Global atmospheric circulation and why deserts sit where they do
Look at a world map of deserts. The Sahara, Arabian, Thar, Kalahari and Australian deserts all sit at roughly the same latitude on both sides of the equator. That is not a coincidence: it is the atmosphere's plumbing.
Part of your national curriculum
- Locational knowledge: Extend locational knowledge and deepen spatial awareness of the world's countries, focusing on Africa, Russia, Asia and the Middle East
- Human and physical geography: Understand weather and climate, including the change in climate from the Ice Age to the present
Lesson overview
What you'll learn in this lesson
Explain how circulation cells create the world's climate belts.
Key learning points
- • The engine: unequal heating
- • The three cells
- • Pressure and wind, in one rule
- • Why the UK's weather is so changeable
This lesson at a glance
- 25 minutes
- 20 parts to scroll through
- 4 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 20
Visual introductionPicture this
Global atmospheric circulation and why deserts sit where they do
Look at a world map of deserts. The Sahara, Arabian, Thar, Kalahari and Australian deserts all sit at roughly the same latitude on both sides of the equator. That is not a coincidence: it is the atmosphere's plumbing.
In a nutshell
Explain how circulation cells create the world's climate belts.
Learning cycle
Part 2 of 20
Learning cycle 1 of 2
Part 1 · The engine: unequal heating
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 3 of 20
Learn
The engine: unequal heating
The Sun's rays hit the equator almost vertically, concentrating energy on a small area. At the poles the same beam spreads across a wider area and passes through more atmosphere, so it delivers less energy. This surplus at the equator and deficit at the poles drives air and ocean movement that constantly tries, and fails, to even the difference out.
Reset break
Part 4 of 20
Pause
That's sitting 1 of 5 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Explore the idea
Part 5 of 20
Learn
The three cells
Hot air rises at the equator, cools, condenses and falls as heavy rain: the low pressure of the tropical rainforest belt. That air travels aloft to about 30 degrees north and south, where it sinks. Sinking air warms and dries, creating high pressure, clear skies and desert. Some of it returns to the equator as trade winds, completing the Hadley cell. The Ferrel and Polar cells repeat the pattern further north, giving wet, unsettled weather around 60 degrees and cold desert at the poles.
Quick check
Part 6 of 20
Quick check
Part 7 of 20
Reset break
Part 8 of 20
Pause
That's sitting 2 of 5 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Learning cycle
Part 9 of 20
Learning cycle 2 of 2
Part 2 · Pressure and wind, in one rule
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 10 of 20
Learn
Pressure and wind, in one rule
Rising air means low pressure, cloud and rain. Sinking air means high pressure, clear skies and dry weather. Wind blows from high to low pressure, and the Earth's rotation deflects it (the Coriolis effect), which is why trade winds blow from the north east in the northern hemisphere rather than straight south.
Explore the idea
Part 11 of 20
Learn
Why the UK's weather is so changeable
The UK sits near 55 degrees north, where warm tropical air meets cold polar air along the polar front. Depressions form on that boundary and are steered across us by the jet stream, so we get a rapid sequence of warm and cold fronts, showers and clear spells within days. The North Atlantic Drift also carries warm water north west, which is why Britain is far milder than Newfoundland at the same latitude.
Reset break
Part 12 of 20
Pause
That's sitting 3 of 5 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Quick check
Part 13 of 20
Quick check
Part 14 of 20
Explore the idea
Part 15 of 20
Worked example
Worked answer: explain why hot deserts are found at about 30 degrees north and south (4 marks)
Air that rose at the equator has already lost its moisture as rainfall there (1). It travels polewards at high altitude, cools, becomes denser and sinks at around 30 degrees (1). Sinking air warms as it compresses, so its capacity to hold water vapour increases and clouds cannot form (1). This gives persistent high pressure, clear skies and very low rainfall, which is the definition of a hot desert climate (1).
Reset break
Part 16 of 20
Pause
That's sitting 4 of 5 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Challenge round
Part 17 of 20
Game · Sort it
Which of these are true?
Drag each card into the right column. Tap a card first if dragging is fiddly.
True
Not true
Challenge round
Part 18 of 20
Game · Recall cards
Rising air at the equator produces
Card 1 of 4
Mastery quiz
Part 19 of 20
Marked quiz
End of lesson quiz: Global atmospheric circulation and why deserts sit where they do
4 questions, marked with the reasoning shown. No timer.
1. Rising air at the equator produces
2. Which cell links the equator and 30 degrees?
3. Why is the UK milder than places at the same latitude in Canada?
4. Sinking air is dry because
Lesson round-up
Part 20 of 20
Lesson round-up
Ready when you are
Quiz score
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Ask LunaPart 1 of 20 · Watch & discover
How this is going
- Not looked at yet
Extend locational knowledge and deepen spatial awareness of the world's countries, focusing on Africa, Russia, Asia and the Middle East
- Not looked at yet
Understand weather and climate, including the change in climate from the Ice Age to the present