Maarg Manthan · Topic 5.18

Tropical Cyclones: Formation Conditions, Paths, Naming Conventions, Early Warning Systems

Geography › Climatology · Topic 5.18

A tropical cyclone is an intense, rotating storm that forms over warm tropical oceans, with a low-pressure eye, spiralling bands of thunderstorm cloud and winds of at least 62 km an hour. It needs sea surface temperature above about 26.5 °C, moist unstable air, a distance of at least 5 degrees from the equator so that the Coriolis force can act, and light winds aloft. Tropical cyclones are called hurricanes in the Atlantic, typhoons in the western Pacific and cyclones in the Indian Ocean. In the north Indian Ocean the India Meteorological Department tracks, names and warns of them.

12 min read · UPSC Prelims · Geography · Climatology

At a Glance

Needs Warm sea above about 26.5 °C and Coriolis force
Structure Eye, eye wall, spiral rain bands
Names Cyclone, hurricane, typhoon by region
Indian warning agency IMD
Biggest killer Storm surge
Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This topic follows the topics on thunderstorms and precipitation and describes the largest and most destructive storm of the tropics. It also covers how storms are named, tracked and prepared for, and it is paired with the topic on temperate cyclones. Previous topic: Types of Precipitation: Convectional, Orographic, Cyclonic, Frontal, Monsoonal. Next topic: tropopause Minimum speed Roughly 110 km per hour Polar front jet…">Jet Streams: Permanent and Temporary Types and Their Impact on Weather.

What Is a Tropical Cyclone?

In short: a tropical cyclone is a giant heat engine over warm seas, whose fuel is the latent heat in sea water vapour.

A tropical cyclone is a warm-core, low pressure storm system, 300 to 1,500 km across, with winds that spiral inward and upward around a centre. In the northern hemisphere the winds blow anticlockwise, and in the southern hemisphere clockwise, because of the Coriolis force, as in Factors Affecting Wind: Pressure Gradient Force, Frictional Force, Coriolis Force. The same storm is called a hurricane in the North Atlantic and northeastern Pacific, a typhoon in the northwestern Pacific, a tropical cyclone in the Indian Ocean and Australia, and willy-willy in northern Australia. It is thus a regional name and not a different kind of storm. The storm is a major hazard for the coasts of India, Bangladesh, the Philippines, China, Japan, the Caribbean and the southeastern United States.

What Conditions Are Needed for a Tropical Cyclone?

Condition	Why it matters
Sea surface temperature of at least 26.5 degrees Celsius, extending to a depth of about 50 m	Gives abundant evaporation and a continuous supply of moisture and heat; a deep warm layer prevents the storm from cooling the surface by mixing
Distance of at least about 5 degrees from the equator	The Coriolis force is almost zero at the equator and cannot spin the winds into a vortex
Low vertical wind shear	Strong change of wind with height tears the storm top away from its base
High humidity in the lower and middle troposphere	Dry air weakens the thunderstorms and the storm
Pre-existing disturbance	A low pressure area, an easterly wave or a monsoon trough gives the first convergence and spin
Unstable atmosphere	Allows deep convection and cumulonimbus clouds

These conditions explain the geography. Cyclones do not form on the equator, in the cold waters of the South Atlantic and the southeastern Pacific where cold currents, as in Major Ocean Currents of the World and Their Effects, cool the sea and the ITCZ lies north of the equator, or over land. Most cyclones form in late summer and autumn when the sea is warmest. In the north Indian Ocean there are two seasons, pre-monsoon in April to June and post-monsoon in October to December, because the strong shear of the summer monsoon suppresses cyclones from July to September.

How Does a Tropical Cyclone Develop?

Formation and development: warm moist air rises over the disturbance, creating a low pressure; air from the surroundings rushes in, is deflected by the Coriolis force and starts to spiral.
Latent heat feedback: as the rising air cools, vapour condenses into cumulonimbus clouds and releases latent heat, which warms the core, lowers the pressure further and draws in more moist air. This positive feedback is the engine, as in Humidity, Saturation, Dew Point, Evaporation and Condensation.
Intensification: the pressure falls and the wind increases. The system moves up from a depression to a deep depression, then a cyclonic storm, and so on. Sometimes the storm intensifies by at least 30 knots in 24 hours, which is called rapid intensification.
Maturity and decay: the storm reaches its peak while over warm water. On landfall, or over cold water, the supply of moisture is cut off and friction increases, so it weakens in a day or two.

