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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
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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 4
• 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
The Intertropical Convergence Zone (ITCZ)
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ITCZ Rainbelt in July
• The ITCZ moves, within the tropics, with the peak of the sun• In July it is at its farthest northern point
WINDITCZ
MSU LUCID: Bilal Butt © MSU Board of Trustees 2010
Adapted from Stock1995. Africa South of the Sahara, p31.
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Rainfall
ITCZ Rainbelt in January
• In January, the ITCZ is at its most southern point• Maritime southwesterly (wet) winds cause heavy rain • Northern Africa is dry
WINDITCZ
Adapted from Stock1995. Africa South of the Sahara, p31. MSU LUCID: Bilal Butt
© MSU Board of Trustees 2010
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Rainfall
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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Source: Tanzania Meteorological Agency 27
Source: Tanzania Meteorological Agency28
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 Surveyhttp://ga.water.usgs.gov/edu/watercyclehi.html
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