Climate in Rwanda

Lessons

Senior 4 · Geography

Climate in Rwanda

3 lessons

Lesson 2 of 3

Factors that Influence Rwanda's Climate, and the Wet and Dry Seasons

25 min

Two puzzles

Lesson 1 left two questions. Why does Rwanda, almost on the equator, have two rainy seasons with dry seasons between them? And why does Nyamagabe receive nearly twice the rain of Kayonza, only about 150 km away? The answers lie in the factors that control climate.

Definition

Intertropical Convergence Zone (ITCZ): the belt of low pressure near the equator where the north-east and south-east trade winds meet; warm, moist air rises there, forming clouds and heavy rain. It moves north and south during the year, following the overhead sun with a lag.

Relief (orographic) rain: rain formed when moist air is forced to rise over high ground and cools. Rain shadow: the drier area on the lee side of high ground.

1. Latitude and the ITCZ: why there are two rainy seasons

Rwanda lies between 1° and 3° south of the equator. The midday sun is high all year, so temperatures change little from month to month (Lesson 1). Meteo Rwanda explains that Rwanda's bimodal rainfall is driven mainly by the ITCZ, which follows the sun as it crosses the equator around 21 March and 23 September. As the rain belt moves north it passes over Rwanda and brings the March–May rains; as it moves back south it brings the September–December rains. Between the crossings, the rain belt is far to the north (June–August) or far to the south (January–February), and Rwanda is drier.

Figure 4: a simplified model, not measured data. The sun is overhead at the equator around 21 March and 23 September; the ITCZ follows with a lag and passes over Rwanda twice a year (Meteo Rwanda). Real rains also depend on winds, relief and the oceans.

2. Altitude

Temperature falls by about 6.5 °C for every 1,000 m of height. This is why the Bugarama valley averages 23–24 °C while the highlands average less than 17 °C (Meteo Rwanda), and why REMA notes temperatures below 0 °C at times in parts of the volcanic region. Altitude is the main reason Rwanda's climate is mild, not hot.

3. Relief: relief rain and rain shadow

Figure 5: relief rain and rain shadow. Meteo Rwanda describes the central and eastern parts of the country as semi-arid because they lie in the rain shadow of the western highlands.

Worked example 1: why air is drier after crossing the ridge

Unsaturated air cools by about 9.8 °C per 1,000 m as it rises (the dry adiabatic lapse rate); once clouds form it cools more slowly, about 6 °C per 1,000 m, because condensation releases heat. Illustrative case: air at 23 °C over Lake Kivu (1,460 m) rises over the Congo–Nile Ridge (2,450 m), forming cloud from 1,800 m, then descends to the central plateau at 1,500 m.

1\,460 \to 1\,800\ \text{m}: \quad 23 - 9.8 \times 0.34 = 19.7\ ^\circ\text{C}

1\,800 \to 2\,450\ \text{m}: \quad 19.7 - 6 \times 0.65 = 15.8\ ^\circ\text{C}

2\,450 \to 1\,500\ \text{m (descending, no cloud)}: \quad 15.8 + 9.8 \times 0.95 = 25.1\ ^\circ\text{C}

The air arrives on the plateau warmer than it started and has lost much of its water as rain on the windward slopes. Warmer air can hold more water vapour before clouds form, so it brings little rain: a rain shadow.

4. Other factors

Factor	Effect on Rwanda's climate
Large water bodies	Meteo Rwanda links the large variation of rainfall to Rwanda's varied topography and the large water bodies near the country; lakes such as Kivu supply moisture and moderate temperatures along their shores
Winds	The trade winds that converge at the ITCZ bring moist air; local valley and lake breezes shape daily weather
Vegetation	Forests such as Nyungwe and Gishwati return moisture to the air by transpiration and keep the ground cool and humid
Ocean–atmosphere events	El Niño and changes in Indian Ocean temperatures alter the rains; Meteo Rwanda cites the floods of the 1997–98 El Niño
Human activity	Towns are warmer than the countryside (urban heat island); deforestation reduces local moisture; global greenhouse gases are changing the climate (Lesson 3)

The wet and dry seasons

Season	Months (approximate)	Characteristics
Long rains (itumba)	March to May	The heaviest rains of the year; April is the wettest month at most stations (Huye 212 mm)
Long dry season	June to August	Little or no rain (Huye 0.4 mm in July); clear skies, cool nights, dusty roads
Short rains (umuhindo)	September to December	Rains return; often heavy in October–November
Short dry season	December to February	Drier spell between the rains; less reliable in the west

REMA describes the same four seasons: a short rainy season from September to November, a longer one from March to May, a short dry season from December to February and a long dry season from June to August (REMA, 2009). The exact dates vary from year to year and from place to place.

