Tropical Cyclone: Formation, Structure, Eye and Key Facts

What is Tropical Cyclone?

A Tropical Cyclone is an intense, rotating storm system that originates over warm tropical oceans between latitudes 5° and 30° on either side of the equator. It has a warm, low-pressure centre called the eye, surrounded by a violent eyewall of thunderstorms and strong spiralling winds. Tropical cyclones draw their energy from the latent heat released when warm, moist oceanic air rises and condenses. They are also called hurricanes in the North Atlantic, typhoons in the Western Pacific, and willy-willies in Australia.

Tropical Cyclone Formation – How Are Tropical Cyclones Formed?

Understanding tropical cyclone formation is essential for UPSC aspirants. A tropical cyclone forms through a step-by-step process that requires several conditions to occur simultaneously.

Step-by-Step: How a Tropical Cyclone Forms

1. Step 1 – Warm Ocean Heating: Warm Ocean water (above 26.5°C) heats the air above it. This warm, moist air becomes less dense and begins to rise rapidly from the ocean surface.
2. Step 2 – Low Pressure Formation: As warm air rises, it leaves behind a region of low pressure at the ocean surface. Surrounding cooler and denser air rushes in to fill this low-pressure zone.
3. Step 3 – Condensation and Latent Heat Release: As the rising moist air cools at higher altitudes, water vapour condenses into clouds and releases latent heat. This latent heat further warms the surrounding air, causing it to rise faster and intensifying the upward motion.
4. Step 4 – Coriolis Force Initiates Rotation: The Coriolis force, caused by Earth’s rotation, deflects the incoming air. This gives the system a spin – anti-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
5. Step 5 – Upper-Level Divergence: At the top of the troposphere (above 7 km), the rising air spreads out horizontally in an anticyclonic (outward) pattern. This outflow maintains the low pressure at the surface and allows the storm to sustain itself.
6. Step 6 – Eye and Eyewall Formation: As the system intensifies, air spirals inward, rises steeply in the eyewall, and subsides in the centre, forming the calm eye. The eyewall contains the highest wind speeds and most intense rainfall.
7. Step 7 – Mature Cyclone: The system reaches maturity with a well-defined eye, eyewall, and spiral rain bands. At this stage it is classified as a tropical cyclone.

Favourable Conditions for Tropical Cyclone Formation

Condition	Required Value / Description
Sea Surface Temperature (SST)	Above 26.5°C to a depth of at least 46 m — provides heat and moisture
Warm Moist Air Supply	Continuous supply of warm, humid air for sustained convection and latent heat release
Coriolis Force	Must be sufficiently strong — absent between 0°–5° latitude; this is why cyclones do not form near the equator
Low Vertical Wind Shear	Wind speed and direction must not change much with height; high shear disrupts the cyclone’s vertical structure
Upper-Level Divergence	Strong divergence at 9-12 km altitude allows rising air to be pumped out, maintaining low pressure at the surface
Pre-existing Low Pressure	A weather disturbance (monsoon trough, easterly wave) to trigger initial rotation
High Mid-Troposphere Humidity	Around 50-60% humidity at 4-7 km altitude to prevent dry air from disrupting convection

Structure of Tropical Cyclone – Parts and Layers

The structure of a tropical cyclone is complex and well-organised. Understanding it helps aspirants answer questions in both UPSC prelims and Mains.

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Horizontal Structure

Component	Description
Eye	Roughly circular calm area at the centre; diameter 30-60 km (range 8–200 km). Has light winds, clear or partly cloudy sky, low pressure, and warm temperatures aloft. No significant rainfall. Sometimes blue sky is visible.
Eyewall	The ring of deep convective clouds surrounding the eye. Contains the highest wind speeds and most intense rainfall. Temperature at eye level may be 10°C warmer than surroundings at 12 km altitude.
Spiral Rain Bands	Long, curved bands of thunderstorm clouds (cumulonimbus) that spiral inward toward the eyewall. Made of nimbostratus and cumulus clouds at the periphery and cumulonimbus near the centre.
Outer Circulation	The outermost region of the cyclone with lighter winds and scattered cloud cover. Isobars are spaced farther apart here.

