Tropical cyclones at Higher Level: An HL-only statement. Hurricanes, typhoons and cyclones are one kind of storm with three names. You will see what makes one, how it draws its energy from a warm ocean, and what it does when it reaches land.
Practise this as you read
- Define a tropical cyclone and say when it becomes a hurricane or typhoon.
- Outline the conditions it needs and explain how it works.
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A spinning storm: A tropical cyclone is a storm system that circulates rapidly around a centre of low pressure.
The points to remember
- A tropical cyclone is a storm system that spins rapidly around a centre of low pressure.
- It forms in the tropics, over warm oceans.
- It is marked by strong winds, heavy rain and high waves.
- It grows in stages: tropical depression, tropical storm (it gets a name), then Category 1 to 5.
- Categories 1-5 (the Saffir-Simpson scale) are set by the sustained wind speed.
Real example: Katrina began as a tropical depression over the Bahamas on 23 August 2005. It became Tropical Storm Katrina, crossed Florida as a Category 1 hurricane, and over the warm Gulf of Mexico reached Category 5, with sustained winds of about 280 km/h.
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A tropical cyclone has a different name in different oceans. The storm is the same; only the name changes.
Hurricane, typhoon or cyclone
- Once sustained winds exceed 119 km/h, it is called a hurricane or a typhoon.
- Hurricane: formed in the North Atlantic, the Caribbean or the north-east Pacific.
- Typhoon: formed in the north-west Pacific, near Japan, China and the Philippines.
- Cyclone: the name used in the Indian Ocean and the South Pacific.
- The name depends on where it forms: the storm itself is the same.
The threshold: Only once the sustained wind is over 119 km/h is it a hurricane or typhoon. Below that it is a tropical storm, however much rain it brings: Tropical Storm Allison (2001) never reached 119 km/h, yet it flooded Houston.
Remember it as: Atlantic hurricane, Asian typhoon, Indian Ocean cyclone.
What a cyclone needs: Tropical cyclones originate in the tropics because that is where the ocean is warm enough to give them their energy.
The points to remember
- Warm ocean: surface water at least about 26.5 °C, warm to a depth of about 50 m.
- Between about 5° and 30° north or south of the equator.
- Not on the equator: the Earth's spin (the Coriolis effect) is needed to make the storm rotate.
- Moist air, and winds that change little with height (low wind shear), so the storm is not torn apart.
- So they form in late summer and autumn, when the sea is warmest.
Real example: the Atlantic hurricane season runs from 1 June to 30 November and peaks in September, when the sea is warmest. Almost no cyclone forms near the equator: the closest ever seen was Typhoon Vamei, which formed only about 150 km north of it, near Singapore, in December 2001.
Why the spin matters: The Coriolis effect makes the storm spin: anticlockwise north of the equator, clockwise south of it.
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A cyclone is a heat engine: its fuel is warm, moist air from the ocean.
How a cyclone works
- Warm water evaporates; the moist air rises and cools.
- The water vapour condenses into cloud, releasing heat (latent heat), so the air rises faster.
- Rising air leaves low pressure at the surface; more air spirals in to replace it.
- The eye in the centre is calm, with sinking air and the lowest pressure; the eyewall round it has the strongest winds.
- Over land or cold water it loses its energy supply, and friction with the land slows it: it weakens.
- Cyclones carry heat from the tropics towards the poles.
Real example: Cyclone Freddy (2023) crossed the whole south Indian Ocean, about 8,000 km, from north-west Australia to Madagascar and Mozambique. Each time it crossed land it weakened; each time it reached warm sea again it strengthened. It lasted 36 days, the longest-lived tropical cyclone ever recorded. Over land in Malawi its rain caused floods and landslides that killed more than 1,000 people.
What they do: Strong winds, a storm surge and heavy rain: three hazards in one storm.
The points to remember
- Strong winds wreck homes, power lines and crops and flatten forests.
- A storm surge, the sea pushed onto the land by wind and low pressure, floods the coast. It kills most.
- Heavy rain brings river floods and landslides, even far inland.
- Ecosystems are hit: coral reefs broken, mangroves and forests stripped, soil washed away.
- Not all bad: the rain refills reservoirs and ends droughts, and the storm mixes and cools the sea.
Real example: Typhoon Haiyan crossed the central Philippines on 8 November 2013 with sustained winds of about 315 km/h, one of the strongest storms ever to reach land. In Tacloban city a storm surge over 5 m high swept inland. More than 6,000 people died and about 4 million lost their homes.
Rain far inland: In March 2019 Cyclone Idai hit Beira, in Mozambique. Its rain flooded a huge area inland, and more than 1,000 people died in Mozambique, Zimbabwe and Malawi.
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How this comes up: Paper 1 case studies of storm-hit islands (Dominica, Mauritius) ask how the storm formed and what it did. Paper 2 can ask you to outline the conditions or explain how a cyclone forms.
Each year about 20 tropical storms form over the Bay of Bengal and the Arabian Sea, but only a few become cyclones.
Outline four conditions needed for a tropical cyclone to form.
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Use the numbers: 26.5 °C, 5° to 30°, 119 km/h: a number makes a point precise and hard to miss.