Tropical Cyclones: Hazard, Impact & Response

A Storm With Many Names

Hurricanes in the Atlantic, typhoons in the western Pacific, cyclones in the Indian Ocean — the names differ but the storm is the same. A tropical cyclone is a deep low-pressure system with sustained winds above 119 km/h, an intense central eye and a spiral of rain bands hundreds of kilometres across. They are the most destructive natural hazards on the planet for low-latitude coasts. Where they form, when they peak and how a country copes are all controlled by physical geography — and by how much money the country has.

How They Form

Tropical cyclones form only between roughly 5° and 30° latitude (rarely on the equator itself). Four conditions must line up. First, sea-surface temperatures must reach 26.5°C or higher down to about 50 m depth, providing the energy. Second, warm ocean air must rise rapidly, cool, condense and release latent heat — powering the storm. Third, low vertical wind shear means upper and lower winds move at similar speeds, so the storm can stack and grow rather than tearing itself apart. Fourth, the Coriolis effect, caused by Earth’s rotation, twists the rising column into a spin (anticlockwise in the Northern Hemisphere, clockwise in the Southern). Once spinning, the storm pulls in more warm moist air from the surrounding ocean, intensifying further.

Anatomy of a Storm

At the centre is the eye: 30–65 km wide, calm, with sinking air and clear skies. Around it is the eyewall, where the strongest winds and heaviest rainfall sit. Spiralling outwards are bands of cloud and rain that can stretch 600 km from the eye. Cyclones are rated on the Saffir-Simpson scale from Category 1 (119–153 km/h) to Category 5 (above 252 km/h), but wind speed alone hides the killer: storm surge, where low pressure and onshore winds push a wall of seawater inland, can drown coastal towns far worse than the wind.

Distribution & Frequency

Cyclones cluster in seven main basins. The North Atlantic and East Pacific seasons run June–November; the western North Pacific (typhoons) is active most of the year. The number of named storms varies year-on-year — a typical Atlantic season produces 14 named storms, of which 7 reach hurricane strength — but a clear trend is emerging: warmer oceans are producing fewer but more intense storms, and rapid intensification (a category jump in 24 hours) has become more common since 2000. Tracks are also shifting polewards as warmer water moves north and south.

Hurricane Katrina, USA (2005) — Developed-Country Impact

Hurricane Katrina struck the US Gulf Coast on 29 August 2005 as a Category 3 storm with a 7-metre storm surge. Around 1,800 people died, most when the New Orleans levees failed and 80% of the city flooded. Damage reached $125 billion, the costliest US disaster on record. Despite the USA’s wealth, the response was widely criticised: poorer Black neighbourhoods of New Orleans had less car ownership and could not evacuate; the Superdome shelter became overcrowded with no air conditioning. Long-term, the city’s population fell from 484,000 (2000) to 343,000 (2010); levees were rebuilt to withstand a 100-year storm; insurance reform reshaped Gulf Coast property markets.

Cyclone Idai, Mozambique (2019) — Developing-Country Impact

Cyclone Idai struck the port city of Beira on 14 March 2019, sweeping across central Mozambique, Malawi and Zimbabwe. Over 1,300 people died and 3 million people were affected. Beira was 90% destroyed in a single night. Idai exposed how poverty multiplies a hazard: weakly built housing collapsed; warning systems reached too few people; only 2,500 hospital beds existed for an affected population of millions; cholera spread through floodwater, killing dozens more. International aid eventually arrived — the UK pledged £22 million — but distribution was slow because roads and bridges had been swept away. Mozambique’s GDP losses were estimated at $2 billion in a country where average annual income is $500.