Climate and weather systems: global atmospheric circulation and tropical storms
OCR J383 Paper 2 tests weather hazards with 4-mark describe and 8-mark evaluate questions. You must understand why tropical storms form AND be able to evaluate their impacts and management responses.
Global atmospheric circulation
The uneven heating of the Earth drives a global pattern of air circulation called the general circulation model:
Three circulation cells (per hemisphere)
| Cell | Location | Characteristic |
|---|---|---|
| Hadley Cell | 0°–30° N/S | Hot air rises at equator (ITCZ), cools, descends at 30° → hot deserts |
| Ferrel Cell | 30°–60° N/S | Surface winds blow poleward; interacts with polar and Hadley cells |
| Polar Cell | 60°–90° N/S | Cold, dense air sinks at poles; flows toward 60° where it meets warmer air |
Inter-Tropical Convergence Zone (ITCZ)
- Hot, moist air converges at the equator; rises, cools and condenses → heavy rainfall (tropical rainforest climates).
- As the air rises it diverges at altitude toward the poles → sinks at ~30°N/S → hot deserts (Sahara, Australian Outback).
Pressure belts and prevailing winds
- Low pressure (rising air, precipitation): at equator and 60°N/S.
- High pressure (sinking air, dry): at 30°N/S and poles.
- Trade winds: blow from high-pressure subtropical zones (~30°) toward the equatorial low — deflected by the Coriolis effect.
- Westerlies: blow from 30° toward 60° latitudes — produce the changeable weather of the UK.
Tropical storms (hurricanes / cyclones / typhoons)
Conditions for formation
Tropical storms need ALL of the following:
- Ocean temperature ≥26°C to a depth of ~50 m — provides heat energy and water vapour.
- Low wind shear (little change in wind speed/direction with altitude) — allows the storm to develop vertically.
- At least 5°–8° latitude from the equator — the Coriolis effect is strong enough to spin the storm.
Structure
- Eye: calm centre, ~50 km wide; low pressure; light winds; clear skies.
- Eyewall: most intense zone surrounding the eye; strongest winds (150–300 km/h); heaviest rainfall.
- Spiral rainbands: extend outward; heavy rain; gusty winds.
- The storm rotates anticlockwise in the Northern Hemisphere; clockwise in the Southern Hemisphere (Coriolis effect).
Energy source and movement
- Storm draws energy from warm ocean surface water (latent heat of evaporation/condensation).
- Moves westward initially (trade winds), then curves poleward as it enters the westerlies.
- Weakens (dissipates) when it moves over cooler ocean water or land (cuts off energy supply).
Naming and intensity (Saffir-Simpson Scale)
| Category | Wind speed | Damage |
|---|---|---|
| 1 | 119–153 km/h | Minimal |
| 2 | 154–177 km/h | Moderate |
| 3 | 178–208 km/h | Extensive |
| 4 | 209–251 km/h | Extreme |
| 5 | >252 km/h | Catastrophic |
Common OCR exam mistakes
- Saying tropical storms form at the equator — they need the Coriolis effect; formation requires 5°+ from equator.
- Confusing the eye and eyewall — the eye is calm; the eyewall has the strongest winds.
- Forgetting that storms weaken over land or cold water — the heat source is removed.
- Not linking global circulation to tropical storm tracks — trade winds steer storms westward initially.
AI-generated · claude-opus-4-7 · v3-ocr-geography