Vertical Structure – Three Layers

Layer	Altitude	Key Features
Inflow Layer	Surface to 3 km	Air flows inward toward the low pressure centre; warm moist air drawn in from ocean surface; drives the storm.
Cyclonic Storm Layer	3 km to 7 km	Main zone of cyclonic activity; intense convection, heavy rainfall, and violent wind speeds occur here.
Outflow Layer	Above 7 km (max at 12 km)	Rising air spreads outward anticyclonically; maximum outflow at 12 km; acts as exhaust system maintaining surface low pressure.

Eye of Tropical Cyclone – The Calm Centre

The eye of a tropical cyclone is one of the most distinctive and frequently examined features in competitive examinations. It appears paradoxically peaceful amid the world’s most violent storms.

The eye is a roughly circular zone of light winds, clear or partly cloudy skies, and fair weather at the centre of a fully developed tropical cyclone. Despite being surrounded by the most violent part of the storm, the eye remains calm because of a process called subsidence – dry air descending from the upper troposphere, which suppresses cloud formation and rainfall within the eye.

Key Facts about the Eye of a Tropical Cyclone

Feature	Details
Diameter	Average 30-60 km; can range from 8 km to over 200 km
Weather inside	Light winds; little or no precipitation; sometimes blue sky visible
Pressure	Lowest surface pressure in the entire cyclone system
Temperature	Warmest temperatures aloft – up to 10°C warmer than surroundings at 12 km altitude; but only 0-2°C warmer at surface
Formation cause	Dynamic centrifuging of air outward into eyewall + forced descent caused by moist convection of eyewall
Eyewall	Ring of tallest convective clouds surrounding the eye; contains highest wind speeds
Eye wall replacement	In intense cyclones, an outer eyewall can form and replace the inner one — temporarily weakening the storm
Danger	Passage of eye over land creates a false sense of safety; winds resume with even greater ferocity after eye passes

Characteristics of Tropical Cyclone

The characteristics of a tropical cyclone distinguish it clearly from other atmospheric systems. These features are frequently tested in UPSC Prelims and Mains examinations.

Characteristic	Description
Origin	Originates exclusively over warm tropical oceans (SST > 26.5°C); between 5°–30° N/S latitudes
Warm Core	The centre is warmer than the surrounding atmosphere — driven by latent heat release. This is its primary energy source
Isobars	Isobars are closely spaced, indicating steep pressure gradients; pressure may fall 14–17 mb per 100 km and sometimes up to 60 mb per 100 km
Wind speed	Highly destructive — wind gusts can exceed 200–300 km/h in severe cyclones
Wind direction	Anti-clockwise in Northern Hemisphere; Clockwise in Southern Hemisphere
No front	Unlike temperate cyclones, tropical cyclones do NOT have a frontal system (warm front or cold front)
Size	Horizontal extent: 500–1,000 km; Vertical extent: 12-14 km
Asymmetry on landfall	The right side of the track (in Northern Hemisphere) has stronger winds and more intense rainfall
Rainfall	Extremely heavy; most rainfall concentrated near the eyewall and in spiral rain bands
Storm Surge	Sea level rises 2–9 metres near the coast due to the combined effect of low pressure and strong onshore winds — deadliest aspect of cyclones
Movement	Eastward to westward in tropics (steered by trade winds); may recurve poleward
Decay	Weakens upon landfall or over cool waters (loses moisture and heat source); also decays in high wind shear environments

Types of Tropical Cyclone – IMD Classification

The India Meteorological Department (IMD) classifies tropical cyclones based on wind speed. This classification is frequently asked in UPSC Prelims.

IMD Category	Wind Speed (km/h)	Description
Low Pressure Area	< 31 km/h	Initial weather disturbance; isobars not closed
Depression	31 – 49 km/h	Organised system with closed isobars; 1–2 isobars
Deep Depression	50 – 61 km/h	Intensifying system; 2–3 closed isobars
Cyclonic Storm	62 – 88 km/h	Named storm; at least 3 closed isobars
Severe Cyclonic Storm	89 – 117 km/h	Intense system causing significant damage
Very Severe Cyclonic Storm	118 – 167 km/h	Major cyclone; extensive damage
Extremely Severe Cyclonic Storm	168 – 221 km/h	Very destructive; large-scale evacuation required
Super Cyclonic Storm	Above 221 km/h	Most intense category; catastrophic damage

Difference Between Tropical Cyclone and Temperate Cyclone

The difference between tropical cyclone and temperate cyclone is a very frequently asked UPSC topic. Understanding both systems and their key contrasts is essential.

