G5: Global Atmospheric Circulation
The atmospheric circulation model, global pressure belts, surface winds and how they influence climate and weather patterns worldwide.
📋 What is Global Atmospheric Circulation?
Definition: Global atmospheric circulation is the large-scale movement of air across the Earth, driven by differences in temperature and pressure. It redistributes heat from the equator towards the poles and is the fundamental system that determines global weather patterns.
The Earth is unevenly heated by the Sun:
- The equator receives the most intense solar radiation because sunlight strikes the surface at a near-perpendicular angle
- The poles receive the least intense solar radiation because sunlight strikes at a low angle and spreads over a larger area
- This temperature difference drives the global atmospheric circulation system
Key Principle:
Warm air rises (creating low pressure at the surface)
Cool air sinks (creating high pressure at the surface)
Air flows from high pressure to low pressure (creating wind)
The circulation system is a giant heat engine transferring energy from the equator to the poles.
Warm air rises (creating low pressure at the surface)
Cool air sinks (creating high pressure at the surface)
Air flows from high pressure to low pressure (creating wind)
The circulation system is a giant heat engine transferring energy from the equator to the poles.
🔄 The Three-Cell Model
The global atmospheric circulation is organised into three convection cells in each hemisphere:
1. Hadley Cell (0°–30°)
- Intense solar heating at the equator causes warm, moist air to rise vigorously (convection)
- As the air rises, it cools and condenses, forming clouds and heavy rainfall — this creates the equatorial low-pressure belt and the wet tropical climate
- At the top of the atmosphere (tropopause), the rising air spreads north and south
- By about 30°N and 30°S, the air has cooled enough to sink back to the surface
- This sinking air creates the sub-tropical high-pressure belt, with warm, dry conditions — this is why the world's major deserts (Sahara, Arabian, Kalahari, Australian) are located at these latitudes
- At the surface, the air flows back towards the equator, but is deflected by the Earth's rotation (Coriolis effect) to create the trade winds
2. Ferrel Cell (30°–60°)
- This is an indirect cell — it is driven by the Hadley and Polar cells rather than by direct heating
- At 30°, some of the sinking air from the Hadley Cell flows polewards at the surface
- This air is deflected by the Coriolis effect to create the westerly winds
- At about 60°N and 60°S, this warm air meets cold polar air, forcing it to rise along the polar front
- This rising air creates the sub-polar low-pressure belt, bringing unsettled, wet weather (this is why the UK at about 55°N has a wet, changeable climate)
- The rising air flows poleward at altitude and equatorward at altitude, completing the cell
3. Polar Cell (60°–90°)
- Cold, dense air sinks at the poles, creating the polar high-pressure belt with cold, dry conditions
- This sinking air flows towards the equator at the surface
- Deflected by the Coriolis effect, these become the polar easterly winds
- At about 60°, this cold polar air meets warmer air from the Ferrel Cell and is forced to rise, creating the sub-polar low-pressure belt
- The rising air flows back towards the poles at altitude, completing the cell
| Cell | Latitude | Rises at | Sinks at | Surface winds |
|---|---|---|---|---|
| Hadley | 0°–30° | Equator (0°) | Sub-tropics (30°) | Trade winds (easterly) |
| Ferrel | 30°–60° | Sub-polar (60°) | Sub-tropics (30°) | Westerlies |
| Polar | 60°–90° | Sub-polar (60°) | Poles (90°) | Polar easterlies |
📊 Global Pressure Belts
The circulation cells create alternating belts of high and low pressure around the Earth:
Low-pressure belts occur where air is rising. Rising air cools, condenses and forms clouds, bringing rain and unsettled weather.
High-pressure belts occur where air is sinking. Sinking air warms and can hold more moisture, preventing cloud formation — bringing dry, settled weather.
High-pressure belts occur where air is sinking. Sinking air warms and can hold more moisture, preventing cloud formation — bringing dry, settled weather.
| Latitude | Pressure | Weather | Resulting climate |
|---|---|---|---|
| 0° (Equator) | Low | Rising air, heavy rainfall | Tropical rainforest (hot and wet) |
| 30° N/S | High | Sinking air, clear skies | Hot deserts (hot and dry) |
| 60° N/S | Low | Rising air, rainfall | Temperate/cool (mild and wet) |
| 90° (Poles) | High | Sinking air, dry | Polar (cold and dry) |
💨 The Coriolis Effect
The Coriolis Effect: Because the Earth rotates, moving air (and water) is deflected from a straight path. In the Northern Hemisphere, air is deflected to the right. In the Southern Hemisphere, air is deflected to the left. This is called the Coriolis effect and it explains why winds don't blow in straight lines from high to low pressure.
