Tropical Cyclone: Meaning, Formation, Structure and Classification

What Is a Tropical Cyclone? Meaning and Regional Names

A Warm-Core, Non-Frontal Low over Tropical Seas

The India Meteorological Department (IMD) treats a low as a tropical cyclone once its central pressure falls 5 to 6 hPa below its surroundings and its maximum sustained wind reaches 34 knots, about 62 km per hour. A tropical cyclone draws its energy from the heat released when water vapour condenses into cloud, which is why it forms only over warm water and weakens soon after it reaches land.

Tropical cyclones form over the oceans between about 30° N and 30° S and do their damage when they reach the coast, through violent winds, very heavy rain and storm surges.

Two terms describe its nature. It is warm-core, meaning that its centre is warmer than the air around it at the same level, and non-frontal, meaning that no warm or cold front is attached to it, unlike the cyclones of the middle latitudes. Its winds are strongest near the surface. Because it forms within a single warm, moist air mass, the temperature differs little across the storm at the surface.

The word cyclone comes from the Greek "cyclos", meaning the coiling of a snake. It was coined by Henry Piddington, who worked as a Rapporteur in Kolkata during British rule, to describe the coiled winds of these storms.

Difference Between Cyclone, Hurricane and Typhoon

A cyclone, a hurricane and a typhoon are the same kind of storm. The scientific term for all of them is tropical cyclone; only the local name changes with the ocean, so the difference between a tropical cyclone and a hurricane is one of place, not of physics.

A storm can even change its name on the way. When a hurricane crosses the International Date Line westward in the northern hemisphere, it becomes a typhoon.

Cyclone and Anticyclone: Wind Direction and the Coriolis Force

Winds turn around a cyclone because the Earth rotates. The rotation sets up an apparent force, the Coriolis force, named after the French scientist Gaspard-Gustave de Coriolis, who set out its mathematics in 1835. It deflects moving air to the right in the northern hemisphere and to the left in the southern hemisphere. When a low starts to form north of the equator, surface winds flow inward to fill it, are deflected to the right, and an anticlockwise rotation begins; south of the equator the deflection is to the left and the rotation is clockwise.

Three properties of the Coriolis force explain where cyclones can form:

• Latitude: The Coriolis force grows with latitude, in proportion to the sine of the latitude, so it is maximum at the poles and absent at the equator.
• Wind speed: The deflection is greater when the wind velocity is high.
• Filling of the low: At the equator, where the force is zero, wind blows straight across the isobars into a low, and the low fills up instead of deepening. That is why tropical cyclones are not formed near the equator.

The force acts on large, long-lived motions such as winds and ocean currents. It is far too tiny to decide which way water turns as it drains from a sink; the shape of the basin and the original motion of the water do that, so both directions occur in either hemisphere.

How Cyclones Are Formed: Conditions, Stages and Basins

Six Conditions for Formation: Warm Sea, Coriolis Force and Low Shear

Tropical cyclones originate and intensify over warm tropical oceans, but warm water alone is not enough. Six conditions have to come together for a weak disturbance to grow into a storm.

Some textbooks list five conditions instead, namely a large sea surface warmer than 27 °C, the Coriolis force, small variations in vertical wind speed, a pre-existing weak low or low-level cyclonic circulation, and upper divergence above the sea-level system. These conditions are necessary but not sufficient, and many disturbances that seem to meet them never develop.

From Easterly Wave to Cyclonic Storm: The Stages of Formation

Cyclone formation usually begins with a small wave in the tropical easterlies. In the tropics, weak pressure waves called easterly waves move from east to west; they have a period of about 3 or 4 days and a wavelength of 2,000 to 2,500 km, and they serve as the seedling circulations of a large share of tropical cyclones. The steps that turn one of them into a storm follow in order:

1. A low forms: Under favourable conditions a low-pressure area forms in the trough of an easterly wave, and air converges into it near the surface.
2. Air rises: Over warm sea, with air being blown off at higher levels, low-level convergence and upper-level divergence together drive moist air upwards.
3. Vapour condenses: The moisture condenses in the middle troposphere and gives out the latent heat of condensation.
4. Pressure falls further: The released heat warms the area, the pressure falls further, and more air converges to replace the rising air.
5. A cyclonic storm forms: The process feeds itself, and the low gradually intensifies into a cyclonic storm.

In the Indian Ocean and the western Pacific, cyclone formation depends less on easterly waves and more on the seasonal movement of the Intertropical Convergence Zone and the monsoon trough.

