What you'll learn
Weather hazards and climate change are linked by the way the atmosphere moves heat around the planet. This guide covers what you need for Edexcel GCSE Geography: the global atmospheric circulation and how it creates the world's climate zones, natural climate change in the past and the evidence for it, human-enhanced climate change and its effects, how tropical cyclones form and the hazards they bring, why their impacts differ between countries, and how countries prepare for and respond to them. It uses Typhoon Haiyan (Philippines, 2013) and Hurricane Katrina (USA, 2005) as contrasting examples.
Typical questions ask you to explain how a tropical cyclone forms, to describe evidence for climate change, to explain why the effects of storms differ, and to evaluate strategies for reducing their impacts. Detailed, specific examples and clear sequences of processes earn the highest marks.
Key terms and definitions
Global atmospheric circulation — The large-scale movement of air around the Earth that transfers heat from the equator towards the poles.
Hadley, Ferrel and polar cells — The three loops of rising and sinking air in each hemisphere.
High pressure — Sinking air, usually bringing dry, settled weather.
Low pressure — Rising air, usually bringing cloud and rain.
Natural climate change — Changes in climate caused by natural factors such as orbital changes, solar output and volcanic eruptions.
Enhanced greenhouse effect — The increase in the natural greenhouse effect caused by human activities releasing extra greenhouse gases.
Greenhouse gases — Gases such as carbon dioxide, methane and nitrous oxide that trap heat in the atmosphere.
Tropical cyclone — An intense low-pressure weather system with very strong winds, called a hurricane, typhoon or cyclone depending on where it forms.
Storm surge — A rise in sea level caused by low pressure and strong winds pushing water onto the coast.
Eye — The calm, clear centre of a tropical cyclone, surrounded by the eyewall.
Saffir–Simpson scale — The scale used to categorise hurricanes from 1 to 5 by wind speed.
Core concepts
Global atmospheric circulation
The Sun heats the equator more strongly than the poles. The atmosphere transfers this heat through three cells in each hemisphere:
- Hadley cell — at the equator, warm air rises, creating low pressure and heavy rainfall (tropical rainforests). The air cools, moves towards the poles and sinks at about 30° north and south, creating high pressure and dry conditions (hot deserts such as the Sahara).
- Ferrel cell — between about 30° and 60°, air moves at the surface towards the poles; at about 60° it meets cold polar air and rises, creating low pressure and unsettled, wet weather (such as in the UK).
- Polar cell — cold air sinks at the poles, creating high pressure and dry, very cold conditions.
Surface winds blow from high to low pressure and are curved by the Earth's rotation, creating the trade winds and the westerlies. Ocean currents also redistribute heat — the North Atlantic Drift keeps the UK milder than other places at the same latitude.
Natural climate change
The Earth's climate has changed many times, including glacial periods and warmer interglacial periods. Natural causes include:
- Orbital (Milankovitch) changes — the shape of Earth's orbit, the tilt of its axis and its wobble change over tens of thousands of years, altering how much solar energy reaches different parts of the Earth.
- Solar output — the amount of energy from the Sun varies slightly, for example with sunspot cycles.
- Volcanic activity — large eruptions release ash and sulphur dioxide that reflect sunlight, cooling the climate for a year or two.
Evidence for climate change
- Ice cores — air bubbles trapped in layers of ice show past carbon dioxide levels and temperatures over hundreds of thousands of years.
- Tree rings — wider rings indicate warmer, wetter years.
- Historical records — diaries, paintings and records of harvests and frost fairs, such as those on the frozen Thames during the Little Ice Age.
- Sea-level and glacier records — retreating glaciers and rising sea levels in recent decades.
- Temperature records — global average temperatures have risen since the late nineteenth century.
Human-enhanced climate change
The natural greenhouse effect keeps the Earth warm enough for life: greenhouse gases absorb heat radiated from the Earth's surface. Human activities increase the concentration of these gases:
- burning fossil fuels for energy, transport and industry releases carbon dioxide;
- deforestation reduces the number of trees absorbing carbon dioxide;
- farming releases methane from cattle and rice paddies and nitrous oxide from fertilisers;
- landfill releases methane.
This enhanced greenhouse effect traps more heat, raising global temperatures.
Effects include rising sea levels (from thermal expansion and melting ice), more frequent extreme weather such as heatwaves, droughts and floods, changes to ecosystems and species distributions, reduced crop yields in some regions, and threats to low-lying coastal communities and island nations. In the UK, effects may include hotter, drier summers, wetter winters and more coastal flooding.
How tropical cyclones form
Tropical cyclones need:
- warm ocean water of at least about 26.5°C, to a depth of around 50 m;
- a location roughly 5–30° from the equator, where the Coriolis effect from the Earth's rotation is strong enough to make the storm spin (it is too weak right at the equator);
- low wind shear, so the storm is not torn apart;
- moist air.
Formation sequence:
- Warm, moist air rises rapidly above the warm ocean, creating low pressure at the surface.
- As the air rises it cools and condenses, forming huge cumulonimbus clouds and releasing latent heat, which powers further rising.
- More air rushes in to replace the rising air, and the Coriolis effect makes it spin.