The Coriolis effect creates the major global wind belts:
- Trade winds (0°–30°) — Surface air flowing from 30° back to the equator is deflected east-to-west, creating north-easterly trade winds in the NH and south-easterly trade winds in the SH
- Westerlies (30°–60°) — Surface air flowing from 30° towards 60° is deflected west-to-east, creating south-westerly winds in the NH and north-westerly winds in the SH
- Polar easterlies (60°–90°) — Surface air flowing from the poles towards 60° is deflected east-to-west, creating north-easterly winds in the NH and south-easterly winds in the SH
🌧️ How Circulation Affects Climate
Example: Desert Locations at 30°
The Sahara Desert (North Africa), the Arabian Desert (Middle East), the Thar Desert (India), the Kalahari Desert (Southern Africa), and the Great Victoria Desert (Australia) are all located at approximately 30° north or south of the equator. This is because the sinking air in the Hadley Cell creates high pressure, which prevents cloud formation and rainfall. These deserts are a direct consequence of global atmospheric circulation.
Example: The UK's Climate at 50°–60°N
The UK lies at about 50°–60°N, in the Ferrel Cell. The prevailing wind is from the south-west (westerlies), bringing moist air from over the Atlantic Ocean. This air rises at the sub-polar low-pressure belt (60°N), cooling and condensing to form clouds and rain. This is why the UK has a mild, wet and changeable climate — it is directly explained by its position in the global atmospheric circulation system.
Seasonal Shifts
The pressure belts and wind belts shift north and south throughout the year as the Sun's position changes:
- In June/July (Northern Hemisphere summer), the Sun is overhead at the Tropic of Cancer — all belts shift north
- In December/January (Northern Hemisphere winter), the Sun is overhead at the Tropic of Capricorn — all belts shift south
- This shift explains seasonal changes, including the monsoon in South Asia — when the ITCZ (Inter-Tropical Convergence Zone) moves north in summer, it pulls in warm, moist air from the Indian Ocean, causing heavy rainfall
The Inter-Tropical Convergence Zone (ITCZ): The ITCZ is the zone where the trade winds from the Northern and Southern Hemispheres meet near the equator. It follows the position of the overhead Sun, migrating north in the Northern Hemisphere summer and south in the Southern Hemisphere summer. The ITCZ brings intense rainfall — when it reaches South Asia in summer, it triggers the monsoon season.
🌊 Ocean Currents and Atmospheric Circulation
Global atmospheric circulation drives and interacts with ocean currents:
- Trade winds push surface ocean water westwards near the equator, creating warm currents like the Gulf Stream
- Westerly winds drive ocean currents from west to east at mid-latitudes
- Warm ocean currents transported polewards warm the air above them, affecting the climate of nearby land — e.g. the Gulf Stream / North Atlantic Drift brings warm water to Western Europe, making the UK milder than expected for its latitude
- Where winds blow offshore, cold water rises from the deep ocean (upwelling), creating cold currents and affecting coastal climates
Example: Why the UK is Warmer Than Labrador
The UK (55°N) and Labrador in Canada (55°N) are at the same latitude, yet the UK has a much milder climate. Average January temperatures: London ~5°C, Labrador ~-15°C. This is because the warm North Atlantic Drift (an extension of the Gulf Stream) carries warm water from the Gulf of Mexico north-eastwards towards Western Europe, warming the prevailing south-westerly winds. Labrador has no such warm current — it receives cold water from the Labrador Current flowing south from the Arctic.
❓ Practice Questions
Q1: Explain why air rises at the equator and sinks at 30°N and 30°S. (4 marks)
Q2: Describe the relationship between global atmospheric circulation and the location of the world's major hot deserts. (3 marks)
Q3: What is the Coriolis effect and how does it influence global wind patterns? (4 marks)
Q4: Explain why the UK has a mild, wet climate. Refer to global atmospheric circulation in your answer. (4 marks)
Q5: Explain how the seasonal shift of pressure belts causes the South Asian monsoon. (4 marks)
Q6: Compare the weather conditions associated with low-pressure belts and high-pressure belts. (4 marks)
✅ Answers
- Air rises at the equator because intense solar heating warms the surface, causing warm air to become less dense and rise vigorously through convection. As it rises, it cools and spreads north and south at the top of the troposphere. By 30°N and 30°S, the air has cooled sufficiently to become denser, so it sinks back towards the surface. This sinking creates the sub-tropical high-pressure belt.
