Climate Controls and Climate Graphs

25 min

Let's begin

Kigali lies only about 2° south of the Equator, yet its average temperature is a comfortable 21.7 °C. Aswan in southern Egypt receives on average about 2 mm of rain in a whole year; Makoua, on the Equator in the Republic of the Congo, receives about 1,640 mm. These numbers are not guesses: they are the official averages for 1991–2020 that each country's weather service sent to the World Meteorological Organization (WMO). In this unit you will use them to describe the climates of Africa and the world, and to explain how climate shapes people's lives. First, the tools: what controls a climate, and how to read a climate graph.

Definition

Climate: the average weather of a place over a long period, usually 30 years.

Climate normal: the 30-year average of a weather element, such as the mean temperature of each month from 1991 to 2020.

Annual range of temperature: the difference between the mean temperature of the warmest month and that of the coolest month.

What controls a climate?

Factor	How it works	African example
Latitude	Near the Equator the noon sun is high all year, so it is hot all year; towards the poles the sun is lower and seasons stronger.	Makoua (0°) is about 25 °C in every month; Tunis (37°N) ranges from 12 °C to 29 °C.
Altitude	Temperature falls by about 6.5 °C for every 1,000 m of height (unit 2).	Kigali (1,490 m) is about 3 °C cooler than Makoua (380 m).
Pressure belts and winds	Where winds meet and rise, along the Inter-Tropical Convergence Zone (ITCZ), rain falls; where air sinks, as near the tropics, skies are dry.	The Sahara lies under sinking air; the ITCZ brings the rainy seasons.
Distance from the sea	The sea warms and cools slowly, so coasts have smaller temperature ranges than inland places.	Coastal Tunis has a smaller range than inland desert stations.
Ocean currents	Warm currents warm and moisten coasts; cold currents cool the air above them and make it dry.	The cold Benguela Current helps keep the Namib coast a desert.
Relief	Mountains force air to rise and rain on the windward side, leaving a rain shadow (unit 2).	The Ethiopian Highlands and the Danakil Desert.

Definition

Inter-Tropical Convergence Zone (ITCZ): the belt near the Equator where the trade winds from the north and south meet. The warm, moist air rises, cools and gives heavy rain. The ITCZ moves north and south during the year, following the overhead sun, and takes the rain belt with it.

Figure 1. The approximate position of the ITCZ in January and July. Drawn by Ganzaa.

Places near the Equator, such as Kigali, are crossed by the ITCZ twice a year, as it goes north and as it comes back south, so they have two rainy seasons. Places near the tropics are reached only once, at the height of summer, so they have one rainy season, and places beyond the ITCZ's reach stay dry.

Royal Meteorological Society (MetLink): Sylvia Knight explains the Intertropical Convergence Zone, why air rises there and how it moves with the seasons. Watch for the link between the ITCZ and the rainy seasons of the tropics.

Reading a climate graph

A climate graph shows a place's average monthly rainfall as bars (read on the left scale) and its average monthly temperature as a line (read on the right scale). Always check which scale belongs to which.

Figure 2. Climate graph of Kigali Airport, 1991–2020. Drawn by Ganzaa from WMO data.

Worked example

Worked example 1: describing Kigali's climate. Use Figure 2.

Temperature: the warmest months are August and September (22.3 °C); the coolest is November (21.2 °C). Annual range: $22.3 - 21.2 = 1.1\,^\circ\text{C}$ , very small. Mean: 21.7 °C, warm but not hot.
Rainfall: total 953.5 mm. Wettest month April (135.5 mm); driest July (10.2 mm).
Seasons: two rainy seasons, March to May and October to December; a long dry season from June to August and a short, less marked drier spell in January and February.
Explanation: the small range comes from the low latitude; the mild temperature from the altitude (1,490 m); the two rainy seasons from the ITCZ passing over twice a year.

Worked example

Worked example 2: altitude check. Makoua is at 380 m with a mean temperature of 25.1 °C. Use the lapse rate to predict Kigali's mean temperature at 1,490 m, and compare it with the real value.

Height difference: $1490 - 380 = 1110\,\text{m}$ .
Expected fall: $6.5 \times \frac{1110}{1000} \approx 7.2\,^\circ\text{C}$ .
Prediction: $25.1 - 7.2 \approx 17.9\,^\circ\text{C}$ .

Comparison: the real mean is 21.7 °C, about 4 °C warmer than predicted. The 6.5 °C rule is an average for the free atmosphere. Temperatures at ground level also depend on cloud, rain and the shape of the land, and Makoua and Kigali are 1,600 km apart, so the rule gives only a rough estimate. Altitude explains why Kigali is cooler, not the exact size of the difference.

Warning

Two scales, one graph. The most common error is to read rainfall from the temperature scale or the other way round. Before you read any value, put your finger on the axis that matches: bars to the left scale (mm), line to the right scale (°C).

Tip

Describe a climate graph in this order: temperature (highest, lowest, range, mean), then rainfall (total, wettest and driest months, seasons), then explain each feature with a factor (latitude, altitude, ITCZ, sea, currents, relief).

Practice

What is the difference between weather and climate?
Name four factors that control the climate of a place.
Explain why temperatures near the Equator are high in every month.
Using Figure 2, give Kigali's total annual rainfall and its driest month.
Explain why Kigali has two rainy seasons.
Calculate the annual range of temperature at a station whose warmest month averages 28.7 °C and coolest month 12.3 °C.
How does a cold ocean current affect the climate of the coast beside it?
Kigali's mean temperature is 21.7 °C at 1,490 m. Estimate the mean temperature at a place 1,000 m higher on a nearby mountain.

Show answersHide answers

Weather is the condition of the atmosphere at a given time; climate is the average weather of a place over a long period, usually 30 years.
Any four: latitude, altitude, pressure belts and winds (ITCZ), distance from the sea, ocean currents, relief.
The noon sun is high in the sky all year, so the ground receives strong heating in every month.
953.5 mm; July (10.2 mm).
The ITCZ, which brings rain, follows the overhead sun: it passes over Rwanda going north (around March to May) and again coming south (around October to December).
$28.7 - 12.3 = 16.4\,^\circ\text{C}$ (these are Tunis's figures).
It cools the air above it, so the air is stable and holds little moisture: the coast is cool and dry, often foggy, as on the Namib coast beside the Benguela Current.
$21.7 - 6.5 \approx 15\,^\circ\text{C}$ (about 15.2 °C).

Sources and credits

Station data: WMO Climatological Standard Normals for 1991–2020, WMO Member Nations (2023), NCEI Accession 0253808, NOAA National Centers for Environmental Information. Kigali Airport data were submitted by Rwanda; Makoua data by the Republic of the Congo; Tunis data by Tunisia.
Intertropical Convergence Zone and Benguela Current: Wikipedia contributors, Wikipedia, CC BY-SA 4.0, adapted by Ganzaa.
Map outline: Natural Earth, public domain.

Video: MetLink: All about the Intertropical Convergence Zone (ITCZ), Royal Meteorological Society, embedded from YouTube.

Figures 1 and 2, the worked examples and the practice exercises are written by Ganzaa.

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

1What controls a climate?

2Reading a climate graph

What controls a climate?

Reading a climate graph