- The system grows into a spiralling storm with an eye of calm, sinking air surrounded by an eyewall of the strongest winds and heaviest rain.
- The storm moves with prevailing winds, usually westwards, and may curve away from the equator.
- When it reaches land or cooler water, it loses its supply of warm, moist air and weakens.
Hazards of tropical cyclones
- High winds — can exceed 250 km/h in the strongest storms, destroying buildings and trees.
- Storm surges — often the deadliest hazard, flooding low-lying coasts.
- Heavy rainfall — causing river flooding and landslides.
- Coastal flooding — from surges combined with large waves.
Contrasting case studies
Typhoon Haiyan, Philippines, 2013. One of the strongest tropical cyclones ever recorded to make landfall, Haiyan struck the central Philippines in November 2013. A storm surge of several metres swept into the city of Tacloban. More than 6,000 people were killed, millions were affected and around a million homes were damaged or destroyed. Electricity, water and communication networks were cut, and many people lost their livelihoods in farming and fishing. Although the government had evacuated hundreds of thousands of people, many did not understand the danger of the storm surge. The Philippines relied heavily on international aid for food, water, shelter and medical care, and recovery took years.
Hurricane Katrina, USA, 2005. Katrina struck the Gulf Coast in August 2005. In New Orleans, much of which lies below sea level, the storm surge caused levees (flood walls) to fail, flooding around 80% of the city. Around 1,800 people died. Hundreds of thousands were displaced and the economic damage was one of the highest of any US disaster. Many people, often the poorest and those without cars, could not evacuate despite warnings, and the emergency response was widely criticised as slow and badly coordinated. Nevertheless, the USA had the resources to rebuild infrastructure and strengthen flood defences afterwards.
Comparing them: Haiyan caused more deaths, reflecting poorer housing, less awareness of storm surge risk and limited resources to respond. Katrina caused greater economic losses because of the high value of property and infrastructure in the USA. Both show that preparation and the quality of the response strongly influence the impacts, and that even wealthy countries can be badly affected when defences fail and the poorest residents are left behind.
Preparing for and responding to tropical cyclones
Prediction and monitoring. Satellites, aircraft and computer models track storms and forecast their path and strength, giving several days of warning.
Warning and evacuation. Clear warnings through TV, radio, phone alerts and sirens; evacuation plans and shelters; making sure warnings explain specific dangers such as storm surges.
Defences and planning. Sea walls, levees and storm-surge barriers; building codes for wind-resistant buildings; raising buildings on stilts; restricting building in the most exposed coastal areas; protecting mangroves that absorb wave energy.
Response. Search and rescue, emergency shelter, clean water, food and medical care, followed by longer-term rebuilding, ideally to higher standards ("building back better").
Worked examples
Example 1: Formation
Explain how a tropical cyclone forms. (4 marks)
Answer: Warm ocean water above 26.5°C heats the air above it, which rises rapidly, creating low pressure (1). The rising air cools and condenses, forming cumulonimbus clouds and releasing heat that powers the storm (1). Air rushes in to replace the rising air and is made to spin by the Coriolis effect (1). The storm grows into a spiralling system with a calm eye, and weakens when it reaches land or cooler water (1).
Example 2: Evidence
Describe two pieces of evidence for climate change. (4 marks)
Answer: Ice cores contain trapped air bubbles that show past carbon dioxide levels and temperatures (1), revealing changes over hundreds of thousands of years (1). Tree rings vary in width (1), with wider rings indicating warmer, wetter growing conditions (1).
Example 3: Explaining different impacts
Explain why Typhoon Haiyan caused more deaths than Hurricane Katrina.
Answer: Many homes in the Philippines were weak and could not withstand the winds or surge, and many people did not understand the danger of a storm surge, so they did not evacuate to safety. The Philippines had fewer resources for emergency response and relied on international aid, which took time to arrive.
Common mistakes and how to avoid them
- Confusing weather and climate. Weather is day-to-day; climate is the long-term average.
- Saying tropical cyclones form at the equator. The Coriolis effect is too weak there.
- Mixing up the natural and enhanced greenhouse effect.
- Listing impacts without explaining why they differ.
- Forgetting storm surges, often the deadliest hazard.
Exam technique for "Weather Hazards and Climate Change"
- For formation questions, give conditions and then a clear sequence.
- In case study answers, include dates, places and specific impacts.
- When comparing countries, link differences to development, preparation and response.
- For evaluations of strategies, consider effectiveness, cost and who benefits.
Quick revision summary
- Global circulation: Hadley, Ferrel and polar cells create rainy equator, dry subtropics, wet mid-latitudes.
- Natural climate change: orbital changes, solar output, volcanoes; evidence from ice cores, tree rings, historical records.
- Enhanced greenhouse effect: fossil fuels, deforestation, farming; effects include sea-level rise and extreme weather.
- Tropical cyclones need warm water above 26.5°C, 5–30° latitude, low wind shear; hazards are winds, surges, rain and flooding.
- Haiyan 2013 (Philippines, over 6,000 deaths) vs Katrina 2005 (USA, around 1,800 deaths, levees failed).
- Preparation: forecasting, warnings, evacuation, defences, building codes, mangroves; response and rebuilding.