What Is the Structure of a Tropical Cyclone?

Eye: the eye of the cyclone is a calm, clear and dry central zone, usually 20 to 50 km wide, where air sinks and warms. The pressure is the lowest here, and winds are light. When the eye passes, there is a deceptive lull before the other side of the eyewall arrives.
Eyewall: a ring of tall cumulonimbus clouds around the eye with the strongest winds and heaviest rain.
Spiral rainbands: curved bands of thunderstorms that extend outward for hundreds of kilometres.
Outflow: at the top, air spreads out at high levels, usually in an anticyclonic pattern.

The most destructive effects are the storm surge, the rise of sea water caused by low pressure and onshore winds, which can raise the water several metres above the normal tide and flood low coasts; the winds; and the heavy rain with floods.

Where Are the Breeding Grounds of Tropical Cyclones?

In short: cyclones breed over warm tropical seas between roughly 5 and 20 degrees latitude, in seven ocean basins.

Basin	Name of the storm	Notes
North Atlantic and Caribbean	Hurricane	June to November; hits the Caribbean and the southeastern United States
Northeast Pacific	Hurricane	Off Mexico and Central America
Northwest Pacific	Typhoon	The most active basin with about a third of all storms; the Philippines, China, Taiwan and Japan are hit
North Indian Ocean	Cyclone	Bay of Bengal and Arabian Sea; only about 5 per cent of global storms but very deadly because of dense coastal populations
Southwest Indian Ocean	Cyclone	Madagascar, Mozambique and the Mascarene islands
Australian region	Cyclone, willy-willy	Northern Australia and the Timor and Coral seas
Southwest Pacific	Cyclone	Fiji, Vanuatu, Tonga and Samoa

The southern Atlantic and the southeastern Pacific are almost free from storms because of cold currents and strong wind shear.

How Do Tropical Cyclones Move?

The cyclone track is the path of its centre. A tropical cyclone is steered by the winds in which it is embedded. In its early life it travels westward, carried by the easterly trade winds, and also drifts poleward because of the way the Coriolis force acts on the vortex. When it reaches the edge of the subtropical high pressure belt, at around 20 to 30 degrees, it often recurves, turning toward the pole and then to the east under the influence of the westerlies, as in General Circulation of the Atmosphere: Hadley Cell, Ferrel Cell, Polar Cell. Typhoons recurve toward Japan, and hurricanes toward the eastern United States. Over land the storm loses its moisture supply and weakens.

In the north Indian Ocean the geography changes the pattern. The storms of the Bay of Bengal usually move west or northwest and strike Odisha, Andhra Pradesh, Tamil Nadu or West Bengal, or recurve to hit Bangladesh and Myanmar. The Arabian Sea storms move toward Oman, Yemen, Pakistan or Gujarat, or turn northwest. The Bay of Bengal produces about four times more storms than the Arabian Sea, because the sea is warmer and has a better supply of low pressure systems from the monsoon trough, and the land around it funnels water into a shallow, flood-prone coast.

How Is the Intensity of a Cyclone Measured?

IMD category	Wind speed (3-minute mean)
Low pressure area	Less than 17 knots
Depression	17 to 27 knots
Deep depression	28 to 33 knots
Cyclonic storm	34 to 47 knots
Severe cyclonic storm	48 to 63 knots
Very severe cyclonic storm	64 to 89 knots
Extremely severe cyclonic storm	90 to 119 knots
Super cyclonic storm	120 knots (about 222 km per hour) and above

One knot is about 1.85 km per hour. The International scale for hurricanes and typhoons in the Atlantic and eastern Pacific is the Saffir-Simpson scale, based on one-minute sustained winds:

Category	Wind speed
Category 1	119 to 153 km per hour
Category 2	154 to 177 km per hour
Category 3	178 to 208 km per hour (major hurricane)
Category 4	209 to 251 km per hour
Category 5	252 km per hour and above

The scales are not directly comparable because of different averaging times. The Saffir-Simpson scale measures wind only, and it does not count the storm surge or rainfall, which often cause more deaths.

