IGCSE Geography / Hazardous environments

How tropical cyclones form

Edexcel International GCSE Geography (4GE1), chapter 3: Hazardous environments, specification point 3.1b. Free lesson.

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Tropical cyclones form when four conditions allow warm, moist air to rise and spin over the ocean. They weaken over land or cooler water because their supply of heat and moisture is cut off.

The four conditions a tropical cyclone needs

All four conditions must be present at the same time. Warm water supplies the storm’s energy, while low pressure and low wind shear let it grow. The Coriolis force makes it spin. These are causes, unlike a characteristic, which is a feature of the storm itself, such as its eye. Ocean temperature: at least 26.5 °C, with warm water down to about 50 m. Warm water evaporates quickly, providing heat and moisture. Atmospheric pressure: low pressure at the sea surface, linked to warm, moist air rising. Wind shear: low wind shear, meaning little change in wind speed or direction…

How the storm forms

Evaporation starts the process by adding warm, moist air above the sea. As that air rises and condenses, heat is released, helping air rise faster. More air rushes in at the surface, and the Coriolis force turns it so the storm rotates.

The storm's features and hazards

A fully formed tropical cyclone has several characteristics, or features. Its warm ocean supplies heat and moisture, while its cloud bands and central eye form part of the storm. It brings three main hazards: strong winds, heavy rain and storm surge. Eye: the calm centre, where air sinks and pressure is lowest. Eyewall: the ring of towering cloud around the eye, with the strongest winds and heaviest rain. Rain bands: curved bands of cloud and rain that spiral towards the centre. Storm surge: strong winds and low pressure push sea water up, which can flood the coast as the…

Names depend on where the storm forms

Tropical cyclone is the general term. The local name depends on the ocean where the storm forms, but all three names describe the same kind of storm. Hurricane: Atlantic and eastern Pacific. Katrina hit the USA in 2005. Typhoon: north west Pacific. Haiyan hit the Philippines in 2013. Cyclone: Indian Ocean and South Pacific. All three have winds of 119 km/h or more.

Why a cyclone weakens over land or cool water

A cyclone gets its energy from warm water evaporating. Over land there is no warm ocean to supply evaporation, heat and moisture. It also weakens over sea cooler than 26.5 °C because the supply of warm, moist air is lost. It is not simply because land is colder than the sea. Without warm, moist air, the storm loses its energy supply and weakens.

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