Sea Surface Temperature Rise and Tropical Cyclones

Sea surface temperature, the temperature of the top layer of the ocean, is the most important factor in a tropical cyclone's development, because warm water is the storm's fuel. The warm layer must also be deep, since the storm stirs up the water beneath it and leaves a cool wake that makes the same area less favourable for the next storm. A cyclone that moves over water much cooler than 26.5 °C weakens.

A rise in sea surface temperature therefore gives a storm more fuel, and its effects are already visible:

• Stronger storms: The global proportion of major tropical cyclones (Category 3 to 5) has likely increased over the last four decades, and the proportion of the most intense ones is projected to rise further with warming.
• Heavier rain: Warmer air holds about 7 per cent more water vapour for each 1 °C of warming, and human-induced climate change increases the heavy rain that comes with tropical cyclones.
• Rapid intensification: IMD defines rapid intensification as an increase of 30 knots in 24 hours. During Cyclone Amphan's rapid intensification, the Bay of Bengal was as warm as 33 °C.
• Arabian Sea: Sea surface temperatures there have risen by 1.2 to 1.4 °C in recent decades. In 2019 eight cyclonic storms formed in the Arabian Sea, a record tied with 1902, and between 1982 and 2019 the number of its cyclones rose by 52 per cent and very severe cyclones by 150 per cent.

The full account of warming seas and cyclones is in the article on cyclones and climate change.

Why No Cyclones Form Near the Equator or in the South Atlantic

Without the Coriolis force, the low pressure of a disturbance cannot be maintained; air rushes in and fills it. For this reason a narrow corridor about 300 km wide on either side of the equator almost never sees cyclones, and most storms form more than about 500 km, roughly 5°, from it. Typhoon Vamei, which formed near Singapore in December 2001 with its centre at 1.5° N, is a rare exception. For the same reason tropical cyclones do not migrate across the equator from one hemisphere to the other.

Tropical cyclones form in the lower latitudes of all oceans except the South Atlantic and the south-east Pacific. In the South Atlantic several things stand in the way:

• Strong wind shear: Very strong vertical wind shear in the troposphere disrupts developing storms.
• No disturbances: There is a lack of weather disturbances favourable for development.
• ITCZ too close to the equator: The Intertropical Convergence Zone drops only one to two degrees south of the equator, not far enough for the Coriolis force to help.
• Cooler water: The tropical South Atlantic is cooler than the tropical North Atlantic.

Hurricane Catarina, which struck southern Brazil on 28 March 2004, is the only recorded hurricane-strength tropical cyclone of the South Atlantic.

Where Tropical Cyclones Form: The Seven Basins

Tropical cyclones form in seven regions of the world, each with its own season. On average about 86 tropical cyclones of tropical storm strength form every year worldwide, and about 47 of them reach hurricane or typhoon strength.

• North Atlantic, Gulf of Mexico and Caribbean: The season officially runs from 1 June to 30 November, with peak activity in early to mid September.
• North-east Pacific: Activity runs from late May or early June to late October or early November.
• North-west Pacific, including the South China Sea: Storms occur all year, with the main season from July to November; this basin produces about 35 per cent of the world's tropical cyclones.
• North Indian Ocean (Bay of Bengal and Arabian Sea): Storms occur from April to December, with a double peak in May and November.
• South-west Indian, South-east Indian and Australian, and Australian and South-west Pacific: These southern basins are active from about late October or November to May.

The seas that see the most storms share the same advantages: They lie between about 5° and 30° from the equator, they hold very warm water, and they are fed by disturbances. The South China Sea lies in the path of storms from the busiest basin; the Bay of Bengal forms its own storms and also receives the remnants of north-west Pacific typhoons; and the Gulf of Mexico lies in the North Atlantic basin, where the trade winds steer easterly waves westward from the African coast towards the Caribbean Sea and North America, seeding many of its storms.

Structure of a Tropical Cyclone: Eye, Eyewall and Rainbands

Size, Height and Wind Profile of a Mature Cyclone

A mature tropical cyclone is marked by strong, spirally circulating wind around a centre called the eye. Around the eye rises the eyewall, and beyond it lie spiral rainbands. On radar a mature cyclone shows an eye, an eyewall, spiral bands, pre-cyclone squall lines and streamers.

• Size: The strongly circulating core is 150 to 250 km across. A storm over Indian seas has a radius of 50 to 100 km at the smallest and about 2,000 km at the largest, and 300 to 600 km on average.
• Height: The storm extends from the surface to a height of about 12 to 15 km, with the eyewall reaching up to the tropopause.
• Wind profile: In the band of maximum winds, 10 to 150 km wide, wind speed falls rapidly towards the eye, but it falls only slowly and irregularly outward from the eyewall.
• Strongest side: The strongest winds are generally on the right side of the storm with respect to its motion, because the storm's forward movement adds to its swirling winds there.

