🌊 Warm water
The sea surface must be at least about 26-27 degrees Celsius, over a large area and to a depth of about 50 m. Warm water evaporates and the rising water vapour is the storm's fuel. This is why storms weaken fast over cool water or land.
Coasts: Opportunities and Hazards » Tropical Storms: Distribution and Impacts
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 2.3.4
- Where tropical storms form and what they are called in each ocean
- The conditions a tropical storm needs, and why few form at the Equator
- The structure of a tropical storm: eye, eyewall and rain bands
- The social, economic and environmental impacts, and how they differ in LICs and HICs
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Unlock This CourseWhat is a tropical storm?
A tropical storm is a huge, spinning storm that forms over warm tropical oceans, with very strong winds, towering clouds and torrential rain. Some are over 500 km across. Once its winds reach about 120 km/h, it is called a hurricane, typhoon or cyclone, depending on where in the world it forms.
Key terms:
- Tropical storm: a powerful, rotating storm that forms over warm tropical oceans.
- Hurricane: the name for a tropical storm in the Atlantic Ocean and the north-east Pacific Ocean.
- Typhoon: the name for a tropical storm in the north-west Pacific Ocean.
- Cyclone: the name for a tropical storm in the Indian Ocean and the South Pacific Ocean.
They are all the same kind of storm. Only the name changes.
Where do tropical storms form?
Tropical storms form between about 5 and 30 degrees north and south of the Equator, over warm oceans. The table shows the local name in each ocean region.
| Ocean region | Local name | Example |
|---|---|---|
| North Atlantic and north-east Pacific | Hurricane | Hurricane Katrina, USA |
| North-west Pacific | Typhoon | Typhoon Haiyan, Philippines |
| Indian Ocean and South Pacific | Cyclone | Cyclone Idai, Mozambique |
Almost none form in the South Atlantic or the south-east Pacific, because the water there is too cold.
What conditions do they need?
🌀 Spin
The Earth's rotation makes moving air turn. This is the Coriolis effect. It is what makes a storm spin. Near the Equator it is too weak to do this.
This explains the pattern. Closer to the Equator than about 5 degrees, the Coriolis effect is too weak, so few storms form right at the Equator even though the sea is very warm. Beyond about 30 degrees, the sea is too cool.
In short, a tropical storm forms when warm moist air rises quickly, cools, forms huge clouds and releases heat. This pulls in more air at the bottom. The Coriolis effect then turns that air, so the storm spins. Storms usually fade once they move over land or cool water, because the fuel is cut off.
The structure of a tropical storm
If you were asked to sketch a tropical storm from above, this is what to label:
- Eye: the calm centre, usually tens of kilometres across. Air sinks here, so the sky is often clear, winds are light and pressure is lowest.
- Eyewall: a ring of tall thunderclouds around the eye. This is where the strongest winds and heaviest rain are.
- Rain bands: curved bands of cloud and rain that spiral outwards, with gusty winds and heavy showers between them.
The eye can fool people. When it passes over, it goes calm for a while, but the other side of the eyewall brings violent winds again.
The impacts of tropical storms
Tropical storms cause damage in several ways:
- Strong winds: destroy buildings, power lines and crops.
- Storm surge: low pressure and strong winds push the sea up and on to the land (see Living on the Coast: Opportunities and Hazards). This often causes most of the deaths.
- Heavy rain and flooding: rivers burst their banks and streets flood.
- Landslides: heavy rain soaks steep slopes, which then slide.
We sort the effects into three types. Short-term impacts happen during and just after the storm. Long-term impacts last months or years.
| Type | Short-term | Long-term |
|---|---|---|
| Social | Deaths and injuries, homes destroyed, people homeless, no clean water, disease risk | Trauma, people move away, schools and hospitals closed for months |
| Economic | Businesses closed, roads and ports blocked, crops and boats lost | High rebuilding costs, lost jobs and tourism, slow economic recovery |
| Environmental | Trees flattened, beaches eroded, salt water floods farmland | Damaged coral reefs and forests, soil ruined by salt, wildlife habitats lost |
LICs and HICs: why the impacts differ
A storm of the same strength can have very different results. In general:
| HICs | LICs | |
|---|---|---|
| Deaths | Fewer, because of strong buildings, warnings and organised evacuation | More, because of weaker housing, poor roads and fewer warnings |
| Costs | Very high in money, but insurance pays much of it | Lower in money, but a bigger share of the country's wealth, and few people have insurance |
| Recovery | Faster, with money and equipment | Slower, often relying on foreign aid |
Real storms, briefly
Hurricane Katrina (2005): hit the USA, an HIC. The storm surge flooded New Orleans, and about 1,800 people died. The cost was well over US$100 billion.
Typhoon Haiyan (2013): hit the Philippines, an MIC. It caused a huge storm surge, over 6,000 deaths and millions of people were made homeless. Many places had little help for days.
Notice that the HIC lost less life but far more money. Even in HICs, poor people can be hit hardest.
Common mistakes
Avoid these slips
- Saying storms form right at the Equator. The Coriolis effect is too weak there.
- Calling the eye the strongest part. It is the calmest part. The eyewall is the strongest.
- Treating hurricanes, typhoons and cyclones as different storms. They are the same type of storm with different names.
- Saying LICs always lose more money. They often lose more lives but less money. Describe both.
Exam-style question
Fig. 1.1 gives made-up data for two tropical storms of similar strength.
| Fig. 1.1 | Storm A | Storm B |
|---|---|---|
| Country type | HIC | LIC |
| Maximum wind speed (km/h) | 210 | 205 |
| Deaths | 40 | 3,200 |
| Damage (US$ billion) | 18 | 2 |
(a) Give the name used for a tropical storm in the north-west Pacific. [1 mark]
(b) Describe two features of the eye of a tropical storm. [2 marks]
(c) Explain why tropical storms rarely form within 5 degrees of the Equator. [3 marks]
(d) Explain why Storm B caused far more deaths than Storm A, even though it caused less damage in money. [6 marks]
Model answer
(a) Typhoon (1).
(b) Any two from: calm, with light winds (1); often clear sky (1); lowest pressure (1); sinking air (1); tens of kilometres across (1).
(c) The Coriolis effect, caused by the Earth's spin, is too weak near the Equator (1). So the air cannot start to spin (1). Without spin, a storm cannot organise itself, even though the water is warm (1).
(d) Level 3 (5-6 marks): a clear, developed explanation using data and several reasons. For example: Storm B is in an LIC, where homes are often weaker and are easily destroyed by winds of 205 km/h (1). There are fewer early warnings and poor roads, so fewer people can evacuate in time (1). Storm surges and flooding kill many people living in low coastal areas (1). Emergency services and hospitals are limited, so the injured and sick are not helped quickly (1). In contrast, Storm A hit an HIC, where strong buildings, warnings and rescue teams kept deaths to 40 (1). Damage in money was lower in B (US$2 billion compared with US$18 billion) because there are fewer expensive buildings and less insured property to lose, not because the storm was weaker (1).
Level 2 (3-4 marks): some reasons, with limited explanation.
Level 1 (1-2 marks): simple statements, such as: LICs are poor so more people die.
Exam tip
In part (d), use the figures in Fig. 1.1 and link each reason to deaths, not just to damage.