Tropical Cyclones

Where tropical cyclones form, and the conditions they need

A tropical cyclone is an intense low-pressure storm that forms over a warm tropical ocean and brings very strong winds, torrential rain and a rise in sea level. The same type of storm has different regional names: hurricanes in the North Atlantic, the Caribbean and the north-east Pacific; typhoons in the western Pacific and the South China Sea; and tropical cyclones in the Indian Ocean, the Bay of Bengal and around northern Australia. A tropical storm is the weaker stage of the same system, and it is upgraded to a cyclone, hurricane or typhoon once its winds are strong enough. They form over the oceans and mostly lose their strength over land, which is why the coast takes the first and worst hit.

Four conditions have to be met together. The surface of the sea must be at least 27 °C, and that warm water must extend down to a depth of about 60 m; the depth matters because the storm churns the ocean as it moves, and if the warm layer is thin the mixing drags cold water to the surface and starves the storm of heat. Cyclones form between 5 and 30 degrees of latitude north or south of the Equator, because the Coriolis effect, the turning of the Earth, has to be strong enough to spin the air into a rotating system; very close to the Equator it is too weak, so no cyclone forms within about 5 degrees of it. Finally, wind shear must be weak. Wind shear is a difference in wind speed or direction between the air near the surface and the air at the top of the storm, and strong shear tears the growing cloud column apart before it can organise itself.

The chain of events begins with the Sun heating the ocean. Water evaporates, so the air just above the sea becomes warm and very humid, and as that air rises it cools and its water vapour condenses into towering cumulonimbus clouds. Condensation releases latent heat, and this is the engine of the storm: the released heat warms the air column so that it expands and keeps rising, the surface pressure falls further, and more air is pulled in from the surrounding ocean and spirals towards the centre over hundreds of kilometres. Because of the Coriolis effect the inflowing air rotates anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The system feeds itself as long as it stays over warm, deep water with little wind shear, which is why cyclones grow stronger over the ocean and weaken once they cross the coast.

• Ocean surface temperature of at least 27 °C.
• Warm water to a depth of at least 60 m, so mixing cannot bring cold water to the surface.
• Between 5 and 30 degrees of latitude north or south, where the Coriolis effect is strong enough to cause rotation.
• Weak wind shear, so the rising air column is not blown apart.
• A source of very humid air, usually a cluster of thunderstorms over the warm ocean.

Inside the storm: the eye, the eyewall and the spiral bands

A mature cyclone is one of the largest and most violent weather systems on Earth, and it is built around a very low pressure centre. Its structure has three parts you must be able to describe and label: the eye at the centre, the eyewall wrapped around the eye, and the spiral rain bands that trail outwards from the eyewall. The whole system may be several hundred kilometres across, but the most destructive weather is concentrated in a ring only tens of kilometres wide.

The eye is the calm centre of the storm. Air is sinking here rather than rising, so the sky may be almost clear, the winds are light and the pressure is at its lowest. An eye is usually a few tens of kilometres across. The calm is what makes the eye so dangerous to people on the ground: they think the storm has passed, come out of their shelters, and are then hit by the second half of the storm, when the wind blows with equal force from almost the opposite direction.

The eyewall is the ring of the tallest and densest cumulonimbus cloud immediately around the eye. Here the pressure gradient is at its steepest, so this is where the strongest winds and the heaviest rain occur and where most wind damage is done. As a cyclone crosses a coast, the leading edge of the eyewall causes a sudden rise in wind speed and the passage of the eye brings a brief, false calm before the winds return from the other side.

The spiral rain bands curve outwards from the eyewall, separated by gaps of lighter cloud. They bring bands of heavy rain and gusty winds that arrive hours before the centre and continue after it has passed, and they can be wide enough to span an entire country as the system moves inland. Small tornadoes are sometimes spawned inside the outer bands, which is why the wind damage in a cyclone is not always in a neat pattern.