Which Cyclones Have Hit India?

Odisha super cyclone (October 1999): struck near Paradip with winds of about 260 km per hour and a surge of several metres; killed around ten thousand people. It led to the creation of the Odisha State Disaster Management Authority, the first of its kind in India.
Phailin (2013): a very severe cyclone in Odisha, with a mass evacuation of about a million people and very few deaths.
Fani (2019): an extremely severe storm that made landfall near Puri in Odisha, again with a large evacuation.
Amphan (2020): a super cyclonic storm in the Bay of Bengal that weakened before landfall near the Sundarbans, and caused huge damage in West Bengal and Bangladesh.
Biparjoy (2023): a very severe cyclone in the Arabian Sea, one of the longest lasting there, which made landfall in Kutch, Gujarat, in June.
Montha (October 2025): a severe cyclonic storm that made landfall in Andhra Pradesh between Machilipatnam and Kakinada, with tens of thousands moved to relief camps.

The deadliest cyclone known was the Bhola cyclone of 1970 in East Pakistan, now Bangladesh, which killed several hundred thousand people. As an analytical point, the Odisha super cyclone and Phailin show that the number of deaths is related more to preparation than to wind speed: a storm of comparable strength killed about ten thousand in 1999 and very few in 2013. The Arabian Sea storms such as Biparjoy are an increasing concern as the sea warms.

How Are Tropical Cyclones Named?

In short: cyclone naming gives a storm a short, neutral name from an agreed list so that warnings are clear and not confused between simultaneous storms.

Naming began in the Atlantic and Pacific, and for the north Indian Ocean the member countries of the WMO/ESCAP Panel on Tropical Cyclones agreed a list in 2000. Naming began in September 2004 with eight countries: Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand. In 2018, five more were added, Iran, Qatar, Saudi Arabia, the United Arab Emirates and Yemen, making 13 members. Each country gives a set of names, and the names are used in order, country by country in alphabetical order, as the storms reach the cyclonic storm stage with winds of 34 knots. A name is never reused.

Rules: the name must be short, easy to pronounce, culturally neutral, not offensive to any group and not already used.
Examples: Amphan (Thailand), Fani (Bangladesh), Biparjoy (Bangladesh), Mocha (Yemen), Remal (Oman) and Montha (Thailand), which hit Andhra Pradesh in October 2025.
Who names: the India Meteorological Department assigns the next name on the list when a system becomes a cyclonic storm.

How Does India Warn Against Cyclones?

The India Meteorological Department, with its headquarters in New Delhi, is one of the six Regional Specialised Meteorological Centres of the World Meteorological Organization and gives cyclone advisories for the north Indian Ocean to the 13 member countries. It tracks the storms with satellites of the INSAT series, Doppler weather radars along the coasts, buoys, ships and numerical models.

Stage	Lead time before expected landfall	Content
Pre-cyclone watch	About 72 hours	Early indication of the development of a cyclone and a possible threat to the coast
Cyclone alert	About 48 hours	Detailed information on track, intensity and areas to be affected
Cyclone warning	About 24 hours	Specific times and places of landfall, expected winds and surge
Post-landfall outlook	About 12 hours before landfall to the end	Likely course and effects inland

Warnings use colour codes, green for no action, yellow for being aware, orange for being prepared and red for taking action. The track forecast is shown with a cone of uncertainty, because the exact path is never certain. State governments act on the warnings by evacuating people, closing ports and warning fishermen. The accuracy of the track forecasts in India has improved greatly, with landfall errors now of only some tens of kilometres at 24 hours, as seen in the forecast of Fani in 2019.

What Are the Effects of a Tropical Cyclone?

