The Inter-Tropical
Convergence Zone
The greatest amount
of rain falls across the
coast of West Africa.
Here rainfall can
exceed 2000mm per
year.
The further north you
go, the less rain, so in
the Sahara desert
there is less than
250mm per year.
Along the coast, rain
falls in every month,
because it has an
equatorial climate.
But just north of the
coast there is one
month dry season.
The dry season
becomes longer the
further north you go.
So in Nigeria, it is 9
months and in the
Sahara it is 12 months
Nigeria
West Africa there are 2 main wind directions – SW and NE
Rainfall is determined by wind direction
South Westerly Winds
These are brought by Tropical Maritime (mT) air streams
They are trade winds
The are onshore winds
The blow from the Atlantic Ocean
These bring rainy, thundery weather
North-Easterly Winds
Brought by Tropical Continental (cT) air streams
They are trade winds
The are offshore winds
They blow from the Sahara Desert
They bring very warm, dry, dusty weather
The SW and NE trade winds blow towards the Equatorial Low Pressure, this is the
ITCZ
The ITCZ moves with the overhead sun, because where the sun is overhead should
be the hottest area.
The ITCZ is over the Tropic of Capricorn in Africa on December 21
Land, heats up a lot quicker than oceans so the ITCZ follows the coast of Africa
instead of going into the Atlantic Ocean
On June 21 the ITCZ is over the Tropic of Cancer
January
23.5 N – Tropic of Cancer
0
23.5 S – Tropic of Capricorn
January
23.5 N –
Tropic of
Cancer
0
23.5 S –
Tropic of
Capricorn
June
23.5 N –
Tropic of
Cancer
0
23.5 S –
Tropic of
Capricorn
Remember!
When the ITCZ is north of countries in Africa, they will
be receiving the rain brought by the Tropcial Maritime
air stream, so countries on the West Coast of Africa
will always have wet weather because the ITCZ is always north
of it!
These countires will have 12 months wet season.
Countries just north of the coast will have 11 months wet
season because the ITCZ is only north of these countries for 11
monhts of the year.
The Sahara desert will have 0 months wet season because the
ITCZ is never north of it!
0 months
wet season
11 months
wet season
12 months
wet season