Changing Rainfall and Water
Availability in East Africa:
Supplementary Material to Lesson 3 of the
“East Africa Climate Change Curriculum Unit”
Available at http://www.eaclipse.msu.edu/teaching_materials.html
Lesson 3 by Dwight Sieggreen & Barbara Naess.
PowerPoint based on materials by Jennifer M. Olson.
The Eaclipse Project is supported by
National Science Foundation Award No. BCS/CNH 0709671.
http://eaclipse.msu.edu
© 2010 Michigan State University Board of Trustees
Lesson 3: Changing Rainfall and Water
Availability in East Africa
Supplementary Material
2
In this lesson you will
• Learn the main factors that affect rainfall
patterns in Africa
• Compare the water cycle in two different
ecosystems
• Use the water cycle to understand where
water is lost from each ecosystem
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Major influences on Africa’s climate zones
1. Intertropical Convergence Zone (ITCZ): As
the ITCZ moves north and south, it
determines the rainy seasons
2. Elevation: The mountains and highlands are
cooler and wetter, and the windward side of
a mountain gets a lot of rain (orographic
effect)
3. Nearness to the coast: there is more rain on
the coast
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The Intertropical Convergence Zone
(ITCZ)
• The ITCZ is a belt of rainfall that governs the rainy
seasons around the equator (between Tropics of
Cancer and Capricorn)
• It is an area of low pressure: the sun heats the earth’s
surface, the surface air warms and expands, and it
rises up like a fountain
• As the hot air rises, it precipitates out water rainfall
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ITCZ Rainbelt in July
Rainfall
ITCZ
WIND
Adapted from Stock1995.
Africa South of the Sahara, p31.
MSU LUCID: Bilal Butt
© MSU Board of Trustees 2010
• The ITCZ moves, within the tropics, with the peak of the sun
• In July it is at its farthest northern point
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ITCZ Rainbelt in January
Rainfall
ITCZ
WIND
Adapted from Stock1995.
Africa South of the Sahara, p31.
MSU LUCID: Bilal Butt
© MSU Board of Trustees 2010
• In January, the ITCZ is at its most southern point
• Maritime southwesterly (wet) winds cause heavy rain
• Northern Africa is dry
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MSU LUCID: Bilal Butt
© MSU Board of Trustees 20108
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Orographic Rainfall
• Elevation affects rainfall. As winds hit a
mountainside on the windward side, air is
forced upwards and cools. The cool air
cannot hold moisture and rain precipitates
out.
• The wind, now dry, flows over the
mountain peak and descends on the
leeward side. It doesn’t rain and leaves
that side of the mountain dry.
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Source: http://rst.gsfc.nasa.gov/Sect14/Sect14_1d.html
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Moist Air Rising and Forming Clouds
Along a Mountain Side
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Mt. Kenya
Windward side
Leeward side - dry
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Proximity to the Coast
• Coastal East Africa receives torrential rains
from moist air masses moving inland from
the Indian Ocean
• This means that coastal areas have
significantly more rainfall than inland areas
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Map source:
U.S. Central
Intelligence
Agency, 2003
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Arusha annual rainfall time series (1961 -2005)
y = -6.2175x + 13146
R² = 0.0799
1800
1600
1400
Rainfall (mm)
1200
1000
800
600
400
200
0
1961
1965
1969
1973
1977
1981
1985
1989
1993
1997
2001
2005
2009
Years
Annual rainfall
Source: Tanzania Meteorological Agency
Linear Trend
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Zanzibar annual rainfall time series (1961 -2005)
y = -0.792x + 3251.6
R² = 0.0007
3000
2500
Rainfall (mm)
2000
1500
1000
500
0
1961
1965
1969
1973
1977
1981
1985
1989
1993
1997
2001
2005
2009
Years
Annual rainfall
Source: Tanzania Meteorological Agency
Linear Trend
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Summary of factors affecting rainfall
• Tropical areas receive highly seasonal
rainfall due to ITCZ
– Some equatorial regions receive almost
constant rainfall
– North & south of equator, there are
mirrored belts of decreasing rainfall
• Rainfall varies according to elevation –
orographic effect
• Rainfall is higher on the coast
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How will rainfall affect vegetation?
• Vegetation growth is
closely associated with
rainfall
• Reliability: In general, the
less rain a region receives
in an average year, the
less reliable the rainfall
• Drought: savanna
livelihood systems must be
capable of withstanding
extreme variability of
rainfall, including droughts
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Review Questions
• What is the Intertropical Convergence Zone
(ITCZ) and how does it affect rainfall in East
Africa?
• What is orographic rainfall and how does it affect
rainfall on mountainsides?
• How does the ocean affect rainfall in locations on
the coast?
• What differences did you see in the change in
average rainfall in Arusha (highland savanna)
and Zanzibar (coast)?
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Source: U.S. Geological Survey
http://ga.water.usgs.gov/edu/watercyclehi.html
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