1. The Inter-Tropical Convergence Zone
Atmosphere Unit
The Inter-Tropical Convergence Zone

2. Learning Intentions The climate and vegetation of West Africa
Air Masses and the ITCZ
The ITCZ
West African rainfall and the ITCZ

3. Climate and Vegetation
West Africa has a tropical climate – temperatures are over 23°C all year.
With increasing distance from the Equator, there is a greater range in temperature, decreasing rainfall and increasingly distinct wet and dry seasons.

4. Climate/vegetation patterns
DESERT
dry all year
dry ‘season’
wet ‘season’
SAVANNA
RAINFOREST
wet all year

5. 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.

6. Equator to 10° N&S The climate is EQUATORIAL
Temperature and rainfall are both high, with little variation all year
This gives rainforest vegetation

7. Further North and South
Tropical Continental or SAVANNA.
Temperatures are high, rainfall is lower
Distinct wet and dry seasons

8. Hot desert In West Africa, this is the Sahara
Daytime temperatures are hot with a large diurnal range
Rainfall is under 250mm.
Vegetation is minimal and farming almost impossible

9. Air Masses
Large masses of air settle over parts of the Earth’s surface and take on the local climate characteristics
These can move, carrying their characteristics to other areas

10. Brings dry, hot, settled weather Brings warm, wet, turbulent
West Africa
Two air masses affect the weather and climate of West Africa cT
Brings dry, hot, settled weather mT
Brings warm, wet, turbulent
weather

11. Tropical Continental (cT)
This air mass is formed over the Sahara desert
It’s associated with the North East Trade winds (Harmattan)
It brings extremely hot temperatures
No rainfall
Poor visibility (dust carried from the desert)

12. Tropical Maritime (mT)
This air mass is formed over the Atlantic Ocean
It’s associated with the South East Trade winds which are deflected by Corioli’s as they pass over the Equator, becoming the South West Monsoon
It brings hot temperatures
High humidity – lots of rain

13. Air Masses and the ITCZ
An area where two different winds meet is called a zone of convergence
Where the North East trades meet the South West Monsoons over West Africa, it is called the Inter-Tropical Convergence Zone
It is a zone of low pressure, associated with a band of rainfall

14. West African Climate and the ITCZ
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.

15. ITCZ seasonal position
ITCZ JANUARY
ITCZ JULY

16. Cont’d
The ITCZ is not stationary but moves north and south of the Equator, following the overhead sun and the thermal equator
In July, the thermal equator is in its most northerly position, bringing rain to West Africa
In January, the thermal equator is in its most southerly position, bringing dry weather to West Africa
WHY?

17. In July, the cT air mass dominates West Africa, so the hot, dry winds of the Harmattan covers most of the area
In January, the mT air mass pushes north into West Africa, bringing the rainy season
As the ITCZ moves south again, the norhtern areas become dry again. Southern coastal areas experience a returning band of rainfall as the ITCZ passes over again, giving a second rainfall peak

19. Sahel region
A major impact of the ITCZ is that it brings rain to the Sahel region of western Africa. This is the area south of the Sahara.
In recent years, the ITCZ has not “arrived” so the very short rainy season has all but disappeared.
This has had a major effect on the landscape and the people

20. The Sahel Region
The “Sahel” area of Africa – located to the south of the Sahara desert

21. West Africa

22. Sahel from the air – few trees or surface vegetation

23. Area of erosion –
Little vegetation
Few trees
Bare soil
Gullies starting to form
Severe gullying causes loss of farmland
Trees chopped down for firewood

24. Effects of desertification:
Villages and farms are abandoned
Lakes and waterholes dry up

25. Preventing Desertification
lattice matting made from twigs and dried grass to stabilise moving dunes

26. Dune fixation with windbreak fences made from branches

28. “Magic Stones” seen from the air
Prevents run-off and erosion

29. There are also human causes of desertification
More crops
overcultivation
More People
More animals
overgrazing
DESERTIFICATION
More firewood
deforestation

30. Decision-making task
“Give a man a fish and he will eat today. Teach a man to fish and he will eat for life” (proverb)
Some people see giving aid to Sahelian countries as a necessity. In your group, discuss advantages/disadvantages of aid and the practical methods for prevention of desertification.

31. Group task Planting trees for firewood/shelter belts
Planting Marram Grass
irrigation
Mechanised farming equipment
Which methods are most appropriate?
fences
GM seeds
Controlled grazing
Windbreaks
Crop rotation

32. 2011 Paper 1 Q1 (amended)
Describe the origin, nature and characteristics of the Maritime Tropical and Continental Tropical air masses in detail.
Describe and explain, in detail, the variations in rainfall within West Africa.

33. Model answer (a) Maritime Tropical (mT)
Origin – Atlantic ocean/Gulf of Guinea, in tropical latitudes
Weather characteristics – hot, high humidity, warm
Nature – unstable
Continental Tropical (cT)
Origin – Sahara Desert, in tropical latitudes
Weather characteristics – hot/very hot, dry, low humidity, warm
Nature – stable, poor visibility

34. Model answer (b)
Highlight the marked contrast in precipitation totals, seasonal distribution and number of days between a very dry north (Gao with only 200 mm in a hot desert climate in Mali) and a much wetter south (Abidjan with 1700 mm in a tropical rainforest climate in the Ivory Coast).
Bobo-Dioulasso in Burkina Faso in central West Africa has an „in-between‟ amount of both rain days and total annual precipitation (1000 mm in a Savannah climate).
Also refer to the variation in rain days and seasonal distribution for each station. Gao with a limited amount of precipitation in summer, Bobo-Dioulasso with a clear wet season/dry season regime and Abidjan with a “twin-peak‟ regime with a major peak in June and a smaller peak in October/November.

35. Explanation should focus on the role of the ITCZ and the movement of the Maritime Tropical and Continental Tropical air masses over the course of the year. For example, Abidjan, on the Gulf of Guinea coast, is influenced by hot, humid mT air for most of the year, accounting for its higher total annual precipitation and greater number of rain days. The twin precipitation peaks can be attributed to the ITCZ moving northwards in the early part of the year and then southwards later in the year in line with the thermal equator/overhead sun.

36. Gao, on the other hand, is under the influence of hot, dry cT air for most of the year and therefore has far fewer rain days and a very low total annual precipitation figure as it lies well to the north of the ITCZ for most of the year. Bobo-Dioulasso again is in an „in-between‟ position, getting more rain days and heavy summer precipitation from June-August when the ITCZ is furthest north.