What Is a Tropical Cyclone?
Weather Guide • Extreme Weather • 7 minute read
A tropical cyclone is a huge, rotating storm that forms over warm ocean water and can bring winds above 250 km/h, torrential rain and a wall of seawater onto the coast. Some are small and weak. The strongest are hundreds of kilometres wide and can affect millions of people. This guide explains what a tropical cyclone is, how it forms, what the different names mean and how to stay safe when one approaches.
Tropical Cyclone, Hurricane and Typhoon: Same Storm, Different Names
These words describe the same type of storm. The name depends only on where it forms.
| Name | Where it is used |
|---|---|
| Hurricane | North Atlantic Ocean, Caribbean Sea and the northeast Pacific |
| Typhoon | Northwest Pacific, including Japan, China and the Philippines |
| Cyclone | Arabian Sea, Bay of Bengal, South Pacific and Australian region |
So a storm in the Arabian Sea that threatens Karachi or the Makran coast is called a cyclone, while the same kind of storm near Florida would be called a hurricane.
How Does a Tropical Cyclone Form?
A tropical cyclone needs several ingredients to come together at the same time. If one is missing, the storm usually fails to grow.
- Warm ocean water: sea temperature of about 26.5°C or higher, reaching down roughly 50 metres. This is the fuel.
- Moist air: humid air in the lower and middle atmosphere lets towering storm clouds keep growing.
- Light upper-level winds: strong wind shear tears a young storm apart, so calm conditions are needed.
- A starting disturbance: a cluster of thunderstorms or a low-pressure area to organise around.
- Distance from the equator: the spin of the Earth (the Coriolis effect) is needed to make the storm rotate, so tropical cyclones almost never form within about 5 degrees of the equator.
Step by step
- Warm sea water heats the air above it, and the moist air rises.
- As the vapour rises and cools, it condenses into clouds and releases heat, which makes the air rise even faster.
- Air at the surface rushes in to replace it, and the Earth’s rotation twists this inflow into a spiral.
- The pressure at the centre keeps falling, the winds grow stronger and an eye can form.
Parts of a Tropical Cyclone
The eye
The eye is the centre of the storm, usually 30 to 65 kilometres wide. Skies can be clear and winds light. It can fool people into thinking the storm has ended, but the violent winds return from the opposite direction once the eye passes.
The eyewall
The eyewall is the ring of tall thunderstorms around the eye. It has the strongest winds and the heaviest rain in the entire cyclone.
Rain bands
Curved bands of showers and thunderstorms spiral outward from the centre. They can stretch far from the eye and bring gusts, heavy rain and even tornadoes long before the main part of the storm arrives.
Which Way Does It Spin?
In the Northern Hemisphere, a tropical cyclone rotates counter-clockwise. In the Southern Hemisphere, it rotates clockwise. Both are caused by the rotation of the Earth.
Stages and Strength of a Tropical Cyclone
A tropical cyclone grows through stages, based on its maximum sustained wind speed.
| Stage | Sustained wind speed |
|---|---|
| Tropical depression | Below 63 km/h |
| Tropical storm | 63 to 118 km/h |
| Category 1 hurricane or typhoon | 119 to 153 km/h |
| Category 2 | 154 to 177 km/h |
| Category 3 (major) | 178 to 208 km/h |
| Category 4 (major) | 209 to 251 km/h |
| Category 5 (major) | 252 km/h or more |
These categories follow the Saffir-Simpson scale. Agencies in the North Indian Ocean use their own terms, such as cyclonic storm, severe cyclonic storm and very severe cyclonic storm, with slightly different wind thresholds. The category only measures wind. A weaker storm can still cause a disaster through rain or flooding.
Why Is a Tropical Cyclone So Dangerous?
Storm surge
The strong winds and the low pressure push the sea up and onto land as a wall of water. Storm surge is often the deadliest part of a tropical cyclone, especially along flat coastlines.
Destructive winds
Winds can tear off roofs, snap power poles, uproot trees and turn loose objects into flying debris.
