INTER-TROPICAL CONVERGENCE ZONE (ITCZ)SUMMARY NOTE:

ITCZA zone of convergence is where winds meetConverging winds include the trade winds which meet at the equator called the ITCZAssociated low pressure: calm, warm conditions

EFFECT OF THE EARTHS ORBIT ON THE ITCZLatitude receiving the most intense heating from the sunThis areas moves between the tropics due to the Earths orbitThe area of maximum heating is known as the thermal equatorSun overhead at the Tropics at the solstices (once in N. Hemisphere and once in the S.)Sun directly over the equator twice a year equinoxesThis migration of the thermal equator shifts the belts of winds and pressure systems north and south over the year

INTER-TROPICAL CONVERGENCE ZONE (ITCZ)SUMMARY NOTE (Cont.):

EFFECT OF LAND AND SEA MASSES ON ITCZRates of heating vary greatly over land and seaTemperature extremes are greater over land it heats and cools at a faster rateThis means that the ITCZ (zone of low pressure) moves at a different rate around the worldAs a result the ITCZ moves further N and S over land than sea

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 thanwinter and summer

23The 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 23N, the Tropic of Cancer.On or around December 20th the the sun is at overhead at 23S, 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.

0EQUATOR23NTROPIC of CANCER23STROPIC of CAPRICORNDecember Winter SolsticeSeptember Autumn EquinoxJune Summer SolsticeMarch Spring Equinox

ITCZ

ITCZ JULYITCZ JANUARY

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 JULYITCZ JANUARY

http://www.cla.sc.edu/geog/faculty/carbone/modules/newmods/africa-itcz/The blue shading on the map shows the areas of highest cloud reflectivity, which correspond to the average monthly position of the ITCZ.

The migration of the inter-tropical convergence zone (ITCZ) in Africa affects seasonal precipitation patterns across that continent. http://www.bbc.co.uk/scotland/learning/learningzone/clips/538/

DESERTSAVANNARAINFORESTdry all yearITCZ moves north in summerdry winterwet summerwet all year

Tropical rainforestsavanna

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.

RAINFORESTGUINEA SAVANNASAHEL SAVANNADESERTrainfall decreasesseasonality increasesvariability increases

Sheet1

LAGOSSOKOTOTIMBUKTU

ABCABCABC

JAN2728652001322022

FEB2941692201324019

MAR2996723101128218

APR281437234101732015

MAY282747633513135518

JUN2646080308941362331

JUL25282802814755327945

AUG2569762623664308157

SEP25140772714559323845

OCT262087628133731323

NOV2769722701828017

DEC2825682501522019

annual2718357428691312923127

A = average monthly temperature C

B = mean monthly rainfall mm

C = % relative humidity at midday

Sheet2

Sheet3

savanna climatetropical summer rain

savanna vegetation

savanna parkland

savanna parkland

savanna parkland

savanna parkland

baobab tree

acacia tree

acacia thorns

desertification

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

3 Marks

Name the air masses

Describe where they come from

Give their characteristics

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.

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?

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.