Tropical storms

Where and when tropical storms form

A is a huge, spinning storm with very strong winds and heavy rain. They have different names in different oceans: in the Atlantic, in the north-west Pacific and cyclones in the Indian Ocean.

They form:

over warm oceans, at least about 27 °C, to a depth of tens of metres;
between about 5° and 30° north and south of the equator. Nearer the equator, the Coriolis effect is too weak to make the storm spin;
in late summer and autumn, when the sea is warmest;
where winds high in the atmosphere are light, so the storm isn't torn apart.

Trade winds carry them westwards. They weaken over land or cooler water, where they lose their supply of warm, moist air.

Key terms

How a tropical storm forms

Warm, moist air rises rapidly from the ocean's surface, leaving below.
More warm, moist air rushes in to replace it, and rises too.
As the air rises, it cools and condenses, forming towering clouds and heavy rain. Condensing releases heat, which powers the storm and makes the air rise faster.
The makes the storm spin: anticlockwise north of the equator.
Cool, dry air sinks in the centre, forming the calm .
The storm grows as long as it stays over warm water. Over land or cool sea, its energy supply is cut off and it dies away.

Remember

Warm sea, rising air, condensation releasing heat, spin from the Coriolis effect.

The structure of a tropical storm

the calm centre, typically tens of kilometres across, where cool air sinks. Skies may be clear, with light winds.
a ring of towering clouds around the eye, where warm air rises fastest. It has the strongest winds and heaviest rain.
bands of cloud and rain spiralling round the storm further out.
At the top, the rising air flows outwards.

A whole storm can be hundreds of kilometres across.

Key terms

Climate change and tropical storms

Warmer oceans could change tropical storms. The so far, summarised by the Intergovernmental Panel on Climate Change (IPCC), suggests:

Intensity: the proportion of the most intense storms has probably increased over the last 40 years, and is expected to keep rising.
Rainfall: warmer air holds more water, so storms are likely to bring heavier rain.
as more of the ocean reaches 27 °C, storms may form, or reach their strongest, further from the equator.
scientists are less sure whether there will be more storms in total; there may even be fewer.

Common mistake

Don't write that climate change will definitely cause more tropical storms. The evidence is strongest for more intense storms and heavier rain.

Exam practice

Plan your own answer first, then open the model answers to see what each level looks like.

6 marks · about 6 minutes

Explain how tropical storms form.

Explain means: Give the reasons why something happens, developing each point with "because" or "so".

Level 1 (Basic)· 2/6 marks (band 1–2)

Tropical storms form over warm seas. The air rises and it makes a storm that spins.

Why this level: Two correct points but no sequence and no explanation of why the air rises or why it spins. To reach Level 2: give the conditions needed and explain the stages in order.

Level 2 (Clear)· 4/6 marks (band 3–4)

Tropical storms need warm oceans, over about 27 °C. The warm sea heats the air, which rises quickly, leaving low pressure at the surface. More air rushes in to replace it. The rising air cools and condenses into huge clouds, which give heavy rain. The Earth's rotation makes the storm spin.

Why this level: A clear sequence with the key condition and the main stages. To reach Level 3: add where storms form (latitude), how condensation releases heat to power the storm, the Coriolis effect by name, and how the eye forms.

Level 3 (Detailed)· 6/6 marks (band 5–6)

Tropical storms form over oceans warmer than about 27 °C, between about 5° and 30° from the equator. Warm, moist air rises rapidly, leaving an area of very low pressure, and more air rushes in to replace it, creating strong winds. As the air rises, it cools and condenses into towering clouds; condensation releases heat, which makes the air rise faster still and powers the storm. The Coriolis effect, caused by the Earth's rotation, makes the storm spin. In the centre, cool air sinks to form the calm eye. The storm keeps growing while it is over warm water, and weakens over land, where its supply of heat and moisture is cut off.

Why this level: Detailed and in a logical order: conditions, rising air, condensation powering the storm, the Coriolis effect, the eye and decay. Correct terms throughout, so this is Level 3.

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