Climate in Rwanda

Lessons

Senior 4 · Geography

Climate in Rwanda

3 lessons

Lesson 1 of 3

Characteristics and Climatic Zones of Rwanda

25 min

A year of weather at Huye

Figure 1 shows four years of measurements at the Huye weather station in the Southern Province, published by NISR. Look first at the temperature panel, then at the rainfall panel. What do you notice about each?

Figure 1: a climate graph for Huye, drawn by Ganzaa as two panels sharing the months. Data: NISR, Statistical Yearbook 2019, tables 5.6.1–5.6.3 (means of the four years 2015–2018; mean temperature is the average of the mean maximum and mean minimum).

Two things stand out. First, temperature hardly changes: the monthly mean stays between 19.4 °C and 21.1 °C, an annual range of only 1.7 °C. Second, rainfall changes a great deal: it has two peaks, in April (212 mm) and November (143 mm), and almost none falls in July. This is the pattern of an equatorial highland climate: steady warmth because the sun is always high, moderated by altitude, and two rainy seasons.

Definition

Weather: the state of the atmosphere at a place for a short time (hours or days). Climate: the average weather of a place over a long period, usually 30 years.

Annual temperature range: the difference between the mean temperatures of the warmest and the coolest months. Bimodal rainfall: rainfall with two peaks (two rainy seasons) in a year.

Worked example 1: describing a climate from its data

Data (Huye, 2015–2018 means). Monthly rainfall: 109.5, 81.3, 133.5, 212.2, 105.6, 11.0, 0.4, 33.1, 56.2, 106.6, 143.4, 101.1 mm. Warmest month: September, 21.1 °C; coolest: April, 19.4 °C.

Solution. Annual rainfall: $\sum = 1\,094\ \text{mm}$ . Annual range: $21.1 - 19.4 = 1.7\ ^\circ\text{C}$ . Share of the year's rain falling from March to May:

\frac{133.5 + 212.2 + 105.6}{1\,093.9} \times 100 = \frac{451.3}{1\,093.9} \times 100 = 41.3\%

And from June to August: $\frac{11.0 + 0.4 + 33.1}{1\,093.9} \times 100 = 4.1\%$ . So more than two-fifths of the rain comes in one season, and the long dry season gets almost none.

Characteristics of Rwanda's climate

Mild temperatures for a country so close to the equator, because of altitude. REMA gives average annual temperatures of 16–20 °C, with 15–17 °C in high areas and below 0 °C at times in parts of the volcanic region (REMA, 2009).
Small annual temperature range, as at Huye.
Temperature varies with height: Meteo Rwanda gives annual means of 20–21 °C in the eastern lowlands and 23–24 °C in the Bugarama valley, 17.5–19 °C on the higher parts of the central plateau, and less than 17 °C in the highlands.
Bimodal rainfall, with rainy seasons centred on March–May and September–December (Meteo Rwanda).
Rainfall decreases from west to east: about 900 mm in the east and south-east to about 1,500 mm in the north-west (REMA, 2009). Meteo Rwanda describes the central and eastern parts as generally semi-arid, in the rain shadow of the western highlands.
Large variability from year to year, with floods (for example during the El Niño of 1997–98) and droughts (Meteo Rwanda).

Climatic zones

Because altitude controls temperature and also affects rainfall, Rwanda's climatic zones follow its relief. Figure 2 shades the country by altitude; Figure 3 compares rainfall at six stations.

Figure 2: drawn by Ganzaa from NASA SRTM 90 m elevations sampled every 0.05° (about 5.5 km), so small valleys and peaks are smoothed out. Boundary: geoBoundaries, CC BY 4.0; Natural Earth.

Figure 3: mean annual rainfall from NISR, Statistical Yearbook 2019, table 5.6.1, using only the years with all twelve months recorded (in brackets). Four years or fewer are too short for a climate average: treat the values as indications.

Climatic zone	Where	Temperature (Meteo Rwanda)	Rainfall
Warm lowlands of the rift (Bugarama)	Far south-west, about 900 m (REMA)	Warmest: 23–24 °C	Moderate
Warm, semi-arid eastern lowlands	East: Bugesera, Kayonza, Kirehe, Nyagatare	20–21 °C	Lowest, about 800–900 mm (Kayonza 787 mm, Bugesera 844 mm in Figure 3); long dry spells
Temperate central plateau	Centre and south: Kigali, Huye, Muhanga	About 17.5–21 °C	Moderate, about 1,000–1,200 mm (Huye 1,094 mm)
Cool, wet highlands	Congo–Nile Ridge, Nyungwe, Gishwati; the Lake Kivu side	Below 17 °C on the high ground	Highest, about 1,300–1,500 mm or more (Nyamagabe 1,438 mm)
Cold, wet volcanic highlands	Virunga volcanoes	Coldest; can fall below 0 °C (REMA)	High

Worked example 2: how cold is it on Karisimbi?