Worked example 2: how much rain falls in the two rainy seasons?

Data (Huye, means 2015–2018, mm). March–May: 133.5, 212.2, 105.6. September–December: 56.2, 106.6, 143.4, 101.1. Annual total: 1,093.9.

\text{MAM} = 451.3\ \text{mm}\ (41.3\%), \qquad \text{SOND} = 407.3\ \text{mm}\ (37.2\%)

\frac{451.3 + 407.3}{1\,093.9} \times 100 = 78.5\%

More than three-quarters of Huye's rain falls in seven months of the year, which is why farming calendars are built around the two rainy seasons.

Warning

The ITCZ is a moving belt, not a fixed line. Do not write that "the ITCZ is over Rwanda in March". It is a broad, irregular zone of rising air whose position changes from day to day; its average movement explains the seasons, but individual years can be early, late, wet or dry.

Rueschhoff Teaches: "ITCZ Migration Animation". Watch the rain belt move north and south through the year, and count how many times it crosses a place just south of the equator, such as Rwanda.

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Practice

Define the ITCZ and explain how its movement gives Rwanda two rainy seasons.
Explain why Rwanda's temperatures are mild although it lies near the equator.
With a labelled diagram, explain the rain shadow in central and eastern Rwanda.
Air at 21 °C at 1,500 m rises dry to 2,000 m. Using 9.8 °C per 1,000 m, find its temperature.
Kayonza's mean rainfall (2015–2017) was 110.2, 130.3 and 52.6 mm in March–May and 68.6, 130.7, 132.6 and 41.7 mm in September–December, with an annual total of 787.1 mm. What percentage fell in the two rainy seasons?
Explain two ways in which Lake Kivu influences the climate of the western shore.
"Relief is more important than latitude in explaining Rwanda's climate." Discuss.

Show answersHide answers

The ITCZ is the low-pressure belt where the trade winds meet and moist air rises, giving rain. It follows the overhead sun north and south with a lag, crossing Rwanda on its way north (March–May rains) and on its way south (September–December rains).
Altitude: most of Rwanda is 1,500–2,500 m high, and temperature falls by about 6.5 °C per 1,000 m.
Moist air from the west is forced up the Congo–Nile Ridge, cools, condenses and rains on the windward slopes; it then descends over the central plateau and east, warming and drying, so those areas receive less rain (see Figure 5).
$21 - 9.8 \times 0.5 = 16.1\ ^\circ\text{C}$ .
MAM $= 293.1$ mm, SOND $= 373.6$ mm; $\frac{293.1 + 373.6}{787.1} \times 100 = 84.7\%$ .
It supplies moisture to the air, increasing humidity and rain nearby; water heats and cools slowly, so it moderates temperatures on the shore; lake breezes can trigger local storms.
Latitude explains the high sun, the small annual range and, through the ITCZ, the two rainy seasons. Relief explains why temperatures are mild and differ from place to place, and why the west is wetter than the rain-shadowed east. Both are essential: latitude sets the pattern through the year, relief sets the differences between places.

Quick check

Why does Rwanda have two rainy seasons a year?

Key points

Latitude: the sun is always high, so temperatures vary little; the ITCZ crosses Rwanda twice, giving rains in March–May and September–December (Meteo Rwanda).
Altitude makes the climate mild: about 6.5 °C cooler per 1,000 m.
Relief gives heavy relief rain on the western highlands and a rain shadow over the central plateau and east.
Water bodies, winds, vegetation, El Niño and human activity also matter.
Four seasons: long rains (March–May), long dry (June–August), short rains (September–December), short dry (December–February).

Sources and credits

Sources: Climatology of Rwanda, Rwanda Meteorology Agency (ITCZ, rain shadow, temperatures, El Niño); Rwanda State of Environment and Outlook Report, REMA, 2009 (seasons, temperatures); Statistical Yearbook 2019, NISR (station rainfall).

Figures 4 and 5, the worked examples (Worked example 1 illustrative) and the exercises were drawn and written by Ganzaa; Figure 5's ground profile is from NASA SRTM data via OpenTopoData.

Video: "ITCZ Migration Animation", Rueschhoff Teaches, embedded from YouTube.

1Two puzzles

21. Latitude and the ITCZ: why there are two rainy seasons

32. Altitude

43. Relief: relief rain and rain shadow

54. Other factors

6The wet and dry seasons