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Weather Hazards » How Tropical Storms Form

What you'll learn this session

Study time: 30 minutes

AQA spec: 3.1.1.3

Where tropical storms happen in the world, and why
How tropical storms link to global atmospheric circulation
The conditions and the steps by which a tropical storm forms
The structure of a tropical storm: eye, eyewall and rain bands

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What is a tropical storm?

Tropical storms form over warm oceans of at least 27 degrees Celsius

A tropical storm is a huge, spinning storm of very strong winds and heavy rain. It forms over warm tropical oceans. The same type of storm has three different names, depending on where in the world it happens.

Key terms:

Tropical storm: a large, rotating low pressure system with very strong winds and heavy rain, formed over warm ocean water.
Hurricane: the name used for a tropical storm in the Atlantic and north-east Pacific oceans.
Typhoon: the name used for a tropical storm in the north-west Pacific, near Asia.
Cyclone: the name used for a tropical storm in the Indian Ocean and around Australia and the South Pacific.

A hurricane, a typhoon and a cyclone are all the same thing. Only the name changes with the ocean.

Global distribution of tropical storms

On a world map, tropical storms form in a band on either side of the equator. They are found roughly 5° to 30° north and south of the equator, mostly within the tropics.

🌊 Where they form

Over warm oceans in the tropics. Common areas include the Caribbean and Gulf of Mexico, the western Pacific near the Philippines, the Bay of Bengal and the seas north of Australia.

❌ Where they do not

Very close to the equator, and in cooler seas far from the tropics. The South Atlantic is almost always too cool, so storms there are extremely rare.

Storms form in the late summer and autumn of each hemisphere. This is when the sea has had time to warm up the most.

Once formed, storms are pushed westwards by the trade winds. Then they usually curve away from the equator. If a storm moves over land or over cooler water, it loses its energy and fades away.

Tropical storms and atmospheric circulation

You have already met the three-cell model of global circulation. Tropical storms are linked to it in two ways.

Low pressure: they form in the tropics, where the Hadley cell has warm air rising near the equatorial low pressure belt. Rising air means low pressure at the surface.
Surface winds: the trade winds blow towards the equator and carry the storm westwards. Further from the equator the westerlies often steer it back towards the east.

This is why storms in the Atlantic usually travel from Africa towards the Caribbean and the east coast of North America.

Conditions needed for a tropical storm

Warm sea water of at least 27 degrees Celsius is a key condition for a tropical storm

A tropical storm needs particular physical conditions. All of these must be present:

Sea surface temperature of 27°C or more. Warm water gives the storm its energy.
Deep warm water. The warm layer must be at least about 50 metres deep, so the storm does not quickly cool the sea beneath it.
Far enough from the equator (at least about 5° north or south). The spin of the Earth is too weak at the equator to make the storm rotate.
Light winds high in the atmosphere. Strong winds would tear the storm apart before it could build.

Why not at the equator?

Winds are turned by the Earth's spin, which is called the Coriolis effect. It is very weak at the equator, so air cannot start spinning there. Away from the equator it makes the air spiral. In the northern hemisphere the storm spins anticlockwise. In the southern hemisphere it spins clockwise.

How a tropical storm forms and develops

This is the sequence of formation. Learn the order, because exam questions ask you to describe it.

The sun heats the ocean surface to 27°C or more. Large amounts of water evaporate.
The warm, moist air rises. This leaves an area of very low pressure at the surface.
Air rushes in from the surrounding higher pressure to replace it. This air is also heated and rises.
As the air rises, it cools and condenses. This forms huge towering storm clouds and releases heat. The heat powers the storm and makes more air rise.
The Coriolis effect makes the whole system spin. Winds get faster as the pressure falls.
When winds reach 119 km/h or more, it is a fully developed hurricane, typhoon or cyclone.
The storm moves across the ocean, pushed by the trade winds. It stays strong while it is over warm water.
When it reaches land, it loses its supply of warm, moist air. Friction with the land also slows it. The storm weakens and dies out.

Structure and features of a tropical storm

A mature tropical storm is usually a few hundred kilometres across. Look at its structure from the centre outwards.

☀ The eye

The centre of the storm, usually about 30 to 50 km across. Air sinks here, so it is calm, dry and often clear, with very low pressure.

🌪 The eyewall

The ring of tall thunderclouds around the eye. It has the strongest winds and the heaviest rain. Air rises very fast here.

🌧 Rain bands

Spiralling bands of cloud further out. They bring heavy rain, gusty winds and thunderstorms, with gaps of lighter weather between.

Key terms:

Eye: the calm centre of a tropical storm where air sinks.
Eyewall: the ring of towering cloud around the eye with the fiercest winds and rain.
Rain bands: spiral bands of cloud and rain around the eyewall.

Air rises in the eyewall and sinks in the eye. If the centre of a storm passed over you, you would get the strong winds of the first part of the eyewall, then the calm of the eye, then strong winds again as the back of the eyewall passes.

Common mistakes

1. Saying the eye has the strongest winds. It is the calmest part. The strongest winds are in the eyewall.
2. Thinking hurricanes, typhoons and cyclones are different storms. They are the same, with different names in different places.
3. Saying storms form at the equator. They form between about 5° and 30° north and south.
4. Forgetting that the storm needs warm sea. It dies over land because it loses its supply of moist air.

Exam-style question

(a) Describe the global distribution of tropical storms. [2 marks]

(b) Explain how a tropical storm forms. [4 marks]

Model answer

(a) They form in a band either side of the equator, roughly 5° to 30° north and south, mostly in the tropics (1). They are found over warm oceans, such as the Caribbean, western Pacific and Bay of Bengal, but not right on the equator (1).
(b) The sea is heated to 27°C or more and water evaporates (1). Warm, moist air rises and leaves low pressure at the surface (1). The air cools and condenses to form huge clouds, which releases heat that keeps the storm powered (1). The Earth's spin makes the air spiral and the winds speed up as pressure falls (1).
(c) The eye is calm (1) with light winds, dry weather and often clear skies (1).

Exam tip

On a 4 mark 'explain' question, write a chain of linked steps using 'so' or 'which leads to'. Four clear steps in the right order score full marks.

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