Problem. Air temperature falls with height at an average rate (the environmental lapse rate) of about 6.5 °C per 1,000 m. A station at 1,700 m has a mean temperature of 20 °C (illustrative). Estimate the mean temperature at the summit of Karisimbi, 4,507 m.

Solution.

\Delta h = 4\,507 - 1\,700 = 2\,807\ \text{m}, \qquad \Delta T = 6.5 \times \frac{2\,807}{1\,000} = 18.2\ ^\circ\text{C}

T_{\text{summit}} \approx 20 - 18.2 = 1.8\ ^\circ\text{C}

A mean close to freezing explains REMA's report of temperatures below 0 °C in parts of the volcanic region, only about 170 km south of the equator.

Warning

Do not call Rwanda's climate "equatorial". Meteo Rwanda points out that although Rwanda lies within the equatorial belt, its climate is not strictly of the equatorial rainy type: altitude makes it cooler, and it has distinct dry seasons. Describe it as a modified (highland) tropical climate.

Mr B: "How to Describe a Climate Graph, GEOGRAPHY BASICS". A step-by-step framework: overall statement, then temperature, then rainfall, with figures. Use the same steps on Figure 1. (His graphs put both on one chart; Figure 1 separates them into two panels.)

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Practice

Distinguish weather from climate.
Describe the climate of Huye from Figure 1, giving figures.
Kayonza's mean monthly rainfall (2015–2017) was 16.2, 69.9, 110.2, 130.3, 52.6, 16.6, 0.1, 17.6, 68.6, 130.7, 132.6, 41.7 mm. Calculate the annual total and the share falling from September to December.
Compare the annual rainfall of Nyamagabe and Kayonza and suggest two reasons for the difference.
Using a lapse rate of 6.5 °C per 1,000 m, estimate the temperature difference between Bugarama (about 900 m) and a ridge at 2,400 m.
Using Figure 2, describe the location of the zone above 2,100 m.
Why are four years of data not enough to define a climate? What would make Figure 3 more reliable?

Show answersHide answers

Weather: the state of the atmosphere for a short time; climate: the average weather over about 30 years.
Warm and even (monthly means 19.4–21.1 °C, range 1.7 °C, annual mean 20.1 °C); 1,094 mm a year, bimodal, with peaks in April (212 mm) and November (143 mm) and a dry season June–August (under 35 mm a month).
Total $= 787.1$ mm; September–December $= 68.6 + 130.7 + 132.6 + 41.7 = 373.6$ mm, which is $\frac{373.6}{787.1} \times 100 = 47.5\%$ .
Nyamagabe 1,438 mm, Kayonza 787 mm, almost twice as much. Nyamagabe is high on the Congo–Nile Ridge, where moist air is forced to rise (relief rain); Kayonza lies low in the east, in the rain shadow of the western highlands.
$6.5 \times \frac{2\,400 - 900}{1\,000} = 6.5 \times 1.5 = 9.8\ ^\circ\text{C}$ cooler on the ridge.
A north–south belt along the west of the country (the Congo–Nile Ridge from Gishwati to Nyungwe) and the volcanoes in the north-west.
Climate is a 30-year average; a few years may include unusually wet or dry years (2018 was very wet at many stations). A longer record, more complete months and more stations would make it reliable.

Quick check

At Huye the monthly mean temperature varies by only 1.7 °C through the year. Why?

Key points

Rwanda has a modified tropical highland climate: mild (about 16–20 °C), with a small annual range and bimodal rainfall (March–May and September–December).
Temperature falls with altitude (about 6.5 °C per 1,000 m): 23–24 °C at Bugarama, below 17 °C in the highlands, below 0 °C at times on the volcanoes.
Rainfall is highest in the west and north-west highlands (about 1,500 mm) and lowest in the semi-arid east (about 800–900 mm).
Climatic zones follow relief: rift lowlands, eastern lowlands, central plateau, Congo–Nile highlands, volcanic highlands.

Sources and credits

Data and sources: Statistical Yearbook 2019, NISR, tables 5.6.1–5.6.3 (station rainfall and temperatures); Climatology of Rwanda, Rwanda Meteorology Agency (Meteo Rwanda); Rwanda State of Environment and Outlook Report, REMA, 2009.

Figures 1–3 were drawn by Ganzaa from these data; Figure 2 from NASA SRTM elevations (public domain) via OpenTopoData, with boundaries from geoBoundaries (CC BY 4.0) and Natural Earth.

Photo: The Welcoming Mist, Dpiskho, CC BY-SA 4.0, via Wikimedia Commons.

Video: "How to Describe a Climate Graph, GEOGRAPHY BASICS", Mr B, embedded from YouTube. The rest of the lesson was written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A year of weather at Huye

2Characteristics of Rwanda's climate

3Climatic zones

A year of weather at Huye

Characteristics of Rwanda's climate

Climatic zones