Atmospheric Circulation Lab
Why the world's hot deserts sit in a belt, why the equator is wet, and why the UK gets the weather it does. Three loops of moving air explain all of it.
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The sun does not share evenly 🌍
Sunlight arrives almost straight down at the equator, so its energy is concentrated on a small area. Near the poles the same beam is spread across a much larger area, and it arrives weaker. That imbalance would keep heating the tropics forever if nothing moved. Something does: the atmosphere and the oceans carry heat away from the equator, and the pattern they move it in explains where the world's rainforests and deserts are, and what kind of weather the UK gets.
Reading the three-cell model 🧭
The next diagram is a slice through the atmosphere from the **equator on the LEFT** to the **pole on the RIGHT**. The straight line across the bottom is the ground. Each loop is a **circulation cell**, and they are named from the equator outwards: **Hadley**, **Ferrel**, **Polar**. **The arrows tell you the rest.** Where a loop's arrow points **up**, air is rising, and rising air leaves **low pressure** at the surface. Where it points **down**, air is sinking, and sinking air presses down as **high pressure**. The circles on the ground mark those pressure points; nothing on the diagram tells you which is which, so work it out from the arrows.
Label the model 🏷️
An interactive activity.
Why a belt of deserts? 🏜️
The Sahara, the Arabian, the Kalahari and the Australian deserts all lie at roughly 30 degrees from the equator. Why?
- Air is sinking there, and sinking air warms and holds its moisture instead of releasing it
- It is the hottest latitude on Earth, so any rain evaporates
- They are too far from the ocean for moist air to reach
- The trade winds blow the clouds away before they can form
What each belt produces 🗂️
Four latitudes, four kinds of weather, one explanation. The last entry is the other half of the system, and the one most often forgotten:
Match each latitude to what you find there
- Around 0 degrees
- Around 30 degrees
- Around 60 degrees
- Around 90 degrees
- dense rainforest and heavy daily rainfall
- hot desert with clear skies almost all year
- changeable, unsettled weather with frequent low-pressure systems
- bitter cold with very little precipitation
Rising against sinking ↕️
Almost every question on this topic comes down to one of these two columns. Learn them as a pair.
Follow one loop 🪜
An interactive activity.
The mechanism 📝
The sun's energy is most concentrated at the _____, so the air there is heated and _____, giving low pressure and heavy rain. That air travels polewards high up, then _____ at about 30 degrees, giving high pressure and the world's hot _____. At the surface, the air returns towards the equator as the _____ winds.
What does the circulation explain? ✅
Select the TWO patterns that global atmospheric circulation directly explains.
- The band of rainforest running around the world near the equator
- The belt of hot deserts at roughly 30 degrees north and south
- The daily cycle of day and night
- The distribution of earthquakes along plate boundaries
Explain three places 🗺️
An interactive activity.
The summary 🖊️
Global atmospheric circulation moves heat and _____ away from the equator. There are _____ cells in each hemisphere: Hadley, Ferrel and Polar. Rising air gives _____ pressure and rain, while sinking air gives high pressure and clear skies. At about _____ degrees the rising air brings the unsettled weather the UK knows well. And heat is carried polewards by ocean _____ as well as by the atmosphere.
Your turn ✍️
An interactive activity.