IGCSE-Hazardous Environments 8-Tropical Cyclones

Match

Match

• Natural Disaster

• Natural Hazard

• Natural event

Catastrophic event caused by nature.

Potential threat from natural processes

Occurrence of a natural phenomenon

• Catastrophic event caused by nature.

• Potential threat from natural processes

• Occurrence of a natural phenomenon

Match

Match the plate boundary

destructive

constructive

conservative

collision

• destructive

• constructive

• conservative

• collision

Multiple Choice

What is the point in the Earth's crust where an earthquake begins?

• Epicentre
• Start Point
• Focus
• Centre

Multiple Choice

Which type of crust is more dense?

• Oceanic
• Continental

Multiple Choice

In which part of the earth's structure do we find convection currents?

• Crust
• Inner Core
• Mantle
• Outer core

Multiple Choice

Which of these plate boundaries does not have volcanoes along it?

• Constructive
• Destructive
• Conservative

​Tropical Cyclones: Formation, Distribution & Characteristics

By Aimee Cooper

Open Ended

What storms have you heard of in the news?
Where did they happen? What damage did they cause?

Open Ended

Key Question: Where do cyclones develop?

​Distribution

Tropical cyclones do not occur everywhere. They form in specific belts of the world’s oceans, between the Tropic of Cancer and the Tropic of Capricorn. They occur between about 5° and 20° latitude north and south of the Equator, where conditions are warm and humid. Importantly, they never form at the Equator.

Terminology

• Tropical storms are also known as:
  Hurricanes → Atlantic Ocean + eastern Pacific (Caribbean, USA, Central America).

• Typhoons → Western Pacific (Japan, Philippines, China).

• Cyclones → Indian Ocean + South Pacific (India, Bangladesh, Madagascar).

• Willy-Willies → Northern Australia (Australian region of the South Pacific/Indian Ocean).

Labelling

Add the names to the correct region

Drag labels to their correct position on the image

Willy-Willies

Hurricanes

Typhoons

Cyclones

Conditions for Formation

Tropical cyclones only form where the sea surface temperature is at least 27°C. Warm water heats the air above, causing it to rise. This rising air carries huge amounts of moisture, leaving behind an area of low pressure at the surface. The constant movement of rising and replacing air creates a powerful cycle.

Conditions for Formation

The rising moist air cools as it rises higher, and the water vapour condenses to form thick storm clouds. This process releases latent heat energy, which makes the cyclone grow stronger. Cyclones also need low wind shear — if upper winds are too strong, they tear the storm apart.

The Coriolis Effect

The Earth’s rotation deflects moving air. This is called the Coriolis effect. It makes cyclones spin anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. At the Equator there is no Coriolis effect, which explains why cyclones cannot form there.

Multiple Choice

Which way would a cyclone spin in Australia?

• Clockwise
• Random direction
• Vertical
• Counterclockwise

Open Ended

Why don’t tropical cyclones form at the Equator?

Formation Process

• Warm, moist air rises from the ocean.

• Low pressure forms at the surface, pulling in more air.

• Rising air cools, condenses and releases latent heat.

• This powers the storm and forms thick clouds.

• The Coriolis effect makes the air spiral.

• An eye forms in the centre of the storm.

​Structure of a Cyclone

​A tropical cyclone has a very distinctive structure. At its centre is the eye, surrounded by the eyewall, with spiral rainbands extending outwards. Each part of the cyclone has different weather conditions.

​The Eye

​The eye of the cyclone is the calmest part. Air is sinking here, so skies may be clear and winds are light. The eye is usually between 30 and 50 km wide.

​The Eyewall

​Surrounding the eye is the eyewall, where the strongest winds and heaviest rainfall are found. This is the most destructive part of the storm, with violent winds, torrential rain and sometimes tornadoes.

​Spiral Rainbands

​Beyond the eyewall are spiral bands of thunderstorms. These can stretch for hundreds of kilometres and bring intense rainfall and flooding.

Open Ended

Which part of the cyclone is most dangerous and why?

Size and Duration

Cyclones are enormous storms. They can be 600–700 km across and stretch over entire countries. On average, they last around 10 days, although some can survive for up to four weeks if they stay over warm ocean water.

Over Land

Cyclones lose strength when they move over land. They are cut off from the warm, moist ocean water that provides their energy. Without this energy source, the winds slow down, rainfall decreases, and the storm eventually breaks apart.

Open Ended

Why do they weaken over land?

​Characteristics of Cyclones

Tropical cyclones have:

• Very low air pressure (often below 950 mb).

• High wind speeds, often above 120 km/h.

• Torrential rainfall leading to floods.

• Thunderstorms and storm surges.

• A spiral structure with an eye in the middle.

​Task: Cyclone Storyboard

​Create a 6-box storyboard showing the life of a cyclone:

1. Warm water fuels rising air.

2. Air spirals due to Coriolis effect.

3. Thick clouds and storms develop.

4. The eye and eyewall form.

5. Landfall causes destruction.

6. The storm weakens over land.

​The Saffir–Simpson Scale

​Cyclones are measured on the Saffir–Simpson scale, which ranges from Category 1 to Category 5. The scale is based on wind speed and gives an estimate of potential property damage and flooding risk. Category 1 storms are fairly weak, but Category 5 storms can be catastrophic.

​The Saffir–Simpson Scale

Open Ended

Which would cause more destruction: a Category 1 or a Category 5 storm? Explain why.

​Storm Surges

​One of the most dangerous effects of cyclones is the storm surge. A storm surge is a sudden rise in sea level caused by low pressure and strong onshore winds. This wall of water can flood coastal towns, farmland and roads in minutes.

