe and explain the

t
pressure belts associated

ha
with these cells.

g
Pa y
si
ph
e, ra
eg og
ll Ge
Co f
u o
hr nt
Ne e
tm
al ar
rl ep
ha D
wa
Ja

A 37
 ASSOCIATED SURFACE WIND
PATTERNS
From what pressure (high or
low) to what pressure does

t
wind blow ?

ha
g
Pa y
si
ph
e, ra
How and why are winds

eg og
deflected in the northern

ll Ge
hemisphere?

Co f
How are winds deflected in u o
hr nt

the southern hemisphere?

Ne e
tm

Make a quick sketch of the

al ar

surface winds
rl ep
ha D
wa
Ja

A 38
POSITION OF THE THREE CELLS
IN DECEMBER

t
Where is the sun overhead in December?

ha
g
Pa y
si
ph
e, ra
What effect does this have on the cells

eg og
and pressure belts?

ll Ge
Co f
u o
hr nt
Ne e
tm
al ar
rl ep
ha D
wa
Ja

A 39
POSITION OF THE THREE CELLS
IN JUNE
Where is the sun is overhead in June?

t
ha
g
Pa y
si
ph
e, ra
eg og
What effect does this have on the cells and pressure belts?

ll Ge
Co f
u o
hr nt
Ne e
tm
al ar
rl ep
ha D
wa
Ja

A 40
IN THE NORTHERN HEMISPHERE:
What is the name of the winds which blow to the Equator?

t
ha
g
Pa y
si
ph
IN THE SOUTHERN HEMISPHERE:

e, ra
eg og
What is the name of the winds which blow to the Equator?

ll Ge
Co f
u o
What is the full name of where these winds meet?
hr nt
Ne e
tm
al ar
rl ep
ha D
wa
Ja

A 41
 THE INTER-TROPICAL
CONVERGENCE ZONE

What are the characteristics

and name of the air mass that

t
ha
develops over the Sahara?

g
Pa y
si
ph
e, ra
Sub-tropical high pressure

eg og
ll Ge
Co f
u o
Equatorial
Inter-Tropical low pressure
Convergence Zone
hr nt
Ne e
tm

What are the characteristics Sub-tropical high pressure

al ar

and name of the air mass that

rl ep

develops over the S. Atlantic?

ha D
wa
Ja

A 42
 THE ITCZ IN DECEMBER
Which air mass influences most of W.Africa in December?

t
What conditions does this bring?

ha
g
Pa y
si
ph
What influence does the other air mass have over West

e, ra
Africa?

eg og
ll Ge
Co f
u o
hr nt
Ne e
tm
al ar
rl ep
ha D
wa
Ja

A 43
 THE ITCZ IN JUNE

Where is the zone of maximum insolation in June ?

t
ha
g
Pa y
si
ph
e, ra
Which air mass has an influence over West Africa ?

eg og
What conditions does this cause ?

ll Ge
Co f
u o
hr nt
Ne e
tm
al ar
rl ep
ha D
wa
Ja

A 44