- The world's major hot deserts (Sahara, Arabian, Kalahari, Australian) are located at approximately 30°N and 30°S because this is where air in the Hadley Cell sinks. Sinking air warms adiabatically and can hold more moisture, preventing cloud formation and rainfall. The resulting high pressure creates hot, dry conditions — ideal for desert formation.
- The Coriolis effect is the deflection of moving air (and water) caused by the Earth's rotation. In the Northern Hemisphere, air is deflected to the right of its direction of travel; in the Southern Hemisphere, it is deflected to the left. This means that instead of blowing in straight lines from high to low pressure, winds curve. Trade winds (flowing from 30° to the equator) become north-easterlies in the NH and south-easterlies in the SH. Westerlies (flowing from 30° to 60°) become south-westerlies in the NH and north-westerlies in the SH. Polar easterlies (flowing from poles to 60°) become north-easterlies in the NH and south-easterlies in the SH.
- The UK lies at 50°–60°N in the Ferrel Cell, where the prevailing surface wind is from the south-west (the westerlies). These winds bring moist air from over the Atlantic Ocean. At about 60°N, this warm moist air meets cold polar air at the polar front and is forced to rise, creating the sub-polar low-pressure belt. The rising air cools, condenses and forms clouds and rain. Additionally, the warm North Atlantic Drift ocean current keeps the UK milder than expected for its latitude. Together, these atmospheric and oceanic circulation features give the UK its mild, wet and changeable climate.
- In the Northern Hemisphere summer (June–September), the Sun is overhead at the Tropic of Cancer. The ITCZ and all pressure belts shift northwards. As the ITCZ moves north over South Asia, it draws in warm, moisture-laden air from the Indian Ocean. This moist air rises, cools and condenses, producing the heavy monsoon rainfall. In winter, the ITCZ shifts south, and dry continental air from the high-pressure zone over the Asian landmass brings the dry season.
- Low-pressure belts occur where air is rising. Rising air cools, condenses and forms clouds, resulting in rainfall and unsettled, changeable weather. The equatorial low pressure brings heavy, daily rainfall (tropical rainforest climate). The sub-polar low pressure at 60° brings frequent rain and storms (temperate climate). High-pressure belts occur where air is sinking. Sinking air warms, can hold more moisture, and prevents cloud formation, resulting in dry, settled, clear weather. The sub-tropical high at 30° brings hot, dry conditions (desert climate). The polar high at 90° brings cold, dry conditions (polar climate).
🎯 Exam Tips
- You must be able to draw and label a cross-section of the global atmospheric circulation model showing all three cells, pressure belts and surface winds
- Learn the three cells (Hadley, Ferrel, Polar) and the latitude boundaries (0°, 30°, 60°, 90°)
- Remember: low pressure = rising air = rain; high pressure = sinking air = dry
- Link the circulation model to real-world climate zones — deserts at 30°, rainforests at 0°, temperate at 50°–60°
- The Coriolis effect deflects RIGHT in the NH and LEFT in the SH — never say it "causes" wind, it only deflects it
- Seasonal shifts explain monsoons — the ITCZ follows the overhead Sun
📝 Exam Technique
Global Atmospheric Circulation Exam Tips:
1. Practise drawing and labelling the full three-cell cross-section diagram: label each cell (Hadley 0°–30°, Ferrel 30°–60°, Polar 60°–90°), all four pressure belts, and all three surface wind belts — this is one of the most common exam tasks.
2. When explaining climate patterns, always follow the chain: cell name → air rising or sinking → pressure belt → weather result. E.g. 'Hadley cell: air sinks at 30° → high pressure → no cloud/rain → desert.' Don't skip steps.
3. For questions about the ITCZ and monsoons, emphasise the seasonal shift: 'In summer, the ITCZ moves north over South Asia, drawing in moist air from the Indian Ocean, causing the monsoon.'
4. Never say the Coriolis effect 'causes' wind — it only deflects wind that already exists due to pressure differences. Write 'deflects' not 'creates.'
