Weather and Climate
Senior 1 · Geography
Weather and Climate
Lesson 4 of 6
24 min
On the bus from Rusizi up through Nyungwe, or on a plane taking off from Kigali, your ears "pop". The air is pressing on the outside of your eardrums less and less as you climb. That push of the air is air pressure, and differences in it are what make the wind blow.
Definition
Atmospheric (air) pressure: the force per unit area exerted by the weight of the air above a place. It is measured with a barometer in hectopascals (hPa) or millibars (mb); 1 hPa = 1 mb. The average at sea level is about 1,013 hPa.
Figure 1. Air pressure falls with height, because there is less air above pressing down (values from the standard atmosphere; real readings vary a few hPa with the weather).
The higher you go, the less air there is above you, so pressure falls: Kigali's pressure is only about 84% of sea-level pressure, and on the summit of Karisimbi it is under 60%. The air is also thinner, which is why climbers breathe hard and why water boils below 100 °C on high ground.
Worked example 1: comparing pressures
What percentage of sea-level pressure (1,013 hPa) is the pressure at Kigali (about 850 hPa)?
So about 16% less air presses down on Kigali than on a beach at Mombasa. Near the ground, pressure falls by roughly 1 hPa for every 8 to 10 m you climb.
Pressure also changes with temperature. Where air is heated it expands and rises, so it presses down less: the pressure at the ground is low. Rising air cools and forms clouds, so low pressure usually brings cloud and rain. Where air cools it becomes denser and sinks: pressure is high, skies are usually clear and the weather dry.
Definition
Wind: air moving horizontally from an area of high pressure to an area of low pressure. The bigger the difference in pressure over a distance, the stronger the wind. Winds are named after the direction they blow from: a westerly wind blows from the west.
Figure 2. Wind is air moving from high to low pressure: the daily breezes of Lake Kivu.
Definition
Lake breeze and land breeze: local winds beside a large lake (or sea). By day the land heats faster than the water, so air rises over the land and cooler air flows in from the lake (a lake breeze). At night the land cools faster, and the breeze reverses (a land breeze). Example: the daily breezes along the shores of Lake Kivu.
Figure 3. A wind vane gives the direction the wind comes FROM; a cup anemometer gives its speed.
Important
The Beaufort scale: estimating wind speed without instruments
| Force | Name | Speed (km/h) | What you see |
|---|---|---|---|
| 0 | Calm | under 1 | Smoke rises straight up |
| 1 | Light air | 1–5 | Smoke drifts; vane does not move |
| 2 | Light breeze | 6–11 | Leaves rustle; wind felt on the face |
| 3 | Gentle breeze | 12–19 | Leaves and twigs move all the time |
| 4 | Moderate breeze | 20–28 | Dust and paper blow; small branches move |
| 5 | Fresh breeze | 29–38 | Small trees sway; small waves on Lake Kivu |
| 6 | Strong breeze | 39–49 | Large branches move; umbrellas hard to use |
| 7 | Near gale | 50–61 | Whole trees move; hard to walk into the wind |
| 8–9 | Gale, strong gale | 62–88 | Twigs break off; roof sheets may lift |
| 10–12 | Storm to hurricane | 89 and over | Trees uprooted; widespread damage |
Worked example 2: converting wind speeds
An automatic weather station reports a wind of 8 m/s during a storm. Convert it to km/h and give its Beaufort force.
1 m/s = 3,600 m per hour = 3.6 km/h, so
28.8 km/h is just above the moderate-breeze band (20–28), so it is force 5, a fresh breeze. Storms in Rwanda's rainy seasons can bring short gusts much stronger than this, strong enough to lift iron roof sheets.
Warning
Direction FROM, not TO. A "south-easterly" wind comes from the south-east and blows towards the north-west. The arrow of a wind vane points into the wind, towards where it comes from.
What causes wind to blow?, by the Met Office (2 min). Watch how uneven heating creates high and low pressure and sets the air moving between them.
Fishermen on Lake Kivu go out at night and return in the morning. Using Figure 2, explain how the night and daytime breezes might once have helped fishermen who used only paddles and sails.
Tip
"Hot air rises, low pressure lies; cool air sinks, high pressure thinks clear." Wind always blows from HIGH to LOW, and is named by where it comes FROM.
Practice
Remember
Sources and credits
Written by Ganzaa: the lesson text, pressure chart, breeze and instrument diagrams, tables, worked examples and practice. Pressures are standard-atmosphere values for each height; the Beaufort bands follow the World Meteorological Organization's scale, rounded to whole km/h.
