Tropical Cyclones

Prerequisites: Pressure Belts, Winds and Jet Streams · Ocean Currents, Temperature and Salinity

The Hook: Same Coast, Same Strength, Different Outcome

Connect: In the News → Concept

In October 1999, a super cyclone struck the Odisha coast and killed thousands of people; a storm surge pushed sea water far inland. In October 2013, Cyclone Phailin, of comparable strength, made landfall on the same coast near Gopalpur. This time, accurate forecasts and a large-scale evacuation kept the death toll to a few dozen.

The storms were alike; the preparation was not. To understand why cyclones are so destructive — and why forecasting and evacuation work — we need to know how a tropical cyclone forms, how it is structured, and what actually kills people when it reaches land.

The Core Concept

A tropical cyclone is an intense low-pressure system that forms over warm tropical seas, with winds spiralling inward — anticlockwise in the Northern Hemisphere and clockwise in the Southern — around a calm centre. It is a giant heat engine powered by the latent heat released when moist air rises and condenses.

Conditions for formation (NCERT)

A large sea surface with temperature above about 27 °C — to supply heat and moisture.
Presence of the Coriolis force — to start the rotation. Since it is zero at the Equator, cyclones rarely form within about 5° of it.
Small variations in vertical wind speed (low vertical wind shear) — so that the vertical column of the storm is not torn apart.
A pre-existing weak low-pressure area or low-level cyclonic circulation.
Upper-level divergence above the system — so that rising air is removed and the surface pressure keeps falling.

Structure

Part	Features
Eye	Calm, often clear centre with descending air and the lowest pressure
Eye wall	Ring of towering cumulonimbus clouds around the eye — the strongest winds and heaviest rain
Spiral rain bands	Bands of cloud and rain spiralling inward towards the eye wall
Outflow	Air spreading out at the top of the storm, often clockwise in the Northern Hemisphere

Life cycle

graph LR
    A[Warm sea above 27 C + weak low] --> B[Moist air rises, condenses, releases latent heat]
    B --> C[Pressure falls, winds strengthen, Coriolis adds spin]
    C --> D[Mature cyclone with eye and eye wall]
    D --> E[Landfall or cold water: moisture supply cut]
    E --> F[Weakens to depression and dissipates]

A cyclone weakens after landfall because it loses its supply of warm, moist air and friction over land slows the winds — but its heavy rain can continue to cause floods far inland.

Regional names

Name	Region
Cyclones	Indian Ocean (Bay of Bengal, Arabian Sea), South Pacific
Hurricanes	North Atlantic, Caribbean and the eastern North Pacific
Typhoons	Western North Pacific and South China Sea
Willy-willies	Traditional name in north-western Australia

Tropical cyclones are almost absent from the South Atlantic and the south-eastern Pacific, where cool sea surfaces (cold currents and upwelling) and strong wind shear inhibit them.

IMD classification (North Indian Ocean)

The India Meteorological Department classifies systems by maximum sustained surface wind speed (3-minute average):

Category	Wind speed (km/h)	Knots
Depression	31–49	17–27
Deep depression	50–61	28–33
Cyclonic storm (named from here)	62–88	34–47
Severe cyclonic storm	89–117	48–63
Very severe cyclonic storm	118–166	64–89
Extremely severe cyclonic storm	167–221	90–119
Super cyclonic storm	222 and above	120 and above

Naming of cyclones in the North Indian Ocean

IMD’s Regional Specialised Meteorological Centre (RSMC), New Delhi names cyclones over the Bay of Bengal and the Arabian Sea, on behalf of the WMO/ESCAP Panel on Tropical Cyclones.
Naming began in 2004, with names contributed by eight countries. The panel now has 13 members: Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, the United Arab Emirates and Yemen.
In 2020, a new list of 169 names (13 from each member) was adopted. Amphan (May 2020) was the last name from the original list.
Names are used in sequence, are given when a system reaches cyclonic storm strength, and are not reused.

What kills: the three hazards

Hazard	Why it is dangerous
Storm surge	Abnormal rise of sea level as strong winds pile water onto the coast, worsened at high tide and on shallow, gently sloping shelves; historically the biggest killer
Strong winds	Destroy houses, trees, power and telecom lines
Heavy rainfall	Floods and landslides, often far inland and for days after landfall

India: why the Bay of Bengal gets more cyclones

The Bay of Bengal sees several times more cyclones than the Arabian Sea, and the east coast is struck far more often than the west.

Warmer sea-surface temperatures and high humidity over the Bay.
Remnants of western Pacific typhoons sometimes cross South-East Asia and re-intensify over the Bay.
The funnel shape of the northern Bay and its shallow continental shelf amplify storm surges along the Odisha, West Bengal and Bangladesh coasts.
The flat, low-lying deltaic coast (Mahanadi, Godavari, Krishna, Ganga–Brahmaputra) lets surges travel far inland.
The Arabian Sea is generally cooler, but studies note a rise in intense Arabian Sea cyclones in recent decades as it warms.

Seasons: two peaks — pre-monsoon (April–May) and especially post-monsoon (October–December). During the monsoon, strong vertical wind shear usually prevents cyclones; monsoon depressions form instead.

Preparedness and mitigation

Forecasting and warning: IMD’s track and intensity forecasts with colour-coded warnings; Doppler weather radars; satellite monitoring.
Evacuation and shelters: multipurpose cyclone shelters and community-based evacuation plans — the core of Odisha’s turnaround after 1999.
National Cyclone Risk Mitigation Project (NCRMP): World Bank-assisted programme for shelters, evacuation roads, embankments and early warning in coastal states.
Nature-based defences: mangroves and coastal shelter belts reduce wind and surge damage; studies of the 1999 Odisha super cyclone found fewer deaths in villages protected by wider mangrove belts.
Policy framework: NDMA’s guidelines on the management of cyclones; Coastal Regulation Zone rules.

