Air in motion · 5 min · 4 steps
The global wind machine
Goal. Explain the pattern of the trade winds, the doldrums and the westerlies from uneven heating and the Earth's spin.
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Steps
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Predict
The Sun heats the equator most. What does the hot air there do?
- Sinks and spreads out
- Rises, cools and drops rain
- Flows straight to the poles along the ground
Show the answer
Rises, cools and drops rain. It rises. As it cools, its water vapour condenses into towering clouds and heavy rain.
Near the equator the trade winds of both hemispheres meet. The hot, moist air rises into a belt of thunderstorms, the Intertropical Convergence Zone, also known as the doldrums.
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The air that rose comes down again around 30° north and south. Sinking air is dry, which is why many great deserts, such as the Sahara, lie near these latitudes. From there the trade winds blow back towards the equator.
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The trade winds do not blow straight towards the equator. Because the Earth spins, moving air is turned to the right in the northern hemisphere and to the left in the southern: the Coriolis effect. So the northern trades blow from the north-east.
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Between about 30° and 60° the westerlies carry storms across the oceans. Compare the live particles with the labels: the real wind changes every day, while the belts are the average pattern.
The common mistake
Myth
Wind blows straight from high pressure into low pressure.
Fact. Air starts moving from high towards low pressure, but the Earth's spin turns it, so large-scale winds blow around highs and lows. Near the ground, friction lets the wind spiral slightly inwards into a low.
Check yourself
Question 1
Why are many deserts found near 30° north and south?
- The Sun is strongest there
- Air sinks there and is dry
- They are far from any sea
Show the answer
Air sinks there and is dry. Air that rose at the equator sinks again near 30°. Sinking air warms and dries, so clouds rarely form.
Question 2
In the northern hemisphere the trade winds blow from the…
- north-east
- south-west
- north-west
Show the answer
north-east. Air flowing towards the equator is turned to the right by the Coriolis effect, so it arrives from the north-east.
Question 3
What turns moving air to the right in the northern hemisphere?
- The Moon's gravity
- The Earth's spin (the Coriolis effect)
- Mountains
Show the answer
The Earth's spin (the Coriolis effect). The Coriolis effect comes from the Earth spinning beneath moving air. It turns winds to the right in the north and to the left in the south.
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Curriculum
- Taiwan 12-year curriculum (108 課綱), natural science
- Ib-Ⅳ-2 · Ib-Ⅳ-3 · EIb-Vc-5
- US Next Generation Science Standards
- MS-ESS2-6
- England national curriculum
- KS3 Geography: weather and climate