​Why Storm Surges Are Deadly

​Storm surges are often the main cause of deaths in cyclones. They drown people, sweep away buildings, contaminate drinking water, and spread disease. For poor coastal communities, storm surges can wipe out homes and livelihoods completely.

Open Ended

Explain why storm surges are so deadly.

​Primary Impacts

​Primary impacts are the immediate effects of a cyclone. These include houses being blown down by strong winds, roads and bridges destroyed by floods, and farmland ruined by saltwater flooding.

​Secondary Impacts

​Secondary impacts are the knock-on effects. These include people becoming homeless, outbreaks of disease due to contaminated water, food shortages from destroyed crops, and economic losses from damaged infrastructure.

Categorize

Options (8)

Roofs blown off houses

Flooded farmland

People drowned by storm surge

Power lines destroyed

Spread of cholera

Food shortages

Homelessness

Roads blocked (slowing rescue)

Categorise the following impacts of tropical cyclones into Primary (immediate, direct) and Secondary (knock-on, indirect).

Primary Impacts

Secondary Impacys

​Naming Cyclones

​Cyclones are given names to make them easy to track and report in the news. They are named alphabetically, alternating between male and female names. Names are reused every 6 years unless they are retired because the storm was especially deadly (e.g. Katrina).

Open Ended

Describe the track of hurricane Katrina

​Answer

Hurricane Katrina formed over the Bahamas and moved westwards across Florida. It then entered the Gulf of Mexico, where it strengthened and curved northwards towards the USA. Finally, it made landfall in Louisiana near New Orleans before weakening as it moved inland.​

Open Ended

Assess the effectiveness of responses to Hurricane Katrina.
(6 marks)

​Answer

Responses to Hurricane Katrina were criticised. The levees and coastal flood protection walls were ageing and neglected, which allowed widespread flooding. The emergency response was slow and not well coordinated, and many people ignored evacuation advice. Problems such as looting also occurred, and many bodies could not be recovered.

In the long term, flood defences were repaired and improved, and buildings were replaced, but poorer residents in flooded areas were forced out as the city became more expensive and gentrified.

Overall, while improvements were made afterwards, the immediate response was ineffective.

​Case Study Introduction

On 8 November 2013, Typhoon Haiyan struck the Philippines. Locally it was known as Typhoon Yolanda. It reached Category 5 strength, making it one of the most powerful tropical cyclones to ever hit land. Wind speeds of over 300 kilometres per hour were recorded.

Multiple Choice

When did Typhoon Haiyan strike the Philippines?

• October 15, 2013
• November 1, 2013
• November 8, 2013
• December 5, 2013

Multiple Choice

What was the maximum wind speed recorded?

• Over 300 Kph

• 150 kph

• 250 kph

• 400 kph

Open Ended

Why might the physical geography of the Philippines make it vulnerable to tropical cyclones?

​Vulnerability of the Philippines

The Philippines is an archipelago of over 7,000 islands in the Pacific Ocean. It is densely populated, and many people live in low-lying coastal areas. This increases vulnerability to storm surges, flooding, and strong winds.

Before Haiyan made landfall, approximately 800,000 people were evacuated. However, 1.9 million people were still left homeless after the storm. The airport was damaged, and many roads were blocked by fallen trees, making access for aid very difficult.

​Preparation

Categorize

Options (15)

Over 6,000 people killed

1.9 million people left homeless

Around 3 million people received emergency food aid

Widespread looting took place in some areas

Schools and health facilities destroyed

Widespread power outages across affected islands

Billions of dollars of damage to homes, businesses, and infrastructure

Crops destroyed, leading to food shortages

Coastal shipping destroyed by the storm surge

Airport damaged, delaying international aid deliveries

Roads blocked by fallen trees and debris

Farmers lost their income due to destroyed crops

Mangroves uprooted along the coast

Coastal ecosystems damaged by flooding and debris

Crops destroyed

Categorise the following impacts into Social, Economic, or Environmental. Some may fit into more than one category, so be prepared to explain your reasoning.

Social

Economic

Environmental

​Immediate Responses

Emergency food, water, and medical supplies were delivered by local authorities, aid agencies, and foreign governments. High-energy biscuits, rice, and clean water were distributed. Access problems slowed down the response, but international aid saved lives.

Open Ended

Why was it difficult to deliver emergency aid after the typhoon?

​Long-term Responses

The government launched a recovery programme called “Build Back Better”. This aimed to rebuild structures with stronger materials to make them hazard-proof. “No-build” zones were introduced in the most at-risk coastal areas. Mangroves were replanted to protect the coast, and a new storm surge warning system was installed.

Open Ended

How did the Philippines’ response to Typhoon Haiyan differ from the USA’s response to Hurricane Katrina?
(Prompt: consider resources, speed of aid, and long-term planning)

​Knowledge Recap

​Cyclones form in the tropics, not at the Equator.
They need warm oceans, moist rising air, low wind shear and the Coriolis effect.
They have a distinctive structure: eye, eyewall, rainbands.
They can cause storm surges and huge destruction.

Open Ended

“Describe the conditions needed for the formation of tropical cyclones.”

Multiple Choice

Identify a way to help plan for tropical cyclone hazards

• A

• B

• C

• D

Open Ended

answer the exam question

Open Ended

suggest two reasons why some populations are more at risk from the impacts of cyclones than others (4)

Lesson Objectives

By the end of this lesson, you should be able to:

• Describe how tropical cyclones form.

• Explain the global distribution of tropical cyclones.

• Identify the main characteristics of tropical cyclones.