Transcript Atmosphere 6 - ITCZ West Africa

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INTER-TROPICAL CONVERGENCE ZONE (ITCZ)
SUMMARY NOTE:
ITCZ
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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
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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
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INTER-TROPICAL CONVERGENCE ZONE (ITCZ)
SUMMARY NOTE (Cont.):
EFFECT OF LAND AND SEA MASSES ON ITCZ
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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
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INTER-TROPICAL CONVERGENCE ZONE (ITCZ)
CASE-STUDY – EQUITORIAL & SAVANNA REGIONS OF AFRICA
Main climatic regions:
1.
Equatorial Climate
2.
Tropical Climate
3.
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
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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.
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June Summer Solstice
23½ºN
TROPIC of CANCER
September Autumn Equinox
March Spring Equinox
0º
EQUATOR
December Winter Solstice
23½ºS
TROPIC of CAPRICORN
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ITCZ
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ITCZ
JANUARY
ITCZ
JULY
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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 JULY
ITCZ JANUARY
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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.
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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/
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DESERT
SAVANNA
dry all year
dry ‘winter’
wet ‘summer’
RAINFOREST
wet all year
ITCZ moves north in summer
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Tropical rainforest
savanna
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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.
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RAINFOREST
0º
GUINEA
SAVANNA
SAHEL
SAVANNA
10ºN
rainfall decreases
DESERT
20ºN
seasonality increases
variability increases
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LAGOS
A
SOKOTO
B
C
A
TIMBUKTU
B
C
A
B
C
JAN
27
28
65
20
0
13
22
0
22
FEB
29
41
69
22
0
13
24
0
19
MAR
29
96
72
31
0
11
28
2
18
APR
28
143
72
34
10
17
32
0
15
MAY
28
274
76
33
51
31
35
5
18
JUN
26
460
80
30
89
41
36
23
31
JUL
25
282
80
28
147
55
32
79
45
AUG
25
69
76
26
236
64
30
81
57
SEP
25
140
77
27
145
59
32
38
45
OCT
26
208
76
28
13
37
31
3
23
NOV
27
69
72
27
0
18
28
0
17
DEC
28
25
68
25
0
15
22
0
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annual
27 1835
74
28
691
31
29
231
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A = average monthly temperature C
B = mean monthly rainfall mm
C = % relative humidity at midday
LAGOS
SOKOTO
TIMBUKTU
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savanna
climate
tropical summer rain
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savanna
vegetation
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savanna ‘parkland’
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savanna ‘parkland’
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savanna ‘parkland’
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savanna ‘parkland’
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baobab tree
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acacia tree
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acacia thorns
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desertification
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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’
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3 Marks
Name the air masses
Describe where they
come from
Give their
characteristics
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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.
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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?
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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.
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