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Global atmospheric circulation
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Why are there rainforests at the equator and deserts at about 30° north and south? Global air circulation explains it.
Uneven heating
The Sun heats the equator more than the poles.
The atmosphere moves heat from the equator towards the poles in three loops (cells) in each hemisphere: the Hadley, Ferrel and Polar cells.
The atmosphere moves heat from the equator towards the poles in three loops (cells) in each hemisphere: the Hadley, Ferrel and Polar cells.
Low and high pressure
At the equator, hot air rises, creating low pressure. Rising air cools, condenses and makes heavy rain, so tropical rainforests grow.
At about 30° north and south, cool dry air sinks, creating high pressure. Little rain falls, so many hot deserts are found here.
At about 30° north and south, cool dry air sinks, creating high pressure. Little rain falls, so many hot deserts are found here.
Winds
Surface winds blow from high pressure to low pressure.
The trade winds blow from about 30° towards the equator. They are curved by the Earth's rotation.
The trade winds blow from about 30° towards the equator. They are curved by the Earth's rotation.
Why is the Sahara desert so dry?
- It lies close to 30° north.
- Air sinks there, creating high pressure.
- Sinking air warms and does not form clouds, so there is very little rain.
Answer: It lies in a zone of sinking air and high pressure.
Rising air at the equator gives low pressure and rain. Sinking air at about 30° gives high pressure and deserts. Three cells in each hemisphere move heat towards the poles.
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