Relief, Climate and Human Activity – Relief in Africa and the World (Senior 3 Geography) | Ganzaa

Relief in Africa and the World

Lessons

Senior 3 · Geography

Relief in Africa and the World

4 lessons

Lesson 4 of 4

Relief, Climate and Human Activity

25 min

Let's begin

Kilimanjaro stands only about 3° south of the Equator, yet ice still caps its summit. In the late 1880s an ice cap of about 20 km² covered the top of Kibo, its highest cone. At its foot, on the plains of Amboseli, it is hot savanna country. Nothing about latitude changes between the foot and the summit; only the height does. This lesson explains how relief changes temperature, rainfall and vegetation, and how it shapes where and how people live.

Relief and temperature

Air is warmed mainly from the ground, and higher up it is thinner and holds less heat. So temperature falls as height rises. On average the fall is about 6.5 °C for every 1,000 m of height (the standard value used by the International Civil Aviation Organization). This is why highland towns in the tropics are mild, and why the highest East African peaks carry ice.

Definition

Lapse rate: the rate at which air temperature falls with height. The average (normal) lapse rate is about $6.5\,^\circ\text{C}$ per 1,000 m.

T_{\text{high}} \approx T_{\text{low}} - 6.5 \times \frac{\text{difference in height (m)}}{1000}

Worked example

Worked example 1: temperature and height. A town on a plain at 500 m has a mean temperature of 26 °C (illustrative figure). Estimate the mean temperature (a) at 2,000 m on a nearby plateau and (b) at 5,500 m near the top of a mountain.

(a) Height difference: $2000 - 500 = 1500\,\text{m}$ . Fall: $6.5 \times \frac{1500}{1000} = 9.75\,^\circ\text{C}$ . Temperature: $26 - 9.75 \approx 16\,^\circ\text{C}$ .
(b) Height difference: $5000\,\text{m}$ . Fall: $6.5 \times 5 = 32.5\,^\circ\text{C}$ . Temperature: $26 - 32.5 = -6.5\,^\circ\text{C}$ .

Answer: about 16 °C on the plateau and below freezing near the summit, which is why snow and ice can last there.

Relief and rainfall

When moist wind meets a mountain range, it is forced to rise. Rising air cools, its water vapour condenses into cloud, and rain falls on the windward side. Having lost its moisture, the air sinks down the leeward side, warms up and becomes drier, so little rain falls there. The dry leeward area is a rain shadow.

Figure 9. Relief rainfall and the rain shadow. Drawn by Ganzaa.

Mountains	Wet windward side	Dry leeward side (rain shadow)
Atlas Mountains	North-west slopes facing moist Atlantic winds	South and east, towards the Sahara
Central highlands of Madagascar	East coast, facing the easterly winds: wet tropical	West and south: thorn forest and desert
Cape Fold Mountains, South Africa	Wettest mountain slopes: up to about 1,500 mm a year	Worcester, in the Breede River valley: about 200 mm a year
Ethiopian Highlands	The highland slopes	The Danakil Desert to the north-east

Definition

Relief (orographic) rainfall: rain formed when moist air is forced to rise over high ground and cools.

Windward: the side of a mountain facing the wind. Leeward: the sheltered side, away from the wind.

Rain shadow: the drier area on the leeward side of high ground.

Relief and vegetation

Because temperature and rainfall change with height, so does vegetation. A high mountain near the Equator has belts of vegetation stacked one above another, rather like a journey from the Equator towards the poles. Kilimanjaro shows them clearly.

Figure 10. Zones of vegetation on Kilimanjaro. Drawn by Ganzaa.