Steering, landfall and decay, and how cyclones are named and classified

A cyclone does not move itself; it is steered by the winds around it, mainly the trade winds near the surface and the sub-tropical high-pressure systems either side of it. In both hemispheres the pattern is similar: the storm drifts westwards at first, carried by the trade winds, then curves polewards and finally turns eastwards as it meets the prevailing winds further from the Equator. Some storms recurve out to sea and never reach land at all. Forecasters use computer models fed by satellite, radar and aircraft data to predict the track, and track forecasts are now good several days ahead; intensity forecasts are much less reliable, because it is harder to measure the ocean heat and the wind structure that actually decide whether a storm will rapidly strengthen.

A cyclone weakens when it loses any of the things it needs. Over land it is cut off from the warm ocean that supplies its moisture and energy, and friction with the surface slows the winds, so it decays within a day or two. It also weakens when it moves over sea cooler than 27 °C, when it runs into strong wind shear, or when it moves into dry air. A decayed storm is not harmless, though: it carries enormous amounts of moisture inland, and the resulting river flooding and landslides can kill many more people than the coastal winds did. Cyclone Idai in 2019 is the classic example, because the heaviest damage was caused by rain and flooding far inland in Zimbabwe and Malawi, not only where it first came ashore in Mozambique.

Cyclones are given names from lists held by the regional bodies of the World Meteorological Organization: that is why the Atlantic, the western Pacific and the Indian Ocean all use different lists. Names make warnings easy to follow when several storms are active at once, and governments can be held to account for what they did about a named storm. The most destructive names are retired after the event and replaced, so it is no longer used again for another storm in that region. Strength is classified by wind speed: the Saffir-Simpson scale runs from category 1 to category 5 for hurricanes in the Atlantic and north-east Pacific, and other regions use similar scales.

The category of a storm is useful but it does not tell you everything. It is based on wind speed, so it says nothing about how much rain will fall, how large the storm is or how big the storm surge will be, and a slow-moving, large, wet storm can cause far more flooding and loss of life than a more tightly wound storm of the same category in the same place. That is a favourite point in exam answers: the hazard is not just wind.

The hazards: wind, storm surge, torrential rain, flooding and landslides

Wind. A mature cyclone can sustain winds strong enough to strip roofs, move cars and turn anything loose into a flying weapon. Damage to buildings and infrastructure includes roofs, walls and windows, but also power lines and pylons, communication masts, roads and bridges, fishing boats, ports and airports, and the loss of electricity and water supply that follows. Crops such as bananas, sugar cane, coconut palms and even mature trees are broken or uprooted, so a single storm can destroy a farmer's income for a year and a plantation's output for several years. Wind damage is worst in the eyewall, and buildings that are not tied to their foundations fail first.

Storm surge. A storm surge is not a wave: it is a dome of water pushed onto the coast by the strong winds and lifted further by the very low pressure at the centre of the storm. In open sea it may be only a metre or so, but in a shallow, funnel-shaped bay, where the water has nowhere to spread out, and when the surge arrives at high tide, it can be several metres deep and drive kilometres inland. Storm surge has drowned more people in tropical cyclones than any other hazard, especially in the low-lying deltas of the Bay of Bengal. It also floods farmland and freshwater wells with salt, and erodes beaches and dunes.

Torrential rain, flooding and landslides. The bands and the eyewall dump huge volumes of rain in a few hours. Rivers that rise in the hills inland flood one or two days after the winds have passed, which is why the flood is often a separate emergency from the wind. On steep slopes, especially where forests have been cleared, the saturated soil slides, burying villages and blocking roads; landslides during and after cyclones are a major killer in the Philippines, Haiti and in the eastern highlands of Zimbabwe and Malawi. The sequence to remember is: wind first, surge at the coast with the centre, and then rain, river flooding and landslides for days afterwards.

