Physical Hazards of Tropical Cyclones

One storm, several physical hazards

A tropical cyclone is a powerful rotating storm that develops over warm tropical seas. Its strong winds and heavy rainfall create several linked physical hazards: high winds, intense rainfall, storm surges, coastal flooding and landslides.

A hazard is something with the potential to cause harm; an impact is what happens when that hazard affects people or environments. For example, intense rainfall is a hazard, while flood damage to a home is an impact. Understanding the physical processes helps explain where and how danger develops.

High winds: damage from moving air

Tropical cyclones produce very strong winds, which can reach around 250 km/h in powerful storms. These winds can uproot trees and damage buildings and infrastructure. Falling trees and wind-blown debris can injure people, so the danger comes both from the force of the wind and from objects it moves.

Wind also creates hazards at sea. It generates large waves and, where it blows towards the shore, pushes seawater towards the coast. This connects the wind hazard to storm surge and coastal flooding.

Intense rainfall: flooding beyond the coast

Tropical cyclones release very large amounts of rain. When rain falls faster than it can soak into the ground or drain away, water collects and flows across the surface. Rivers can overflow, and flash flooding can develop rapidly. Fast-flowing floodwater can damage property and injure or drown people.

Rainfall hazards are not confined to the point where the cyclone reaches the coast. Heavy rain can cause extensive flooding well inland. A place away from the sea may therefore escape storm surge but still face dangerous floodwater.

Storm surge: seawater pushed towards land

A storm surge is a storm-driven rise in sea level. Its main cause is strong winds pushing seawater towards the coast, where it piles up. The cyclone's low air pressure also allows sea level to rise, but this contribution is smaller than the wind's effect.

Storm surge is not the same as an individual wave. The surge raises the water level, while wind-generated waves travel on top of it. These waves can damage sea defences or spill over them, adding to the flood risk.

The danger also depends on timing. If a surge arrives at high tide, the sea is already higher, so flooding may be more extensive. The storm's size, strength and direction of approach, together with the shape of the coastline and seabed, influence the surge's height.

Coastal flooding: water spreads across low-lying land

Coastal flooding happens when water covers normally dry land near the coast. Storm surge can drive seawater inland, while intense rainfall adds further floodwater. Low-lying coastal land can be flooded over a large area.

This distinguishes the process from its result: storm surge raises the sea level; coastal flooding is the inundation of land. Seawater can contaminate farmland and freshwater supplies, while surge and waves can erode beaches and damage sea defences.

Landslides: rainfall destabilises slopes

Intense rainfall can saturate soil on steep slopes. As the soil becomes waterlogged, it may no longer remain stable and can slide downhill under gravity. This is a landslide.

The connection is important: rainfall does not only create floodwater. It can also trigger the movement of soil and debris, threatening buildings and routes below a slope.

Assessing which hazards pose the greatest danger

Wind speed alone does not describe the full danger of a tropical cyclone. At a low-lying coast, a surge arriving at high tide may make coastal flooding especially serious. Inland, intense rainfall may create flash floods; on steep slopes, the same rainfall may trigger landslides.

The hazards can also reinforce one another. Strong winds push seawater towards land, waves act on top of the raised sea level, and heavy rainfall adds water. Assessing danger therefore means considering the hazards together, their timing and the physical setting—not assuming that the strongest winds always cause the greatest harm.