The Eye of a Cyclone: Diameter, Temperature and Pressure

The eye of a cyclone is a roughly circular area of comparatively light winds and fair weather at the centre of a severe tropical cyclone. In a fully developed storm the eye is a cloud-free region of calm winds, usually 30 to 60 km across, though eyes range from 8 km to over 200 km. There is little or no rain, and sometimes blue sky or stars can be seen.

The eye has the lowest surface pressure in the storm and the warmest temperatures aloft. At a height of 12 km it may be 10 °C or more warmer than the surrounding air, though only 0 to 2 °C warmer at the surface. The warmth comes from sinking air. The air in the eye descends slowly, is compressed and warms, and the sinking usually stops about 1 to 3 km above the sea instead of reaching the surface.

Only some cyclones develop an eye. Before a storm reaches very severe cyclonic storm strength of 64 knots, a uniform shield of cold cloud tops usually hides the centre. Once it reaches that strength, an eye can usually be seen on satellite images.

Eyewall, Spiral Rainbands and the Central Dense Overcast

The eyewall is the roughly circular ring of deep convection around the eye, where there is a strong spiralling ascent of air up to the tropopause. It is the area of the highest surface winds in the storm, reaching as high as 250 km per hour, and torrential rain falls here. This wall cloud region is about 10 to 150 km wide.

• Spiral rainbands: Long, narrow bands of convection lined up with the horizontal wind. They seem to spiral into the centre, and trains of cumulus and cumulonimbus clouds drift outward along them.
• Central dense overcast: The cirrus cloud shield spread by the thunderstorms of the eyewall and rainbands. A nearly circular one shows low vertical wind shear, a sign that the storm can strengthen.
• Concentric eyewalls: In some of the most intense storms an outer ring of convection forms around the eyewall and contracts inward. The inner eyewall weakens and is replaced by the outer one, and the cyclone itself weakens for a short time while this happens.

Energy, Movement and Landfall of Tropical Cyclones

Tropical Cyclone as a Heat Engine: Latent Heat and Energy

A tropical cyclone can be compared to a heat engine. The energy input comes from warm water and humid air over tropical oceans, and the energy is released as water vapour condenses into droplets and rain in the towering cumulonimbus clouds around the centre. With a continuous supply of moisture from the sea the storm grows stronger; on reaching land the moisture supply is cut off and the storm dissipates.

• Efficiency: Only a small share of the released energy, about 3 per cent, is converted into the kinetic energy of the winds.
• Scale: A mature cyclone releases energy equivalent to that of 100 hydrogen bombs.
• Heat transport: Tropical cyclones take heat stored in the ocean to the upper atmosphere, where upper-level winds carry it towards the poles. This keeps the polar regions warmer and the tropics cooler than they would otherwise be.

Movement of Tropical Cyclones: Track, Speed and Recurvature

Tropical cyclones move from east to west, the opposite of temperate cyclones, which move from west to east. In the northern hemisphere they usually move west-north-westward or north-westward at an average of 15 to 20 km per hour, or 360 to 480 km a day. Some then turn north, slowing to 10 km per hour or less, and a large share of these later turn north-east and move fast, at 25 km per hour or more.

IMD groups cyclones by their speed of movement:

• Slow-moving: 10 to 14 km per hour.
• Moderately moving: 15 to 25 km per hour.
• Fast-moving: More than 25 km per hour.

The place where a tropical cyclone crosses the coast is its landfall. Cyclones that cross latitude 20° N generally recurve, turning north and then north-east, and these recurving storms are more destructive. Slow-moving, large cyclones give more rain than fast-moving, small ones, and more than 90 per cent of a storm's rainfall falls within 200 km of its centre.

Fujiwhara Effect: When Two Cyclones Interact

The Fujiwhara effect occurs when two nearby cyclones move around each other and close the distance between them. It is named after the Japanese meteorologist Sakuhei Fujiwhara, who described it in a 1921 paper. Tropical cyclones usually interact when they are within about 1,400 km of each other.

The two centres circle a point between them, anticlockwise in the northern hemisphere and clockwise in the southern. If the two storms are of equal size they orbit that point or push each other aside; if they are unequal, the larger one dominates and the smaller circles around it. Storms that come close often spiral in and merge, and a merger can build a larger cyclone.

• Effect on movement: Tracks bend, stall or swing aside. In 2009 Typhoon Parma stalled near the Philippines while it interacted with Typhoon Melor, and in 2017 Hurricane Irwin changed course to the north-west before it died out.
• Effect on intensity: One storm can grow at the other's expense. In July 2017 Hurricane Hilary became stronger while Hurricane Irwin became weaker.
• Indian Ocean cases: Cyclones Diamondra and Eunice interacted in 2015 in the central Indian Ocean, and Cyclones Seroja and Odette in 2021 in the eastern Indian Ocean.