Impacts on people, property and economies

On people. Most deaths in a tropical cyclone are caused by drowning, in the storm surge at the coast or in the freshwater floods afterwards, followed by injuries from flying debris and collapsing buildings. Homes are destroyed or damaged, so large numbers of people have to be evacuated, and livestock have to be moved or are lost, which removes both a family's food and its savings. Drinking-water supplies are contaminated when flood water carries sewage, animal carcasses and salt into wells and pipe networks, and when treatment works are flooded. The result is water-related disease: cholera and typhoid spread quickly where people have no clean water or sanitation, and leptospirosis can follow contact with flood water. Standing water is also a breeding ground for mosquitoes, so malaria and dengue cases rise in the weeks after the storm. Damaged clinics, cut roads and the trauma of losing a home damage human health long after the winds have died down.

On crops, livestock and habitats. Crops are flattened, broken, uprooted or killed by salt water, so a cyclone can wipe out a whole rice, banana, coconut or sugar harvest and leave the soil too salty to replant for a season or more. Grazing is lost and draught animals are killed, which sets back the next planting. Coastal habitats take a heavy hit: mangroves are stripped and uprooted, coral reefs are smashed by waves and smothered by silt and freshwater run-off, seagrass beds are scoured, and nesting beaches and seabird colonies are destroyed. Since mangroves and reefs are the nurseries for many fish, the loss of habitat reduces fish catches for years afterwards, which matters most to the people who depend on fishing for their food and income.

On the economy. The financial losses come in layers: the immediate cost of clearing debris and repairing roads, bridges, ports, airports, power lines and water systems; the cost of rebuilding homes and businesses; lost earnings from destroyed crops, factories, shops and tourism; and the loss of export income when a port or a plantation is out of action for months. Insurance pays for part of the loss where people and firms are insured, but cover is often thin in poorer countries, so the cost falls on households and on the government budget, which can push a country into debt. Two storms of the same strength therefore have very different consequences: a rich country loses money, while a poorer country may lose years of development as well, because it has less money in reserve, fewer insured properties, weaker building standards and less capacity to rebuild to a higher standard.

• Deaths and injuries: drowning in the surge and floods, then debris and collapsing buildings.
• Evacuation of people and livestock, and the loss of livestock, tools and stored food.
• Contaminated drinking water leading to cholera, typhoid and leptospirosis, and mosquito-borne disease as water stands.
• Loss of crops to wind and salt, and loss of grazing and draught animals.
• Damaged mangroves, coral reefs, seagrass and beaches, so fishing and tourism incomes fall for years.
• Financial losses: repair and rebuilding costs, lost production and exports, insurance claims and government debt.

Managing the impacts: monitoring, warning, preparation, buildings, evacuation and shelters

Monitoring. Cyclones are watched from space, from the air and from the sea. Geostationary satellites take images of the cloud pattern every few minutes, so the growth, the position and the track of the storm can be followed; Doppler radar on the coast measures the rain and the wind inside the storm as it approaches; aircraft fly into the storm and drop instrument packages (dropsondes) to measure pressure, temperature, humidity and wind at different heights, and automatic buoys measure the sea temperature below. All of this feeds computer models that forecast the track and, less reliably, the strength. The limitation is at the heart of the system: track forecasts are much better than intensity forecasts, and the data are thinnest over the open ocean, exactly where the storm spends most of its life.

Warning and communication. A national meteorological service issues warnings to the government and the emergency services, and they are passed on to the public by radio and television, by mobile phone alerts and text messages, by loudspeakers and sirens, and, in the most successful programmes, by trained village volunteers who go house to house. In coastal Bangladesh the Cyclone Preparedness Programme has tens of thousands of volunteers who spread warnings and move people to shelters, and its value is measured in lives: the same country now loses far fewer people to a similar storm than it did before the programme existed. The limitation is not the forecast but the last kilometre: a warning that does not reach the fishing boats at sea, or that arrives without transport, shelter or trust, saves nobody. False alarms are a real cost too, because people are less willing to leave next time.