Feature	Tropical Cyclone	Temperate (Extra Tropical) Cyclone
Also known as	Hurricane, Typhoon, Willy-Willy	Extratropical Cyclone, Mid-latitude Cyclone, Western Disturbance
Origin	Warm tropical oceans (5°–30° N/S)	Mid and high latitudes (35°–65° N/S), land or sea
Energy source	Latent heat from warm ocean evaporation	Temperature contrast between warm and cold air masses (baroclinicity)
Core temperature	Warm core (centre is warmer than surroundings)	Cold core (centre is colder than surroundings)
Frontal system	No fronts – non-frontal in nature	Well-developed warm front and cold front
Size	Smaller (500–1,000 km)	Much larger (2,000–3,000 km or more)
Wind speed	Very high; more destructive	Moderate; less destructive
Movement	East to West (trade winds); may recurve poleward	West to East (westerlies); more predictable
Season	Confined to specific season (summer/post-monsoon)	Can occur throughout the year
Rainfall	Very heavy; concentrated in eye region and rain bands	Widespread moderate rainfall across a large area
Over land	Weakens rapidly	Can persist and even intensify
Example	Cyclone Nivar, Tauktae, Biparjoy	Western Disturbances affecting northern India

Extra Tropical Cyclone – Formation and Features

An extra tropical cyclone forms outside the tropics, in the mid and high latitudes. Unlike tropical cyclones, which draw energy from warm ocean water, extra tropical cyclones form due to the temperature contrast (baroclinicity) between warm tropical air and cold polar air.

Formation of Extra Tropical Cyclone (Norwegian Model)

1. Polar Front Development: Warm tropical air and cold polar air meet along the polar front, a boundary between the two contrasting air masses.
2. Wave Formation: A disturbance creates a wave along the polar front. The warm air begins to push northward (in the Northern Hemisphere) and the cold air pushes southward, forming a wave-like bulge.
3. Warm and Cold Fronts: A warm front forms on the leading edge (warm air advancing) and a cold front on the trailing edge (cold air advancing). Low pressure develops at the apex of the wave.
4. Deepening: The low pressure deepens; winds spiral inward; the cyclone intensifies. The cold front moves faster than the warm front.
5. Occlusion: The faster-moving cold front catches up with the warm front, forming an occluded front. The warm air sector is lifted off the ground entirely.
6. Decay: As the warm air is fully lifted, the temperature contrast diminishes, and the cyclone decays.

Key Features of Extra Tropical Cyclone

• Firstly, cold core system: The centre of an extra tropical cyclone remains colder than its surrounding areas. Therefore, it differs fundamentally from tropical cyclones, which are warm-core systems. 
• Secondly, presence of frontal system: It develops along well-defined warm and cold fronts. Consequently, it shows clear temperature contrasts within the system. 
• Moreover, larger spatial extent: It covers a much larger geographical area, often extending 2,000-3,000 km or more, thus affecting wide regions simultaneously. 
• Additionally, west-to-east movement: It generally moves from west to east under the influence of prevailing westerlies. Hence, its path is comparatively more predictable. 
• Furthermore, year-round occurrence: It can form during any season of the year, unlike tropical cyclones, which are season-specific.
• Finally, Western Disturbances: Extra tropical cyclones that originate over the Mediterranean Sea and bring winter rainfall to northwestern India are known as Western Disturbances. 

Why Do Tropical Cyclones Occur Rarely in Brazil? (South Atlantic)

This is a direct UPSC Prelims 2015 question. In the South Atlantic and South-Eastern Pacific regions in tropical latitudes, tropical cyclones rarely originate. Understanding why is essential for UPSC aspirants.

Hurricane Catarina (2004) is the only recorded South Atlantic hurricane in history — a near once-in-a-lifetime event. Tropical Storm Akara (2024) was another rare recent example.

