Weather and Climate
Senior 1 · Geography
Weather and Climate
Lesson 2 of 6
24 min
In Kigali, school starts in cool air, often around 16 °C, and by early afternoon it is close to 27 °C. Yet the average temperature hardly changes from January to December. Temperature tells two stories: how it changes through each day, and how it changes (or does not) through the year. To tell them, we need two numbers every day: the highest and the lowest.
Definition
Temperature: how hot or cold the air is, measured with a thermometer in the shade, in degrees Celsius (°C). The Sun heats the ground, and the ground heats the air above it.
A weather observer cannot watch a thermometer all day. Instead, the Stevenson screen holds a maximum and minimum thermometer, which remembers the highest and lowest temperatures since it was last reset. The best-known type is Six's thermometer.
Figure 1. A Six's thermometer records the highest and lowest temperatures since it was last reset (schematic).
Definition
Maximum and minimum thermometer: a thermometer that records the highest (maximum) and lowest (minimum) temperatures reached since it was last reset, usually over 24 hours. In Six's thermometer, small steel indices are pushed by the mercury and stay at the furthest points; they are read at their lower ends and reset with a magnet.
| Term | How it is calculated |
|---|---|
| Mean daily temperature | |
| Daily (diurnal) range | maximum minimum |
| Mean monthly temperature | sum of the mean daily temperatures number of days |
| Annual range | mean of the hottest month mean of the coldest month |
| Mean annual temperature | sum of the 12 monthly means |
Worked example 1: one day in Figure 1
The indices show a maximum of 27 °C and a minimum of 16 °C.
Worked example 2: a week at a school weather station
| Day | Mon | Tue | Wed | Thu | Fri |
|---|---|---|---|---|---|
| Maximum (°C) | 27 | 28 | 25 | 24 | 26 |
| Minimum (°C) | 16 | 17 | 16 | 15 | 16 |
| Mean (°C) | 21.5 | 22.5 | 20.5 | 19.5 | 21.0 |
(a) Mean temperature for the five days:
(b) Highest and lowest temperatures of the week: 28 °C (Tuesday) and 15 °C (Thursday), so the range for the week is .
These are example readings of the kind a school station records.
Worked example 3: Kigali's annual range
At Kigali Airport the warmest months (August and September) have a mean of 22.3 °C and the coolest (November) 21.2 °C (1991–2020 averages).
The daily range in Kigali is about 11 °C, ten times the annual range. Near the Equator, "night is the winter of the tropics": the difference between day and night is far bigger than between the seasons.
| Factor | Effect | Example |
|---|---|---|
| Latitude | Near the Equator the noon Sun is high all year, so its heat is concentrated; towards the poles the Sun is low and its heat is spread out | Kigali is warmer than Johannesburg's winter |
| Altitude | Air cools by about 6.5 °C for every 1,000 m of height | Nyungwe and the Virungas are cold; the Bugarama plain is hot |
| Distance from the sea | Water warms and cools slowly, so coasts have smaller ranges than places far inland | Mombasa's temperatures are more even than Nairobi's |
| Cloud cover | Clouds block sunshine by day and trap heat at night: cloudy days are cooler, cloudy nights warmer | Clear dry-season nights in July bring Kigali's lowest minima |
| Aspect | Slopes facing the Sun are warmer than shaded slopes | Important on steep hills and in deep valleys |
| Winds and air masses | Winds bring warm or cold air from elsewhere | Cool air drains down valleys at night |
| Vegetation and towns | Forests shade the ground and stay cooler; concrete and roofs store heat | City centres are warmer than nearby farmland |
Definition
Lapse rate: the rate at which air temperature falls with height, on average about per 1,000 m (0.65 °C per 100 m).
Worked example 4: a climb up Bisoke
A group leaves the park entrance at about 2,500 m, where it is 17 °C, and climbs to the crater rim of Mount Bisoke at about 3,700 m. Estimate the temperature at the rim.
That is why guides tell visitors to bring warm clothes, even though they are only about 160 km from the Equator.
Important
Case study: temperature across Rwanda
Rwanda is close to the Equator, so the Sun is strong all year, but the country is high. Altitude therefore controls its temperatures: the low Bugarama plain and parts of the eastern lowlands are the warmest places, Kigali and the central plateau are mild, and the Congo–Nile ridge and the Virungas are cool, with frost possible on the highest summits. Because the annual range is so small, Rwandans describe the year by its rains, not by hot and cold seasons.
Ask the Bureau: How is temperature measured?, by the Bureau of Meteorology, Australia (2 min). Watch what is inside the white box and why it is built the way it is.
Two students measure the air temperature at the same time: one holds a thermometer in the sun on the concrete assembly ground and reads 34 °C; the other reads 26 °C in the school's Stevenson screen. Which reading is the air temperature, and why do they differ so much?
Tip
Mean = add and halve. Range = biggest minus smallest. Always give the unit (°C), and for the lapse rate remember "6.5 per thousand".
Practice
Remember
Sources and credits
The lesson text, Six's thermometer diagram, 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.