Landfall and Weakening of Tropical Cyclones over Land

A few hours after landfall, a tropical cyclone begins to weaken rapidly. The main reason is not friction but the loss of the moisture and heat that the ocean provided. Without them the storm cannot keep up its thunderstorms near the centre, and without that convection it cannot survive.

Friction acts in two directions at once. The rougher land surface of bushes, trees and houses lowers the sustained winds, while the extra turbulence brings faster air down to the surface in short bursts, so the gusts become stronger. There are exceptions. The Odisha super cyclone of October 1999 remained practically stationary over coastal Odisha and kept cyclonic storm intensity for 24 hours after landfall.

A tropical cyclone can also end at sea. It weakens over water much cooler than 26.5 °C or under strong wind shear, and as it recurves poleward and eastward it often changes into an extra-tropical cyclone, a transition that is one of the most difficult problems in forecasting.

Classification and Naming of Tropical Cyclones

IMD Classification of Cyclonic Disturbances: Depression to Super Cyclonic Storm

IMD classifies the low-pressure systems of the Indian region by the maximum sustained wind speed, averaged over 3 minutes, and by the pressure deficit, measured as the number of closed isobars at 2 hPa intervals. The pressure criteria are used when the system is over land, and the wind criteria when it is over the sea.

By the isobar count, a system is a low with one closed isobar, a depression with two, a deep depression with three, and a cyclonic storm with four or more. From satellite images, IMD estimates intensity with the Dvorak technique, which assigns a "T number" from T1.0 to T8.0 in steps of 0.5; T2.5 corresponds to a cyclonic storm. Other warning centres use other scales, such as the Saffir-Simpson scale for Atlantic hurricanes, which is why the same storm can carry different labels.

How Tropical Cyclones Are Named in the North Indian Ocean

Tropical cyclones are named so that forecasters and the public can talk about warnings without confusion, especially when a storm lasts a week or more or when more than one storm is active in the same basin at the same time. The first proper names were given by an Australian forecaster early in the twentieth century. North Atlantic storms received women's names from 1953, and men's names were added in 1979. Since 1 January 2000 the north-west Pacific has used Asian names contributed by the members of the WMO's Typhoon Committee, most of them names of flowers, animals, birds, trees or foods rather than of people.

Tropical cyclones of the north Indian Ocean have been named since October 2004. IMD, as the Regional Specialised Meteorological Centre in New Delhi, names a storm when it intensifies into a cyclonic storm, with 3-minute sustained winds of at least 34 knots.

The names come from a list adopted by the WMO/ESCAP Panel member countries in April 2020. The Panel has 13 member countries: Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, the United Arab Emirates and Yemen.

• Order of countries: The member countries are listed alphabetically, and each contributes one name to every column of the list.
• Order of use: The names are used one after another, column by column, starting from the first row of column one and running to the last row of column thirteen.
• Used once: Each name in the table is used only once.
• Storms from the Pacific: A storm that moves into the basin from the western Pacific keeps its original name, but if it weakens into a deep depression and re-intensifies here, it gets a new name.

India's names on the list include Gati, Tej, Murasu and Aag, and Bangladesh's include Nisarga, Biparjoy and Arnab.

Life Cycle of a Tropical Cyclone: From Disturbance to Dissipation

A tropical cyclone passes through recognisable stages. A disturbance first becomes a low, then a depression and a deep depression; if conditions stay favourable it becomes a cyclonic storm, and the strongest go on to severe, very severe, extremely severe or super cyclonic strength. After landfall, or over cooler water, it weakens back through the same stages and dissipates or turns extra-tropical.

• How many grow: Of all the disturbances in the north Indian Ocean, 35 per cent intensify into cyclones, 16 per cent into severe cyclones and 7 per cent into very severe cyclones.
• How long they last: A tropical cyclone over the north Indian Ocean lives for 5 to 6 days and holds hurricane intensity for 2 to 4 days, against a global average of 6 days.
• Longest in Indian seas: The longest-lived tropical cyclones of the Indian seas lasted 14 days, in November 1886 and November 1964.

The stages from disturbance to maturity and decay are explained in full in the article on the stages of a tropical cyclone.

Tropical Cyclones in India: Bay of Bengal and Arabian Sea

Cyclone Seasons and Frequency in the North Indian Ocean

India's peninsula lies between two seas that breed cyclones, the Bay of Bengal and the Arabian Sea. The north Indian Ocean sees about 5 tropical cyclones a year, out of the world's 86 or so, and the Bay of Bengal has more of them than the Arabian Sea in a ratio of 4 to 1.

