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8/2/2019 ATMO 4 itcz

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ATMOSPHERE

CASE STUDY AREA- the ITCZ in AFRICA

You will need to be able togive very detailed answersto a question on this areain an assessment.

This case study shows the way that the movement ofthewind belts between their summer and winterpositions has a profound effect on the lives indeed

the very survival- of people who live in the Sahel zoneof northern Africa.


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ATMOSPHERE

vThe ITCZ is an areawhere two air masses

meet- it brings heavyrain to the areas itpasses over.

vIt happens all roundthe world between theEquator and about 20degrees N/S.

vIts full name is theInter Tropical

Convergence Zone. It ispart of the Hadleyconvection cells, and hasthe Doldrums within itsboundary.


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ATMOSPHERE

The ITCZ does

not stay in thesame area allyear round butmigrates to thenorth and then

back south again.It is thismovement thatmatters so muchto the people and

animals of thearea.

The AFRICAN ITCZ REGION

Discussthisdiagram

Learnthesetwo

boxes!


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ATMOSPHERE

The trade winds ( mT air mass) comeinto the zone from the Atlantic,south of the Equator, and havetravelled over oceans; they are there-fore carrying a lot of moisture.This is their position in January.

Once in the hotter latitudes, they areenergised into huge towering cumulo-nimbusthunderclouds. These can be anything up to 10kmsacross, and groups of clouds can form covering

1000kms. In between the clusters are often sunnycloud-free areas.

The clusters are particularly found over land, not sea.


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WetwarmmT air

HEAVY RAINS

ATMOSPHERE

S N

Gulf ofGuinea

Coastalareas-equatorialclimate

Inland areas-savannaclimate type

Sahara-Desertclimatetype

Moves this way

HotdrycTair

Harmattan wind

IN JANUARY

Copy diagram


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ATMOSPHERE

In January, the sun is

overhead near the Tropicof Capricorn, in thesouthern hemisphere.

The ITCZ zone of

meeting air lies well tothe south, as seen here.

The rains brought by thezone are confined to thevery coastal areas of

Nigeria, Togo, Ghana andtheir neighbours.

ITCZJANUARY

Case study area


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ATMOSPHERE

Here, the moist warm mT air from the Atlantic Oceanmeets the hot dry cT air coming out of the SaharaDesert. Note that the desert is an area of HIGH pressure in winter andparticularly strong winds blow AWAY from such areas.

The cT wind is called the HARMATTAN and is extremely

hot, dry and dusty. Of course, the Harmattan cannot bring any rainto the area it travels over, and all the northern part of Africa is

influenced totally by it.No crops can grow.

In the south, however, the wet moist winds from the

sea are forced upwards over the land where they drenchthe land in life-giving torrential rains. Here crops can beeasily grown, providing the soil is fertile enough and not washed away.


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ATMOSPHERE

During the spring, the ITCZ moves slowly northwards,the Harmattan losing its dominance over the land bitby bit.

Places further and further inland get the rains that

they so desperately need for people, crops andanimals alike. The slight drawback is that the further the ITCZtravels north, the less water it can bring to the rainy area, so crops tothe north get less than those to the south.

By mid spring, places like northern Ghana, northern

Benin and central Nigeria are getting the rains. Thewells are re-filled, the grass for feeding animals starts to grow and

farmers can start off their crops.


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WetwarmmT air

HEAVY RAINS

ATMOSPHERE

S N

Gulf ofGuinea




Moves this way

HotdrycTair

Harmattan wind

IN JANUARY

Copy diagram

Rememberthis?


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ATMOSPHERE

S N

Gulf ofGuinea




WetwarmmT air

HotdrycTair

Harmattan wind

IN JULY

Moves this way

HEAVY RAINS LIGHT RAINS

Copy diagram

Compare the January and July diagrams.


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ATMOSPHERE

By July the ITCZ has reached as farnorth as it will go, reaching central

Mali, northern Niger and northernChad. There the Harmattan is confined to thevery edges of the Sahara desert, the weakest it

gets all year.

This is the Sahel zone, the area mostprone to DESERTIFICATION

Without these rains most years, the desert will takeover yet more land, forcing poverty-stricken, hungry

people to migrate southwards into other peoplesterritory, with their thirsty cattle and goats.

ITCZ JULY


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ATMOSPHERE

After July, the belt moves back down south

again, giving a second rainy spell to the luckyareas in its path.

No more rain will fall this far inland until nextyear!

The people rely on these rains that arrive within aweek or two of the same time every year.

Recently, there have been several years wherethe rain has not got as far inland as normal !


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Watch!

ATMOSPHERE

SecondLot ofrain

SN


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ATMOSPHERE

Look carefully at these three graphs.

Without using an atlas, can you say roughly whereeach place is likely to be using only the rainfallpattern. Use words like inland, coastal, further,Sahel.

Can you say why?

2 31


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ATMOSPHERE

A question asking about the ITCZ, with graphs andmaps, will need you to do the following things;explain-

They arelooking

for.


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ATMOSPHERE

END

The people rely on these rains that should arrive

within a week or two of the same time every year.Recently, there have been several years wherethe rain has not got as far inland as normal !

What do you think has been the result of this?

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