1. INTER-TROPICAL
CONVERGENCE ZONE
(ITCZ)

2. INTER-TROPICAL CONVERGENCE ZONE (ITCZ)
SUMMARY NOTE:
ITCZ
A zone of convergence is where winds meet
Converging winds include the trade winds which meet at the equator – called the ITCZ
Associated low pressure: calm, warm conditions
EFFECT OF THE EARTH’S ORBIT ON THE ITCZ
Latitude receiving the most intense heating from the sun
This areas moves between the tropics due to the Earth’s orbit
The area of maximum heating is known as the thermal equator
Sun overhead at the Tropics at the solstices (once in N. Hemisphere and once in the S.)
Sun directly over the equator twice a year – equinoxes
This migration of the thermal equator shifts the belts of winds and pressure systems north and south over the year

3. INTER-TROPICAL CONVERGENCE ZONE (ITCZ)
SUMMARY NOTE (Cont.):
EFFECT OF LAND AND SEA MASSES ON ITCZ
Rates of heating vary greatly over land and sea
Temperature extremes are greater over land – it heats and cools at a faster rate
This means that the ITCZ (zone of low pressure) moves at a different rate around the world
As a result the ITCZ moves further N and S over land than sea

4. INTER-TROPICAL CONVERGENCE ZONE (ITCZ)
CASE-STUDY – EQUITORIAL & SAVANNA REGIONS OF AFRICA
Main climatic regions:
Equatorial Climate
Tropical Climate
Hot Deserts
These zones merge into one another
Key difference is the rainfall rather than temperature
Therefore seasons tend to be wet season and dry season rather than
winter and summer

5. These two dates are the solstices.
23º
The most intense heating of the sun, occurring at the so-called thermal equator, annually moves between the tropics.
On or around June 20th each year the sun is overhead at 23½ºN, the Tropic of Cancer.
On or around December 20th the the sun is at overhead at 23½ºS, the Tropic of Capricorn.
These two dates are the solstices.
Twice a year, at the equinoxes, on or around March 20th and September 20th the overhead sun crosses the equator.
This annual north to south and back again "shift" of the thermal equator shifts the belts of planetary winds and pressure systems to the north and to the south as the year turns.

6. 23½ºN 0º 23½ºS June Summer Solstice TROPIC of CANCER
September Autumn Equinox
March Spring Equinox 0º
EQUATOR
December Winter Solstice
23½ºS
TROPIC of CAPRICORN

7. ITCZ

8. ITCZ
JULY
ITCZ
JANUARY

9. The location of the ITCZ varies throughout the year
The ITCZ over land moves farther north or south than the ITCZ over the oceans
due to the variation in land temperatures.
ITCZ JANUARY
ITCZ JULY

10.
The blue shading on the map shows the areas of highest cloud reflectivity,
which correspond to the average monthly position of the ITCZ.

11. The migration of the inter-tropical convergence zone (ITCZ) in Africa affects seasonal precipitation patterns across that continent.

12. ITCZ moves north in summer
DESERT
dry all year
SAVANNA
dry ‘winter’
wet ‘summer’
RAINFOREST
wet all year
ITCZ moves north in summer

13. Tropical rainforest
savanna

14. The further North
of the Equator in tropical Africa:-
the lower the annual rainfall
the more the rainfall is concentrated in the summer months
the more variable the rainfall.

15. seasonality increases variability increases
GUINEA
SAVANNA
SAHEL
SAVANNA
DESERT
RAINFOREST 0º
20ºN
10ºN
rainfall decreases
seasonality increases
variability increases

16. LAGOS
SOKOTO
TIMBUKTU

17. savanna
climate
tropical summer rain

18. savanna
vegetation

19. savanna ‘parkland’

20. savanna ‘parkland’

21. savanna ‘parkland’

22. savanna ‘parkland’

23. baobab tree

24. acacia tree

25. acacia thorns

26. desertification

27. INTER-TROPICAL CONVERGENCE ZONE (ITCZ)
CASE-STUDY – EQUITORIAL & SAVANNA REGIONS OF AFRICA
Using pages 19  26
Answer questions 4  6 on page 39 under the heading ‘Tropical Africa:
Climate’

28. 3 Marks
Name the air masses
Describe where they
come from
Give their
characteristics

29. Air Mass A: Tropical Continental or cT
Origin: Over the Sahara Desert (ie large land mass in
tropical latitudes)
Nature: Warm, dry, stable air
Air Mass B: Tropical Maritime OR mT
Origin: Over Atlantic Ocean (Gulf of Guinea) in tropical
latitudes
Nature: Warm, moist, unstable air
Allow one mark for each correctly named air mass,origin
and nature. Candidates who incorrectly name (or omit the
name) of the air mass can still gain credit for the correct
origin and/or nature of the air mass.

30. 12 Marks
Describe the climate graphs:
i. Highest rainfall
ii. Least rainfall
iii. Rain all year
iv. One peak or two
Give figures
Explain why this happens.
Match movement of ITCZ to
rainfall
i. Dry = cT
ii. Wet = mT
iii. Peak Rainfall =
ITCZ overhead
Why does the ITCZ move?

31. Description should be able to focus on the marked contrasts in
rainfall amounts between a very dry north (with only 250 mm per
annum) and a much wetter south (where some coastal areas receive over
2000 mm per annum) as shown on the map provided. Candidates could also
refer to the graphs and note the variations between the three stations with
Tombouctou in the north and Jos “in the middle” experiencing obvious wet
and dry seasons whilst coastal Lagos in the south has a vastly greater annual
rainfall total, no dry months and a “twin peak” regime.
Explanation ought to concentrate on the role of the I.T.C.Z and its
associated Tropical Maritime air mass (warm, moist and unstable) and
Tropical Continental air mass (warm, dry and stable). For example, Lagos –
On the coast of the Gulf of Guinea – is influenced by warm moist Tm air for
Most of the year. This accounts for its much higher annual rainfall total.
The twin rainfall peaks can be attributed to the I.T.C.Z migrating
northwards and then southwards again later in the year, following the
overhead sun or thermal equator. Tombouctou, in contrast, lies well to the
north of the I.T.C.Z in January and is under the influence of the hot, dry Tc
air from the Sahara Desert. In May/June the I.T.C.Z. moves north bringing
moist Tm air and rainfall to Jos and, to a much lesser extent, Tombouctou
which lies closer to its point of maximum extent.