Storm surge: the sea rises by several metres as the storm pushes water ashore, flooding low coasts. It is the biggest killer, especially when it coincides with high tide. The funnel shape of the Bay of Bengal coast amplifies it.
High winds: damage kutcha houses, trees, power lines and crops, and can generate flying debris.
Heavy rain and floods: rainfall of several hundred millimetres in a day, with flooding of rivers and landslides in the hills.
Salinisation: sea water ruins farmland, wells and ponds for years.
Economic and social effects: loss of life, fishing boats, crops and infrastructure; the poor and fishing communities suffer most.
Some benefits: cyclone rain can end a drought, fill reservoirs and cool the sea, but these benefits seldom outweigh the losses.

Why Is This Topic Important?

Conditions Asked in Prelims and Mains
Geography Why not near the equator
Climate Stronger, faster-intensifying storms

What Came Next?

The next topic describes jet streams, the fast winds high in the troposphere. Next topic: Jet Streams: Permanent and Temporary Types and Their Impact on Weather.

Key People and Terms

Tropical cyclone Rotating warm-core storm over tropical seas
Eye Calm centre
Eyewall Ring of the strongest winds
Storm surge Sea level rise driven by the storm
Rapid intensification Gain of 30 knots in 24 hours
Wind shear Change of wind with height

Exam Corner

Points to Remember

Sea temperature must be at least 26.5 degrees Celsius.
No cyclones within about 5 degrees of the equator.
Winds spiral anticlockwise in the north.
The eyewall has the strongest winds.
Storm surge kills the most people.

Do Not Confuse With

Eye and eyewall Calm centre versus fiercest ring
Hurricane and typhoon Same storm, different ocean
Storm surge and tidal wave Storm-driven versus tide-driven or tsunami

Memory Hook

Warm sea, spin, calm shear, a first spark: latent heat winds the engine, the eye stays quiet.

Mains Angle

Questions ask for conditions and structure. These points add depth.

Energy Latent heat feedback
Limits Cold currents and equator
Trend Stronger storms with warming

A Question You May Face

An original practice question, not a past paper question.

“Explain the conditions for the formation of tropical cyclones and why they do not form near the equator or in the South Atlantic.”

How to Answer

Conditions Warm sea, Coriolis, low shear.
Process Latent heat feedback.
Exceptions Equator and cold currents.
Trend Warming and intensification.

GS Relevance

Prelims: conditions for formation, structure, paths, naming, IMD warnings, intensity scales, Indian cyclones. GS Paper 1: tropical cyclones; GS Paper 3: disaster management.

Frequently Asked Questions

What is a tropical cyclone?

A tropical cyclone is an intense, rotating, low pressure storm that forms over warm tropical oceans, with spiral rainbands, a ring of strong winds and a calm eye. It gains energy from the latent heat released when vapour from the warm sea condenses.

What conditions are needed for a tropical cyclone to form?

A tropical cyclone needs sea surface temperatures of at least 26.5 degrees Celsius to about 50 metres deep, a distance of more than about 5 degrees from the equator for the Coriolis force, low wind shear, high humidity and an initial disturbance.

Why do cyclones not form on the equator?

The Coriolis force, which is caused by the rotation of the Earth, is zero at the equator and very weak close to it. Without it, the inflowing air cannot be deflected to spiral around a low pressure centre, so a cyclonic vortex cannot form there.

Which is the most active cyclone basin?

The northwestern Pacific is the most active tropical cyclone basin, producing around one third of all storms in the world, where they are called typhoons. They affect the Philippines, China, Taiwan, Japan and Vietnam, and the season peaks from July to October.

Who names cyclones in the north Indian Ocean?

The India Meteorological Department assigns names to cyclones in the north Indian Ocean from a list provided by the 13 member countries of the WMO/ESCAP Panel on Tropical Cyclones. The naming system began in 2004, and each name is used only once.

PYQ Practice — Statement Analysis

1 Tropical cyclones need sea surface temperatures above about 26.5 degrees Celsius.

True

Warm water supplies heat and moisture.

2 Cyclones commonly form on the equator.

False

The Coriolis force is too weak there.

3 The eye of a cyclone has the strongest winds.