Reasons Why Tropical Cyclones Are Rare in Brazil / South Atlantic

Reason	Explanation
1. ITCZ Does Not Shift South	The Intertropical Convergence Zone (ITCZ) stays at or near the equator in the Atlantic. It does NOT shift sufficiently southward over the South Atlantic, unlike the Indian Ocean basin where the ITCZ and Monsoon Trough regularly shift far enough from the equator. Without the ITCZ, there is no large-scale vorticity (spin) and convergence to trigger cyclone formation.
2. High Vertical Wind Shear	The South Atlantic experiences persistently high values of vertical wind shear throughout the year. Strong wind shear disrupts the vertical structure of a developing storm, preventing it from organising into a cyclone.
3. Relatively Cool Sea Surface Temperatures	SSTs in the South Atlantic are generally slightly cooler than the threshold of 26.5°C, even during the southern summer. The cold Benguela Current further keeps coastal waters cool.
4. Absence of Tropical Waves	In the North Atlantic, tropical waves generated over West Africa regularly trigger cyclone formation. No such organised wave train exists over the South Atlantic.
5. Weak Coriolis Force at Low Latitudes	The ITCZ in the South Atlantic rarely shifts more than 1-2 degrees south of the equator. At such latitudes, the Coriolis force is too weak to initiate cyclonic rotation.
6. No Official Warning System	Because cyclones are so rare, Brazil historically had no official tropical cyclone warning system – reflecting the region’s perceived immunity to such storms.

Indian Tropical Cyclone – Cyclone Nivar, Tauktae, Burevi, Biparjoy, Gulab

India is one of the most cyclone-prone countries in the world, with about 10% of the world’s tropical cyclones affecting its coastline. The Bay of Bengal generates more cyclones than the Arabian Sea due to higher sea surface temperatures and a more favourable monsoon trough.

Stay updated with cyclone developments, climate events, and disaster updates through regularly updated Current news.

Cyclone Nivar – November 2020

Parameter	Details
Origin	Bay of Bengal
Named by	Iran
Landfall	Near Puducherry (Karaikal), Tamil Nadu — 25–26 November 2020
Classification	Very Severe Cyclonic Storm — wind speeds 100–120 km/h at landfall
States affected	Tamil Nadu, Puducherry, Andhra Pradesh, Telangana, southern Karnataka
Damage	Approximately Rs 5,000 crore; heavy floods in Chennai
Response	NDRF deployed 1,200+ personnel; large-scale evacuation; Army rescue teams deployed
UPSC relevance	Third name from the new 2020 IMD list; good example of IMD’s 48-hour advance warning system

Cyclone Tauktae — May 2021

Parameter	Details
Origin	Arabian Sea
Named by	Myanmar (means ‘gecko’ in Burmese)
Landfall	Near Diu-Una, Gujarat coast — 17 May 2021
Classification	Extremely Severe Cyclonic Storm — wind speeds 185 km/h; among strongest to hit Gujarat in decades
States affected	Kerala, Karnataka, Goa, Maharashtra, Gujarat
Impact	17 deaths on a barge off Mumbai coast; widespread damage in Gujarat; disrupted COVID-19 vaccination drives
Significance	One of the most intense cyclones ever recorded in the Arabian Sea; intensified rapidly due to unusually warm sea surface temperatures
UPSC relevance	Rapid intensification, climate change link, impact on COVID response — all important angles

Cyclone Burevi — December 2020

Parameter	Details
Origin	Bay of Bengal
Named by	Maldives
Landfall	Near Rameswaram, Tamil Nadu — 4 December 2020
Classification	Cyclonic Storm — slightly weaker than Nivar
States affected	Tamil Nadu, Sri Lanka, Kerala
Significance	Unusual — followed closely after Cyclone Nivar (November 2020); two cyclones in quick succession in the same season
UPSC relevance	Named by Maldives; example of post-monsoon Bay of Bengal cyclones; close spacing of two cyclones in a single season

Cyclone Biparjoy — June 2023

Parameter	Details
Origin	Arabian Sea
Named by	Bangladesh — ‘Biparjoy’ means ‘disaster’ or ‘calamity’ in Bengali
Landfall	Near Jakhau Port, Saurashtra-Kutch coast, Gujarat — 15 June 2023
Classification	Extremely Severe Cyclonic Storm — wind speeds up to 150 km/h
States affected	Gujarat, Rajasthan; also affected Pakistan’s coastal areas
Scale	One of the longest-lasting cyclones in the Arabian Sea; completed 13+ days over the sea before landfall
Significance	Fourth major cyclone to affect Gujarat in five years; massive evacuation of 37,700+ people from coastal districts
UPSC relevance	Long duration cyclone; Arabian Sea warming; disaster management; Gujarat’s increasing cyclone frequency

