THE THREE CELL MODELFORMATION OF THE HADLEY CELL 1

1
CELLS AND HEAT TRANSFER
A single cell model like in diagram 9 is too
simplistic- we can find three interlinked cells!
THE THREE CELL MODEL-FORMATION OF THE HADLEY
CELL (1)
Insolation in tropical areas causes warm air to
rise and spread polewards, carrying heat energy.
INSOLATION
Nb lots of new terms to learn!
2
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.
SOLAR ENERGY
Heat energy is therefore transferred from the
Equator to sub-tropical latitudes.
3
FORMATION OF THE POLAR CELL (1)
Intensely cold, dense air sinks at the poles,
then blows as surface winds towards the Equator.
4
FORMATION OF THE POLAR CELL (2)
At about 60ºN and 60 ºS, the cold polar air is
warmed in contact with the earths surface.
This warmed air rises and returns polewards,
carrying heat energy.
This circular motion is called the POLAR CELL.
5
FORMATION OF THE FERREL CELL
Unlike the Hadley and Polar Cells, the Ferrel
Cell is not driven by differences in heat energy.
The Ferrel Cell is caused by friction where air
is in contact with the other two cells.
(The Hadley Cell drags air down at about 30ºN and
S.
The Polar Cell causes an uplift at about 60ºN and
S. )
6
Polar Cell
THE THREE CELLS TOGETHER
Ferrel Cell
Hadley Cell
Hadley Cell
Ferrel Cell
Polar Cell
7
It is well worth making sure that you can see the
rising and falling pattern of these cells. The
next section uses this to work out wind
directions on the surface of the earth. There are
handouts of the next few slides use them and
diagram 13 to answer the later questions on the
screen.
8
ASSOCIATED PRESSURE BELTS
HANDOUT
Rising air at the equator causes the equatorial
belt of low pressure
Polar high pressure
Mid latitude low pressure
Descending air at about 30ºN and 30ºS causes the
sub-tropical belt of high pressure
Sub-tropical high pressure
Equatorial low pressure
Rising air at about 60ºN and 60ºS causes a
mid-latitude belt of low pressure
Sub-tropical high pressure
Mid latitude low pressure
Descending air at the poles causes the polar high
pressure areas
Polar high pressure
9
ASSOCIATED SURFACE WIND PATTERNS
Winds always blow from high pressure to low
pressure. They are deflected because of the
Coriolis Force which come about because of the
rotation of the earth. ( see later slide)
HANDOUT
Polar high pressure
Mid latitude low pressure
Sub-tropical high pressure
Winds in Northern Hemisphere are deflected to the
right.
Equatorial low pressure
Winds in the southern hemisphere are deflected to
the left.
Sub-tropical high pressure
Mid latitude low pressure
These wind belts shift seasonally. (See next
section)
Polar high pressure
10
Answer these five questions fully in sentences.

• Q1. What kind of pressure has rising air?
• Low pressure has rising air.
• Q2. What kind of pressure has falling air?
• High pressure has falling air.
• Q3. Name two latitudes with rising air.
• The Equator and 60 degrees N/S have rising air.
• Q4. Name two latitudes with falling air.
• 30 degrees N/S and the Poles have falling air.

11

• Q5. Explain the two factors that make the NE
  trade winds blow in the direction they do.
• ( You will need to be able to do this for any of
  the winds in diagram 13 !)
• i) Air is flowing from a high pressure area at 30
  degrees north towards a low pressure area at the
  Equator.

ii). The winds are being deflected by the
Coriolis force/ effect to the right as the area
is in the northern hemisphere.
12
Rossby waves and the Jet Stream
Not far above our heads in the northern UK is
where the Ferrel and Polar cells meet.
Here is where powerful waves of turbulence occur
called Rossby waves.
This turbulence is mainly responsible for the
series of depressions and anticyclones that
happen over Britain.
Read about these on pages 10 and 11, then answer
the questions on the next slide.
13

• Q1. In what direction do Rossby waves flow in the
  northern hemisphere?
• They flow towards the east in the northern
  hemisphere.

Q2. What is the name of the contact zone between
the two cells? A. It is called the Polar Front.
Q3. In what way is stage 4 like the formation of
a rivers ox-bow lake ? A. Loops of turbulence
get cut- off and isolated from the main flow.
14
Q4.In what way is the Ferrel cell thought to be
less effective in atmospheric heat transfer? A.
It seems that winds and ocean currents transfer
more heat than this weak atmospheric cell.
THE CORIOLIS FORCE.
What happens when you set off in a plane to fly
to somewhere? The earth turns beneath you, and
you have to keep adjusting your direction !
Watch.
15
Got the idea? Note the runners path is curved!
16
Pages 14 and 15 are very technical ! The
information is of the kind that can give extra
marks in an extended answer. Ignore it at your
peril !
We suggest that you read it at home where you can
think deeply about it !
You will need to have a basic understanding of
why the wind belts are in different places at
different times of the year.
END