Relief and people

How relief affects it	Example
Settlement. In the tropics, highlands are cooler and often wetter, with fertile volcanic soils, so many are densely settled; steep slopes, cliffs and flood-prone valley floors are avoided.	The Ethiopian Highlands and the highlands round the East African lakes are among Africa's most crowded rural areas.
Farming. Cool, wet highlands suit crops such as tea; steep slopes need terraces to hold the soil; lowland plains suit large-scale farming and machines.	Terraced hillsides in Rwanda (photo in unit 1); the wide wheat plains of North America.
Transport. Mountains and escarpments are barriers: roads must zigzag, follow valleys and cross at passes, or tunnel through; plains are cheap to build on.	Rift valley escarpments in Kenya; tunnels and passes through the Alps.
Energy. Rivers falling steeply from plateaus give sites for hydro-electric power.	The Inga sites on the lower Congo, where the river falls off the plateau towards the sea.
Tourism. Mountains, volcanoes and rift lakes attract visitors.	Climbing Kilimanjaro; gorilla trekking in the Virunga volcanoes.
Hazards. Steep, soaked slopes slide; volcanoes erupt; rivers in narrow valleys flood quickly.	On the night of 2–3 May 2023, heavy rain caused floods and landslides in Rwanda's Western, Northern and Southern provinces that killed more than 130 people (Government of Rwanda).

Warning

Relief alone does not decide. Do not write "people live in the highlands because they are high". Explain the chain: height → cooler, wetter climate → fewer mosquitoes, good farming → more people. And name the cost too: the same steep slopes bring soil erosion and landslides.

Worked example

Worked example 2: an "evaluate" answer. "Rwanda's relief is more of an advantage than a disadvantage." Discuss.

Advantages: most of Rwanda lies between 1,500 m and 2,500 m (Rwanda Development Board), which gives a mild climate near the Equator; the highlands and the Virunga volcanoes attract tourists.

Disadvantages: steep slopes suffer soil erosion and landslides, as in May 2023; roads are costly to build and must wind around hills; flat land for farms, factories and airports is scarce.

Judgement: a good answer weighs both sides and reaches a view, e.g. "an advantage overall, provided slopes are protected by terraces and trees and people are moved away from high-risk zones".

Tip

Three links to remember. Height lowers temperature (about 6.5 °C per 1,000 m); slopes facing the wind get more rain; and temperature plus rain decide the vegetation and the farming. Exam questions on relief and climate almost always test one of these.

Practice

State the average rate at which temperature falls with height.
A place at 1,000 m has a mean temperature of 24 °C. Estimate the mean temperature at 3,000 m on a mountain nearby.
With the help of a labelled diagram, explain how relief rainfall forms.
What is a rain shadow? Give one example from Africa.
Name the vegetation zone found between 1,800 m and 2,800 m on Kilimanjaro, and explain why vegetation changes with height.
Give two ways in which mountains hinder transport, and one way engineers overcome them.
Explain why many tropical highlands in Africa are densely populated.
"Relief is more of an advantage than a disadvantage for Rwanda." Do you agree? Give two arguments on each side and a conclusion.

Show answersHide answers

About 6.5 °C for every 1,000 m.
Rise: 2,000 m; fall: $6.5 \times 2 = 13\,^\circ\text{C}$ ; temperature $\approx 24 - 13 = 11\,^\circ\text{C}$ .
Moist wind from the sea meets high ground and is forced to rise; the air cools; vapour condenses into cloud; rain falls on the windward slope. Draw as in Figure 9 with the wind, the rising air, the cloud, the rain, the windward and leeward sides labelled.
The dry area on the leeward side of high ground, where sinking air warms and gives little rain. Examples: the Breede River valley and Karoo behind the Cape Fold Mountains; western and southern Madagascar; the Danakil Desert beside the Ethiopian Highlands.
Rainforest. Temperature falls with height, and rainfall changes too, so each belt has the conditions that suit a different set of plants.
Steep slopes make roads and railways climb slowly and wind; ranges block direct routes; snow and landslides close roads. Engineers build zigzag roads, use passes and valleys, or dig tunnels.
They are cooler than the lowlands, often have reliable rain and fertile (often volcanic) soils, and have fewer mosquitoes and less malaria, so farming supports many people.
For: a mild highland climate near the Equator; tourism (volcanoes, gorillas). Against: soil erosion and landslides (more than 130 deaths in May 2023); costly roads; little flat land. Conclusion: your own reasoned view, e.g. advantage overall if slopes are managed.

1Relief and temperature

2Relief and rainfall

3Relief and vegetation

4Relief and people

Relief and temperature

Relief and rainfall

Relief and vegetation

Relief and people