The north Indian Ocean has a double peak of cyclone activity, a primary peak in November and a secondary peak in May; May to June and October to November produce cyclones of severe intensity. Few cyclones form during the south-west monsoon, from July to September, because strong westerly winds below 5 km and very strong easterly winds above 9 km create large vertical wind shear. In that season the zone of origin shifts to the north Bay of Bengal, and depressions forming along the monsoon trough cross the Odisha and West Bengal coast within a day or two, too soon to grow strong.

How these storms fit into India's seasons is explained in the articles on Bay of Bengal and Arabian Sea cyclones and the cyclone and monsoon interaction.

Cyclone Hazards in India: Strong Winds, Heavy Rain and Storm Surge

A cyclonic storm brings three dangers, namely very strong winds, very heavy rain that causes floods, and the storm surge. The force of a cyclone falls with distance from the sea, so it is the coastal areas that are struck by severe cyclonic storms with an average velocity of 180 km per hour.

• Strong wind: Winds of a cyclonic storm, 62 to 88 km per hour, damage kutcha houses and break branches; those of a severe cyclonic storm uproot trees and damage pucca houses; those of very severe and super cyclonic storms cause widespread damage and a total breakdown of communications.
• Heavy rain: Rain of about 50 cm a day is common with a cyclonic storm, and within about 50 km of the centre it can reach about 85 cm a day, enough to cause flash floods.
• Storm surge: A storm surge is an abnormal rise of sea level as the cyclone crosses the coast. Storm surges are by far the greatest killers in a cyclone; the 1970 cyclone over the Bay of Bengal killed at least 3,00,000 people, mostly through its storm surge in the low-lying deltas.

The east coast bears the brunt. The frequency of severe cyclonic storms is greatest for Andhra Pradesh and that of cyclones for Odisha, while on the west coast Gujarat is the most vulnerable. The thickly populated deltas of the Godavari, Krishna and Kaveri are preferred targets, and the coastal areas of Bengal and Bangladesh are the most prone to storm surge, with surges of about 10 to 13 m. The same October and November depressions and cyclones give the Coromandel coast the bulk of its rainfall.

Tropical Cyclones Compared with Temperate Cyclones and Tornadoes

Difference Between Tropical Cyclone and Temperate Cyclone

A temperate cyclone, also called an extra-tropical or mid-latitude cyclone, forms along the polar front beyond the tropics, where cold and warm air masses meet. It shares the low centre and the cyclonic winds of a tropical cyclone, and little else.

The tropical cyclone is more destructive because its winds are much stronger and it strikes crowded coasts. A tropical cyclone often turns into an extra-tropical cyclone as it recurves poleward and eastward, and occasionally the reverse happens.

Difference Between Cyclone and Tornado

A tornado is a rapidly rotating column of air that reaches from the ground to the base of a cumulonimbus or cumulus cloud, often seen as a funnel. It is a far smaller and shorter-lived vortex than a cyclone. Most tornadoes have winds below 180 km per hour, are about 80 m across and travel several kilometres before they die out, although the most extreme exceed 480 km per hour and 3 km in width.

The United States has by far the most tornadoes of any country, but they also occur in Bangladesh and adjacent eastern India.

Sources

• NCERT: Fundamentals of Physical Geography (Class XI), Atmospheric Circulation and Weather Systems
• NCERT: India: Physical Environment (Class XI), Climate
• NCERT: India: Physical Environment (Class XI), Natural Hazards and Disasters
• India Meteorological Department (RSMC New Delhi): Frequently Asked Questions on Tropical Cyclones
• NOAA JetStream: Tropical Cyclone Introduction
• NASA Space Place: How Do Hurricanes Form?
• IPCC: Sixth Assessment Report, Working Group I, Summary for Policymakers
• Wikipedia: Tropical cyclone
• Wikipedia: Eye (cyclone)
• Wikipedia: Tropical cyclogenesis
• Wikipedia: North Indian Ocean tropical cyclone
• Wikipedia: Tropical cyclones and climate change
• Wikipedia: Rapid intensification
• Wikipedia: Tropical cyclone naming
• Wikipedia: Fujiwhara effect
• Wikipedia: South Atlantic tropical cyclone
• Wikipedia: Hurricane Catarina
• Wikipedia: Jet stream
• Wikipedia: Tornado
• Wikipedia: Extratropical cyclone
• Wikipedia: Cyclone Arnab
• UPSC: Civil Services Examination question papers