Weather and Climate
Senior 1 · Geography
Weather and Climate
Lesson 6 of 6
25 min
Most Rwandan farmers plant twice a year: once as the short rains begin in September and October, and again when the long rains arrive in March. The whole farming calendar, as well as school holidays, road works and even the power supply from hydroelectric stations, follows the rhythm of Rwanda's climate. In this lesson you will read that rhythm from a climate graph and explain it.
Figure 1. Kigali's climate graph: two rainy seasons, two drier seasons and an almost flat temperature line.
| Month | Rainfall (mm) | Mean temperature (°C) | Month | Rainfall (mm) | Mean temperature (°C) |
|---|---|---|---|---|---|
| January | 84.2 | 21.9 | July | 10.2 | 21.4 |
| February | 83.9 | 22.0 | August | 30.2 | 22.3 |
| March | 129.1 | 21.7 | September | 64.8 | 22.3 |
| April | 135.5 | 21.5 | October | 108.8 | 21.9 |
| May | 88.7 | 21.5 | November | 121.2 | 21.2 |
| June | 20.3 | 21.3 | December | 76.4 | 21.5 |
Source: World Meteorological Organization climate normals for Kigali Airport, 1991–2020, published by NOAA and tabulated in the Wikipedia article "Kigali". The values are plotted exactly as published (to 0.1 mm and 0.1 °C); none has been rounded.
Definition
Climate graph: a graph that shows the average monthly rainfall (as bars) and the average monthly temperature (as a line) of one place over a long period, usually 30 years, on the same chart with two vertical scales.
Worked example 1: describing Kigali's climate from the graph
(a) Total annual rainfall: add the 12 monthly values:
(The published annual total is 953.5 mm; the small difference comes from rounding each month to 0.1 mm.)
(b) Wettest and driest months: April (135.5 mm) and July (10.2 mm).
(c) Annual temperature range: .
(d) Mean annual temperature: .
Description: Kigali has a warm, mild climate with almost no temperature seasons (range about 1 °C) but two clear rainy seasons, March to May and October to December, separated by a long dry season from June to August and a shorter, less dry one in January and February.
Worked example 2: how much of the rain falls in the two rainy seasons?
About seven-tenths of Kigali's rain falls in just six months.
| Factor | How it affects climate | In Rwanda |
|---|---|---|
| Latitude | Near the Equator the Sun is high all year: warm, with little seasonal change in temperature | At about 2°S, Rwanda has almost no hot or cold season |
| Altitude | Temperature falls about 6.5 °C per 1,000 m | Rwanda is much cooler than lowland places at the same latitude |
| Relief | Mountains force air to rise and rain on the windward side; the leeward side is drier | The Congo–Nile ridge and Nyungwe are wet; the east is drier |
| Distance from the sea and large lakes | Water evens out temperatures and supplies moisture | Lake Kivu adds moisture to the west; Rwanda is about 1,000 km from the ocean |
| Prevailing winds and air masses | Winds from the sea bring moisture; winds from dry land bring drought | Moist easterly air from the Indian Ocean and air from the Congo Basin bring rain |
| Movement of the rain belt | The belt of rising air near the overhead Sun (the Intertropical Convergence Zone) brings rain as it passes | It crosses Rwanda twice a year, giving two rainy seasons |
| Vegetation | Forests add moisture by transpiration and keep local temperatures lower | Nyungwe and Gishwati help keep the west cool and moist |
Definition
Intertropical Convergence Zone (ITCZ): a belt of low pressure near the Equator where winds from the northern and southern hemispheres meet and warm, moist air rises, producing cloud and heavy rain. It moves north and south during the year, following the overhead Sun, so places near the Equator get rain when it passes over them.
Important
Case study: why Rwanda has two rainy seasons
The rain belt (ITCZ) follows the overhead Sun. It passes over Rwanda moving northwards around March to May, bringing the long rains, and passes back southwards around October to December, bringing the short rains. Between these passes, from June to August, it lies to the north and Rwanda has its long dry season. The rains do not arrive on the same date every year: in some years they are late or weak, bringing drought, especially in the drier east; in others they are very heavy, bringing floods and landslides in the steep west and north.
Within Rwanda, rainfall generally increases from east to west: the western highlands commonly receive well over 1,300 mm a year, while parts of the eastern lowlands receive under 1,000 mm (approximate figures).
| Activity | Effect |
|---|---|
| Farming | Two main seasons: Season A (September to February) and Season B (March to June), plus Season C in the marshlands (July to September). Late or failed rains cut harvests; hail damages crops. |
| Energy and water | Hydroelectric stations and water supplies depend on rainfall; dry seasons lower river flows. |
| Transport and building | Heavy rain causes landslides that block roads; roofs, gutters and drains must cope with storms. |
| Health | Malaria-carrying mosquitoes breed more in warm, wet lowlands than in cool highlands; floods spread water-borne diseases. |
| Tourism | Gorilla trekking and safaris are easier in the dry seasons, so visitor numbers peak then. |
| Hazards | Floods and landslides in the rainy seasons; droughts in the east; lightning during thunderstorms. |
How does the climate system work?, by the Met Office (4 min). Watch for how the Sun's uneven heating of the Earth drives the winds and the rain belts.
A new school in the Eastern Province is planning a vegetable garden. Using the climate graph and the case study, advise the school when to plant, how to store water, and what extra risk it faces compared with a school in Musanze.
Tip
Describing a climate graph? Use "TRS": Temperature (mean and range), Rainfall (total, wettest and driest months), Seasons (how many and when), each with a figure from the graph.
Practice
Remember
Further reading
Books and resources for this unit
Sources and credits
The lesson text, climate graph, tables, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