Heavy rain and flooding
A slow-moving storm can drop huge amounts of rain. Flash floods and landslides can strike far inland, long after the winds have weakened.
Tornadoes
Rain bands can spawn short-lived tornadoes, mainly on the side of the storm where winds are strongest.
Where and When Do Tropical Cyclones Form?
Tropical cyclones form over warm tropical oceans. The main basins are the North Atlantic, the northeast and northwest Pacific, the North and South Indian Ocean and the Australian and South Pacific region. The Atlantic hurricane season officially runs from 1 June to 30 November. In the North Indian Ocean, activity peaks twice a year, before the monsoon (May and June) and after it (October and November).
Tropical Cyclones in Pakistan
Pakistan is not hit by cyclones as often as India or Bangladesh, but the risk is real. Storms form over the Arabian Sea and can threaten the coast of Sindh and Balochistan, including Karachi, Thatta, Gwadar and Ormara. Even when a cyclone does not make landfall, it can bring rough seas, strong winds, heavy rain, high tides and flooding. Cyclone Biparjoy in June 2023 is a recent example: it moved close to the Sindh coast, and thousands of people were moved to safer areas as a precaution. Fishermen are usually advised to stay out of the sea whenever a cyclone develops.
How Does a Tropical Cyclone End?
A tropical cyclone weakens when it loses its fuel. This happens when it moves over cold water, when it reaches land and loses the supply of warm moist air, or when strong wind shear breaks up its structure. A storm over land can still cause severe flooding for days, even after its winds have fallen.
Tropical Cyclone Safety Tips
- Follow official warnings from your national weather service, such as the Pakistan Meteorological Department, and the instructions of local authorities.
- Evacuate early if you are told to, especially if you live near the coast or in a low-lying area.
- Prepare an emergency kit with drinking water, dry food, medicines, a torch, a power bank, a radio and copies of important documents.
- Secure loose objects, shut windows and stay away from glass.
- Do not go near the sea. Waves and surge can reach far beyond the beach.
- Stay indoors, even if the eye passes over and it suddenly feels calm.
- Never walk or drive through flood water.
For official forecasts and tracking maps, you can visit the National Hurricane Center. For updates in your area, check the live weather and weather maps on WeatherWorld.
Do Tropical Cyclones Get Names?
Yes. Naming storms makes warnings clearer and easier to remember, especially when more than one storm is active. Each region uses its own rotating lists of names agreed by the World Meteorological Organization. Names of especially deadly or costly storms are usually retired and never used again.
Is Climate Change Affecting Tropical Cyclones?
Research suggests that a warmer climate is making the strongest tropical cyclones more intense and increasing the amount of rain they produce. Rising sea levels also make storm surge more damaging. Scientists are less certain whether the total number of storms will change. This is an active area of study, and the details differ from one ocean basin to another.
Frequently Asked Questions
What is the difference between a tropical cyclone and a tornado?
A tropical cyclone is hundreds of kilometres wide, forms over the ocean and lasts for days. A tornado is only a few hundred metres wide, forms over land from a single thunderstorm and lasts for minutes.
Why is the eye of a tropical cyclone calm?
Air sinks in the centre of the storm, which warms and dries it and keeps clouds from forming. The strongest winds are in the eyewall just outside.
Can a tropical cyclone form on land?
No. It needs warm ocean water as fuel, so it dies out soon after moving inland.
How long does a tropical cyclone last?
Most last from several days to about two weeks, although some live longer if they stay over warm water.
Can we stop a tropical cyclone?
No. A strong one releases energy on a scale far beyond anything humans can control. The best defence is early warning, preparation and timely evacuation.
Final Thoughts
A tropical cyclone is one of the most powerful weather systems on Earth. It is born over warm seas, organised by the spin of the planet and carried toward land by the winds around it. Understanding how it forms and what it brings, from storm surge to flooding rain, helps you take warnings seriously and act early. Keep an eye on official forecasts during cyclone season, prepare before the storm approaches and never take a calm eye as a sign that the danger has passed.