Tropical cyclones are rotating low-pressure systems that develop over warm tropical oceans.
Sea-surface temperatures of about 27°C or higher must extend through a sufficiently deep layer to provide sustained heat and moisture.
A pre-existing zone of converging winds supplies rising humid air and clusters of thunderstorms.
Condensation releases latent heat, warming the air column and causing further uplift and falling surface pressure.
More air flows towards the low-pressure centre, and the Coriolis effect turns this inflow into a rotating circulation.
Low vertical wind shear allows the circulation to remain vertically aligned and strengthen.
The sequence diagram connects warm ocean water, evaporation, latent-heat release, low pressure and Coriolis-driven rotation.
Common Mistake
Warm water alone is not enough for hurricane formation.
Strong wind shear can disrupt a developing storm even when the ocean is warm.
A Positive Feedback Strengthens the Storm
Falling central pressure steepens the pressure gradient and increases wind speed.
Faster surface winds increase evaporation from the warm ocean.
More water vapour condenses in the eyewall and releases more latent heat.
The heated air rises faster, pressure falls further and the cyclone can intensify while favourable conditions persist.
Note
The eye contains descending air and relatively calm conditions.
The surrounding eyewall contains the strongest uplift, rain and winds.
Distribution Follows Physical Controls
Tropical cyclones form mainly between about 5° and 30° latitude because Coriolis force is too weak close to the equator.
They occur in the North Atlantic, north-east and north-west Pacific, north and south Indian Ocean, and south-west Pacific basins.
The same hazard is called a hurricane in the Atlantic and north-east Pacific and a typhoon in the north-west Pacific.
Activity usually peaks when tropical seas are warmest, although the timing differs between ocean basins.
Example
Cyclone Amphan crossed the Bay of Bengal in May 2020 and caused severe coastal flooding in India and Bangladesh.
The example shows why densely populated deltas face high exposure to tropical-cyclone hazards.
Storm Surge Drives Coastal Flooding
A storm surge is an abnormal rise of sea level generated mainly when strong onshore winds push seawater towards the coast.
Low atmospheric pressure contributes to the rise, while large waves add destructive energy above the surge level.
A shallow continental shelf, a funnel-shaped bay and landfall near high tide can increase the depth and reach of flooding.
Surge water can overtop or breach defences, erode beaches and dunes, contaminate soils with salt, and damage wetlands.
Wind, waves and intense rainfall add roof damage, river flooding, landslides and infrastructure failure.
Common Mistake
Storm surge is not the same as a tsunami.
A surge is driven by a storm’s winds and pressure, while a tsunami is usually generated by sudden displacement of seawater.
Case Study: Hurricane Katrina, USA, 2005
Katrina strengthened over the Gulf of Mexico before making its main Gulf Coast landfall on 29 August as a Category 3 hurricane.
Storm-surge flooding reached about 25 to 28 feet above normal tide level along parts of the Mississippi coast.
Surge overtopped and breached flood defences around New Orleans, inundating much of the city.
The official National Hurricane Center update records 1,392 fatalities, including direct, indirect and indeterminate causes.
Damage extended across homes, transport, energy infrastructure, wetlands and coastal settlements in Louisiana and Mississippi.
Older residents, households without private transport and communities in low-lying districts faced greater evacuation and recovery barriers.
The disaster shows how an extreme physical event becomes a catastrophe through exposure, uneven vulnerability and defence failure.
Exam technique
Separate the formation sequence from the factors that control global distribution.
For coastal impacts, link storm surge to coastal shape, elevation, defences and population exposure.
Use Katrina to connect physical processes with unequal social and economic consequences.
Case study
Essay use
Use Hurricane Katrina to show that cyclone intensity alone does not determine disaster severity because exposure, protection and social vulnerability mediate impacts.
Evidence
Katrina made its main Gulf Coast landfall on 29 August 2005 as a Category 3 hurricane, produced a 25–28-foot storm surge on parts of the Mississippi coast and caused 1,392 recorded fatalities.
Analysis
Surge overtopped and breached flood defences around New Orleans, converting a coastal hazard into extensive urban flooding and cascading damage to housing, transport, energy and wetlands.
Evaluation
Older residents, low-income households and people without private transport faced greater evacuation and recovery barriers, so national loss totals conceal unequal neighbourhood-scale outcomes.
Risk Changes Along the Coast
Risk is highest where intense cyclones meet low-lying coasts, dense settlement and limited evacuation capacity.
Wetlands, dunes and barrier islands can reduce wave energy, so their loss removes a layer of natural protection.
Warnings, evacuation routes, building standards and maintained defences can reduce losses without removing the hazard.
Active recall
What four environmental conditions allow a tropical cyclone to strengthen?
Why do tropical cyclones rarely form within 5° of the equator?
What physical factors can increase a storm surge?
How did defence failure worsen Katrina’s impacts in New Orleans?
