What this lesson teaches
I can explain how global atmospheric circulation shapes tropical rainfall.
Syllabus 9173, Topic 2.1, 1. Tropical Climates. How does global atmospheric circulation affect rainfall in the tropics?
Make a guess
Many of the world's great deserts lie around 20 to 30 degrees north and south of the equator. Why there?
- The sun is weakest there.
- They are farthest from the sea.
- Air sinks there, warms and dries out.
- Rising air there carries moisture away.
Show the answer
Air sinks there, warms and dries out.
Air that rose at the ITCZ spreads polewards high up, then sinks in the subtropics. Sinking air warms as it is compressed, which stops clouds forming.
Rising moist air near the ITCZ brings rain, sinking air in the subtropics brings dryness, and both belts shift with the seasons.
High sun angles concentrate energy on a small area. So the tropics take in more solar energy than they lose, a heat surplus. This surplus drives air circulation that carries heat towards the poles. Global atmospheric circulation covers large parts of the Earth and includes lasting patterns like Hadley cells. Synoptic-scale circulation covers regional high- and low-pressure systems, such as tropical cyclones.
At the intertropical convergence zone (ITCZ), trade winds meet and moist air rises. As it rises, it expands and cools. Water vapour condenses into clouds and releases heat, which drives the air higher. So the ITCZ has frequent heavy rain.
High up, the air spreads towards the poles, then sinks in the subtropics. Sinking air warms and becomes drier, which stops clouds forming. This helps explain deserts there. At the surface, air flows back to the ITCZ as trade winds, turned westward by Earth's spin.
The ITCZ and Hadley cells shift towards the summer hemisphere, with a delay. Places under the rain belt most of the year are wetter. Places it reaches only in one season have wet and dry seasons.
- Heat surplus
- The tropics gain more energy than they lose.
- ITCZ
- Where trade winds meet and moist air rises: heavy rain.
- Subtropics
- Sinking air warms and dries: deserts.
Step by step
Heat surplus
Strong heating powers air movement.
Rise and sink
Rising air cools and rains; sinking air warms and dries.
Seasonal shift
The rain belt moves with the seasons.
Real examples to use
The Sahel's short wet season
In the Sahel, on the southern edge of the Sahara, most rain falls from about June to September, when the ITCZ moves north over West Africa. For the rest of the year sinking air and dry northeast winds bring a long dry season. A late or weak northward shift can mean drought.
Worked example: A savanna wet season
A place north of the equator gets summer rain when the ITCZ moves north. When the rain belt moves south, sinking air and drier winds take over. This explains the wet and dry seasons.
Watch out for this
The ITCZ stays exactly over the equator.
It shifts with the seasons and is not a perfect straight line.
Check your understanding
Why does sinking subtropical air often prevent rain?
- It expands and cools as pressure rises, so clouds clear.
- It warms as it is compressed, so its relative humidity falls.
- It carries the Sun's energy away from the subtropics.
Show the answer
It warms as it is compressed, so its relative humidity falls.
Yes. Warmer, drier air does not form clouds easily.