Global Atmospheric Circulation
Understanding global atmospheric circulation is essential for explaining why different parts of the world experience different weather patterns, and why tropical storms form where they do. This lesson covers the three-cell model, pressure belts, and surface winds — all key components of the AQA GCSE specification.
Why Does the Atmosphere Circulate?
The atmosphere circulates because of unequal heating of the Earth's surface by the Sun:
- At the equator, the Sun's rays hit the Earth at a high angle (close to 90 degrees), concentrating energy over a small area. This means the equator receives more heat per unit area.
- At the poles, the Sun's rays hit at a low angle, spreading energy over a larger area. The poles receive less heat per unit area.
- The atmosphere and oceans redistribute this heat from the equator towards the poles through circulation cells and ocean currents.
The Role of the Coriolis Effect
The Coriolis effect is caused by the Earth's rotation:
- Moving air is deflected to the right in the Northern Hemisphere.
- Moving air is deflected to the left in the Southern Hemisphere.
- This deflection explains why winds do not blow in a straight line from high pressure to low pressure — they curve.
Exam Tip: The Coriolis effect is the reason tropical storms rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. You may be asked to explain this in an exam question about tropical storms.
The Three-Cell Model
Global atmospheric circulation can be explained using a simplified three-cell model. Each hemisphere has three circulation cells:
The following diagram shows the three-cell model of atmospheric circulation:
graph TD
A[Equator — Intense heating] --> B[Warm air rises]
B --> C[Hadley Cell]
C --> D[Air sinks at ~30° N/S]
D --> E[Ferrel Cell]
E --> F[Air rises at ~60° N/S]
F --> G[Polar Cell]
G --> H[Air sinks at poles]
D --> I[Surface winds return to equator — Trade Winds]
F --> J[Polar Front — low pressure, rain]
1. Hadley Cell (0° – 30°)
- The largest cell, closest to the equator.
- Warm, moist air rises at the equator due to intense solar heating, creating an area of low pressure known as the Inter-Tropical Convergence Zone (ITCZ).
- As the air rises, it cools, and moisture condenses, producing heavy convectional rainfall — this is why tropical rainforests are found near the equator.
- The air moves poleward at high altitude, cooling further.
- At around 30° N and S, the air has cooled enough to sink, creating areas of high pressure.
- This sinking air is dry, producing hot desert climates (e.g. the Sahara Desert, the Arabian Desert).
- Surface winds blow back towards the equator as the trade winds (deflected by the Coriolis effect to become the NE and SE trade winds).
2. Ferrel Cell (30° – 60°)
- A mid-latitude cell driven by the Hadley and Polar cells.
- Surface winds blow from the subtropical high pressure (30°) towards the subpolar low pressure (60°).
- These winds are deflected by the Coriolis effect to become the prevailing westerlies (SW winds in the Northern Hemisphere).
- At around 60° N and S, warm air from the Ferrel cell meets cold air from the Polar cell, creating an area of low pressure and frontal rainfall.
- Air rises at 60°, splits, and flows back at high altitude to complete the cell.
3. Polar Cell (60° – 90°)
- The smallest cell, closest to the poles.
- Cold, dense air sinks at the poles, creating areas of high pressure.
- This cold air flows towards 60° as the polar easterlies.
- At around 60°, it meets the warmer air from the Ferrel cell, is forced to rise, creating low pressure and precipitation.
Pressure Belts and Surface Winds
The three-cell model creates alternating bands of high and low pressure around the Earth:
| Latitude | Pressure | Climate Effect | Surface Winds |
|---|
| 0° (Equator) | Low | Rising air, heavy rainfall, tropical rainforests | Trade winds converge here |
| 30° N and S | High | Sinking air, dry conditions, hot deserts | Trade winds flow towards 0° |
| 60° N and S | Low | Rising air, frontal rainfall, changeable weather | Westerlies flow towards 60° |
| 90° N and S | High | Sinking air, very cold, dry polar regions | Polar easterlies flow towards 60° |
How This Affects the UK
The UK sits at approximately 50–60° N, in the zone influenced by the Ferrel cell:
- The prevailing wind direction is from the south-west, bringing warm, moist air from the Atlantic Ocean.
- The UK experiences low pressure systems that bring frequent rainfall, especially in winter.
- The jet stream (a fast-flowing band of air at the boundary of the Ferrel and Polar cells) steers weather systems across the UK, causing our changeable weather.
Exam Tip: If asked why the UK has changeable weather, reference the jet stream, the prevailing south-westerly winds, and the low-pressure zone at 60°N. These are all linked to the three-cell model.
The Inter-Tropical Convergence Zone (ITCZ)
The ITCZ is the zone where the trade winds from the Northern and Southern Hemispheres converge near the equator. Key points:
- It is characterised by intense solar heating, rising air, and heavy convectional rainfall.
- The ITCZ shifts seasonally — moving north in the Northern Hemisphere summer (June–August) and south in the Southern Hemisphere summer (December–February).
- This seasonal shift causes wet and dry seasons in tropical regions and is linked to monsoon patterns.
- The ITCZ is important for understanding where tropical storms form — they develop over warm ocean water in the ITCZ zone.
Ocean Currents and Heat Transfer
Ocean currents work alongside atmospheric circulation to redistribute heat:
- Warm currents carry heat from the equator towards the poles (e.g. the Gulf Stream brings warm water from the Caribbean to north-west Europe, keeping the UK warmer than other countries at the same latitude).
- Cold currents carry cold water from the poles towards the equator.
- Together, atmospheric and oceanic circulation help to moderate global temperatures and create distinct climate zones.
Linking to Weather Hazards
Global atmospheric circulation directly explains:
- Where tropical storms form — over warm ocean water (above 27 °C) in the ITCZ zone, between 5° and 30° from the equator.
- Why deserts exist at 30° N and S — sinking dry air from the Hadley cell.
- Why the UK has changeable weather — position at the meeting point of the Ferrel and Polar cells.
- Why the equator is wet — intense solar heating causes rising air and convectional rainfall.