Structure and features of tropical cyclones

In appearance, a tropical cyclone is like a huge whirlpool - a gigantic mass of revolving moist air.

Tropical cyclones (or storms) are between 482-644 kilometres wide and 6-8 km high. They move forward at speeds of 16-24 km/h, but can travel as fast as 65 km/h. The Coriolis force caused by the rotation of the Earth causes the tropical cyclone to spin.

The central part of the tropical cyclone is known as the eye. The eye is usually 32-48 km across. It is an area of light wind speeds and no rain. It contains descending air.

Large towering cumulonimbus clouds surround the eye. These are caused by warm moist air condensing as it rises. This leads to very heavy rainfall and wind speeds of up to 320 km/h.

How tropical cyclones develop

  • Cyclones form between approximately 5° and 30° latitude. Because of easterly winds they initially move westward.
  • They occur most commonly in early autumn as this is when sea temperatures are at their highest, temperatures having built up over the summer.
  • The air above the warm ocean is heated. Once the ocean water reaches at least 27°C, the warm air rises quickly, causing an area of very low pressure.
  • As the air continues to rise quickly it draws more warm moist air up from above the ocean leading to strong winds.
  • The rapidly rising warm air spirals upwards, cools, condenses and large cumulonimbus clouds form.
  • These clouds form the eye wall of the cyclone and produce heavy rainfall.
  • In the centre of the cyclone, cold air sinks forming the eye of the cyclone - here, conditions are calm and dry.
Tropical storms form where sea temperature is over 27 °C. The eye in the centre is formed by spiralling currents of rising warm air, with cold air sinking in the middle.
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When tropical cyclones reach a land surface, they begin to lose their energy and die out. This is because they are no longer receiving heat energy and moisture from the ocean, which is needed to drive them.

What are the hazards associated with tropical cyclones?

When a tropical cyclone arrives at a coastline, it is potentially fatal and can cause damage to property. This is because tropical cyclones bring with them:

  • Storm surges – these are huge surges of high water up to 3 metres in height that sweep inland from the sea, flooding low-lying areas.
  • Strong winds – winds of over 120 km/h (75 mph) blow inland, which are capable of causing significant damage and disruption, for example by tearing off roofs, breaking windows and damaging communication and transport networks.
  • Torrential rain and flooding – the warm, humid air associated with a tropical cyclone produces very large amounts of rainfall, often in excess of 200 mm in just a few hours. This can cause short-term flash flooding, as well as slower river flooding as the cyclone moves inland.