Buildings and infrastructure. Building codes make the difference between a roof that stays on and a roof that becomes a missile. The standard measures are to fix the roof to the walls and the walls to the foundations with metal straps and tie-downs, to use reinforced concrete columns and better mortar, to fit shutters or impact-resistant glass, to anchor water tanks and solar panels, and to raise floors above the expected flood and surge level or to put buildings on stilts. Roads, bridges, power lines and water systems are strengthened in the same way. The lessons were learnt the hard way: after Cyclone Tracy destroyed most of the buildings in Darwin, Australia in 1974, far stricter cyclone building codes were introduced across northern Australia, and newer houses rode out later storms that damaged older ones. The limitations are cost and enforcement: stronger buildings cost more to build, older buildings may need expensive retrofitting, and in informal settlements there is no code, no inspection and no money to follow one.

Preparation, evacuation and shelters. Disaster preparation means a written national or district plan, drills in schools and workplaces, a stockpile of food, water, fuel, medicine and tarpaulins, trained emergency rescue teams with boats, cutting tools and radios, and an agreed chain of command. Evacuation is phased: the most exposed areas first, usually 24 to 48 hours before landfall, along planned routes to designated safe buildings on high ground, with arrangements for livestock where possible. Cyclone shelters are purpose-built public buildings on stilts or on the highest ground, with clean water, toilets and separate space for women, and in quiet seasons many are used as schools or community halls. The limitations are human as much as technical: shelters can be too few, too far away, or poorly maintained; people may refuse to leave their homes, their animals or their belongings; and transport may not exist for those with the least money. A shelter that cannot be reached in time protects nobody, which is why drills and public information are as important as the concrete.

After the storm: rebuilding, insurance, aid and coastal defences

The response after landfall is search and rescue, then restoring water, power, roads and communications, clearing debris and providing temporary shelter, food and medicine before disease spreads. Rebuilding is the stage that decides the impact of the next storm, and the principle is to build back better: replace damaged houses to a stronger standard, move the most exposed houses off the surge zone, and rebuild the water supply and the drainage so they can survive the next event. The limitation is that rebuilding is slow, expensive and easily skipped when people are desperate, so the same weak houses are often rebuilt in the same dangerous place.

International aid arrives as emergency relief in the first weeks and as money, materials and expertise for reconstruction afterwards, from governments, United Nations agencies and non-governmental organisations. It saves lives, but it has limits: aid is pledged slowly and often arrives after the emergency peak, it is short-term, it may be tied to buying goods from the donor country, and it can weaken local responsibility if it is seen as an alternative to planning. Insurance is the other form of financial protection: households and firms buy cover, insurers pay claims, and governments can use risk-pooling schemes such as the Caribbean Catastrophe Risk Insurance Facility and the Pacific Catastrophe Risk Insurance payout that followed Cyclone Winston in Fiji in 2016, which release money quickly after a storm. The limitations are who can pay: premiums are high, insurers withdraw from the most exposed areas or refuse to renew cover, and the poorest households, who lose everything, are almost never insured.

Coastal defences divide into hard and soft engineering. Hard defences are sea walls, embankments, groynes, storm surge barriers and upgraded drainage, such as the sea walls and surge barriers that protect Japanese and Dutch coastal cities. They can stop a surge reaching a city, they last for decades and they protect the roads, ports and power stations behind them, but they are very expensive to build and to maintain, they can increase erosion further along the coast because they cut off the movement of sediment, they give a false sense of security because a storm can overtop or breach the design height, and a poorer country may have to borrow to pay for them. Soft defences work with natural systems: mangrove planting, dune and beach restoration, coral reef protection, land use zoning that keeps new hospitals and housing out of the surge zone, and planned retreat from the most exposed land. Mangroves are cheap to plant and to look after, they absorb wave energy, they trap sediment and raise the land, they shelter young fish, and they give communities timber and food, which is why replanting programmes such as those along the coast of Vietnam have reduced both wave damage and the cost of maintaining the dykes behind them. The limitations are that mangroves take years to mature, a strip has to be wide to make much difference to a large surge, and the land they need is often wanted for farms, shrimp ponds and housing, so the forest is cleared as fast as it is planted.