1. Practise drawing and labelling the full three-cell cross-section diagram: label each cell (Hadley 0°–30°, Ferrel 30°–60°, Polar 60°–90°), all four pressure belts, and all three surface wind belts — this is one of the most common exam tasks.
2. When explaining climate patterns, always follow the chain: cell name → air rising or sinking → pressure belt → weather result. E.g. 'Hadley cell: air sinks at 30° → high pressure → no cloud/rain → desert.' Don't skip steps.
3. For questions about the ITCZ and monsoons, emphasise the seasonal shift: 'In summer, the ITCZ moves north over South Asia, drawing in moist air from the Indian Ocean, causing the monsoon.'
4. Never say the Coriolis effect 'causes' wind — it only deflects wind that already exists due to pressure differences. Write 'deflects' not 'creates.'
⚠️ Common Errors
Watch Out!
Students often think The Ferrel cell is driven by direct heating like the Hadley cell. Wrong: The Ferrel cell is driven by direct heating like the Hadley cell Correct: The Ferrel cell is an indirect cell — it is mechanically driven by the Hadley and Polar cells, not by direct solar heating. Air sinks at 30° because of the Hadley cell's descending air, and rises at 60° because cold polar air undercuts warmer Ferrel cell air at the polar front.
Students often think The Coriolis effect makes wind blow from high to low pressure. Wrong: The Coriolis effect makes wind blow from high to low pressure Correct: The pressure gradient makes air move from high to low pressure. The Coriolis effect only deflects this moving air — to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection is what creates the trade winds and westerlies rather than straight-line winds.
Students often think Deserts are always hot because they are near the equator. Wrong: Deserts are always hot because they are near the equator Correct: Major hot deserts like the Sahara are at 30° north and south, NOT at the equator. The equator has tropical rainforest because the Hadley cell's rising air creates low pressure and heavy rainfall. Deserts form at 30° where the Hadley cell's sinking air creates high pressure and dry conditions.
✍️ Model Answer
Full-Mark Response
9 marks: Explain how global atmospheric circulation influences the climate of two contrasting regions of the world. Use named examples.
Global atmospheric circulation directly determines climate by controlling where air rises (creating low pressure and rainfall) and where air sinks (creating high pressure and dry conditions). The Sahara Desert (approximately 30°N) has a hot, arid climate because it sits beneath the descending limb of the Hadley cell. Air that rose at the equator, carrying moisture, has cooled and spread poleward at altitude. By 30°N, this air sinks back towards the surface, warming adiabatically as it descends. This warming increases the air's capacity to hold moisture, preventing cloud formation and rainfall. The resulting persistent subtropical high-pressure belt gives the Sahara its average annual rainfall of less than 25 mm and temperatures regularly exceeding 40°C. In contrast, the UK (approximately 55°N) has a mild, wet and changeable climate because of its position in the Ferrel cell. The prevailing surface wind is the south-westerly, driven by the pressure gradient between the subtropical high at 30°N and the sub-polar low at 60°N. These westerlies carry moist air from over the Atlantic Ocean. At 60°N, this warm moist air meets cold polar air at the polar front and is forced to rise, creating the sub-polar low-pressure belt with its associated clouds, rainfall and unsettled weather. Additionally, the warm North Atlantic Drift (an extension of the Gulf Stream, driven by atmospheric circulation patterns) keeps the UK milder than expected for its latitude — London (55°N) averages 5°C in January compared to Labrador in Canada (also 55°N) which averages -15°C. These two contrasting regions demonstrate that global atmospheric circulation, not just latitude, determines climate.
Mark scheme: 3 marks for explaining one region's climate using cell/pressure belt mechanism, 3 marks for explaining the contrasting region with mechanism, 3 marks for clear links to named atmospheric circulation features with specific data
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge): Know the three circulation cells, four pressure belts (0° low, 30° high, 60° low, 90° high), three wind belts and the ITCZ. AO2 (Understanding): Explain the physical mechanism — why rising air creates low pressure and rain, why sinking air creates high pressure and drought, and how the Coriolis effect deflects winds. AO3 (Analysis): Link atmospheric processes to real-world climate zones and explain causal chains. Grade 9 answers use precise terms like 'adiabatic warming', 'polar front' and 'subtropical high', and connect the ITCZ migration to the South Asian monsoon with specific seasonal detail.
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