Cyclone Gulab — September 2021

Parameter	Details
Origin	Bay of Bengal
Named by	Pakistan (gulab means ‘rose’ in Urdu/Hindi)
Landfall	Near Kalingapatnam, Andhra Pradesh — 26 September 2021
Classification	Cyclonic Storm
States affected	Odisha and Andhra Pradesh — heavy rainfall, flooding, crop damage
Unique feature	After crossing land, the remnants re-emerged into the Arabian Sea and intensified into Cyclone Shaheen
UPSC relevance	First cyclone to make landfall on the east coast during September in 15 years; the rare land-to-sea transition and re-intensification is a very important concept

Naming of Tropical Cyclones – How Are Indian Cyclones Named?

The naming of tropical cyclones in the North Indian Ocean region is governed by the WMO/ESCAP Panel on Tropical Cyclones. The India Meteorological Department (IMD) is the designated Regional Specialised Meteorological Centre (RSMC) for the North Indian Ocean.

Key Facts about Cyclone Naming

• 13 member countries contribute names: Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, UAE, and Yemen.
• A new list of 169 names (13 names each from 13 countries) was issued by IMD in 2020.
• Names are used sequentially regardless of basin (Bay of Bengal or Arabian Sea).
• Once a name is used, it is retired and not repeated.
• Names must be culturally neutral, short (maximum 8 letters), easy to pronounce, and not offensive.
• India’s 13 names: Gati, Tej, Murasu, Aag, Vyom, Jhar, Probaho, Neer, Prabhanjan, Ghurni, Ambud, Jaladhi, Vega.

Impacts and Disaster Management – Tropical Cyclones in India

Tropical cyclones cause multiple types of damage to coastal and inland areas. Aspirants must understand these for Disaster Management (GS Paper III) answers.

Types of Impact

Impact Type	Description
Strong Winds	Destroy buildings, infrastructure, uproot trees, damage power and communication lines; gusts above 200 km/h are catastrophic
Storm Surge	Most deadly impact — sea level rise of 2–9 m near coast due to low pressure + onshore winds; inundates coastal areas
Heavy Rainfall and Flooding	Extreme rainfall causes flash floods, river flooding, and waterlogging; affects large inland areas even after landfall
Tornadoes	Strong cyclones can spawn smaller tornadoes in their rain bands, causing localised extreme destruction
Landslides	Heavy rainfall on hilly coastal areas (Western Ghats, Odisha hills) triggers landslides
Economic Loss	Crop damage, fisheries loss, infrastructure damage — billions of rupees in losses per major cyclone
Disruption	Transportation, power supply, communication, healthcare, and supply chains are severely disrupted

India’s Cyclone Management Systems:

• Firstly, Indian Meteorological Department (IMD): The IMD issues cyclone warnings 48-72 hours in advance. Moreover, it monitors cyclones using Doppler weather radars and advanced satellite systems to ensure timely alerts. 
• Secondly, National Disaster Management Authority (NDMA): The NDMA formulates national guidelines for cyclone risk mitigation. Therefore, it strengthens preparedness and policy coordination across states. 
• Additionally, National Disaster Response Force (NDRF): The NDRF deploys trained rescue and relief teams both before and after cyclone landfall. Consequently, it minimizes casualties and damage. 
• Furthermore, Odisha Model: Odisha has developed an internationally recognized cyclone preparedness system. As a result, it reduced deaths from over 10,000 during the 1999 Super Cyclone to near zero in later cyclones.
• Finally, Cyclone Shelters: India has constructed thousands of cyclone shelters along the eastern coast, especially in Andhra Pradesh, Odisha, and West Bengal. Thus, it ensures safe evacuation and community protection during severe cyclonic events.

Conclusion:

In conclusion, tropical cyclones are one of the most powerful and destructive natural phenomena on Earth. A thorough understanding of tropical cyclone formation – from warm ocean heating and low pressure development to the role of the Coriolis force and upper-level divergence — is essential for UPSC, APSC, and State PCS aspirants.

Source:

World Meteorological Organization

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