Memory Anchors

Formation conditions — “WC-LPU”: Warm sea (>27 °C) · Coriolis · Low wind shear · Pre-existing low · Upper divergence.

IMD ladder — the “62 rule”: a system becomes a named cyclonic storm at 62 km/h; a super cyclone at 222 km/h. Between them: severe (89), very severe (118), extremely severe (167).

Names by ocean: “Hurricanes Atlantic, Typhoons Pacific-west, Cyclones Indian” — HA-TP-CI.

Spin rule: same as all lows — anticlockwise in the North, clockwise in the South.

For Prelims

Facts worth a mark

Tropical cyclones need sea-surface temperatures above about 27 °C, the Coriolis force, low vertical wind shear, a pre-existing low and upper-level divergence.
They do not form at the Equator (no Coriolis force) and are almost absent in the South Atlantic and south-eastern Pacific.
The eye is calm with descending air; the eye wall has the strongest winds.
IMD names a system at cyclonic storm stage (62–88 km/h); super cyclonic storm is 222 km/h and above.
North Indian Ocean naming: 13 member countries; list of 169 names adopted in 2020; names are not repeated.
Storm surge is usually the deadliest hazard.
Peak cyclone seasons in India: April–May and October–December.

For Mains

Dimensions: physical mechanism; regional distribution and India’s vulnerability (east vs west coast); impacts (lives, livelihoods, agriculture, salinisation, infrastructure); climate change (warmer seas, rapid intensification, more intense Arabian Sea storms); preparedness (forecasting, evacuation, shelters, mangroves, insurance).
Keywords: latent heat, vertical wind shear, storm surge, landfall, rapid intensification, impact-based forecasting, zero-casualty approach, multipurpose cyclone shelter, bio-shield.
Value-addition: Odisha’s shift from the 1999 super cyclone to Phailin (2013) and Fani (2019) is widely cited as a model of early warning and evacuation; the remaining challenge is reducing economic losses and protecting livelihoods.

Common Traps

Trap #1: 'Cyclones form most easily at the Equator because it is hottest'

Warm water is necessary but not sufficient. The Coriolis force is zero at the Equator, so the system cannot start rotating. Most cyclones form between about 5° and 20°–30° latitude.

Trap #2: 'The eye is the most dangerous part of the cyclone'

The eye is calm, sometimes with clear skies. The eye wall around it has the strongest winds. The danger is that people come out during the lull, only to be hit by the other side of the eye wall.

Trap #3: 'The monsoon season is the peak cyclone season in India'

Strong vertical wind shear during the south-west monsoon usually suppresses cyclones. The peaks are pre-monsoon (April–May) and post-monsoon (October–December).

In the News

May 2020 — Super Cyclone Amphan in the Bay of Bengal made landfall near the West Bengal–Bangladesh coast; its name was the last on the original North Indian Ocean list.
June 2023 — Cyclone Biparjoy, a long-lived Arabian Sea cyclone, made landfall near Jakhau port in Kachchh, Gujarat.

Practice

Prelims

UPSC Practice · Prelims GS I Qtc-1

Which of the following conditions favour the formation of tropical cyclones?

Sea-surface temperature above about 27 °C
Strong vertical wind shear
A pre-existing weak low-pressure area
Presence of the Coriolis force

Select the correct answer using the code given below.

A 1 and 2 only
B 2 and 3 only
C 1 and 4 only
D 1, 3 and 4 only

Solution

Conditions 1, 3 and 4 favour formation. Strong vertical wind shear (condition 2) is unfavourable — it tilts and tears apart the developing column of the storm. Cyclones need low wind shear.

Answer: D

UPSC-pattern practice question

UPSC Practice · Prelims GS I Qtc-2

According to the India Meteorological Department’s classification, a system over the North Indian Ocean with maximum sustained winds of 62–88 km/h — the stage at which it is first given a name — is called a:

A Deep depression
B Severe cyclonic storm
C Cyclonic storm
D Very severe cyclonic storm

Solution

Cyclonic storm — 62–88 km/h (34–47 knots). Below it is the deep depression (50–61 km/h); above it are the severe (89–117), very severe (118–166), extremely severe (167–221) and super cyclonic storm (222 and above) categories.

Answer: C

UPSC-pattern practice question

UPSC Practice · Prelims GS I Qtc-3

Consider the following statements about the naming of tropical cyclones in the North Indian Ocean:

Names are assigned by the Regional Specialised Meteorological Centre in New Delhi.
The list adopted in 2020 contains names contributed by thirteen countries.
A name is reused after a gap of ten years.
Names are assigned when a system reaches the depression stage.

How many of the above statements are correct?

A Only one
B Only two
C Only three
D All four

Solution

1 — correct. IMD’s RSMC New Delhi names cyclones on behalf of the WMO/ESCAP Panel on Tropical Cyclones.
2 — correct. The panel has 13 members; the 2020 list has 169 names (13 each).
3 — wrong. Names on the list are used once and not reused.
4 — wrong. A name is given only when the system reaches cyclonic storm strength (62 km/h or more), not at depression stage.

So two statements are correct.

Answer: B

UPSC-pattern practice question

Mains

UPSC Practice · Mains GS I 15 marks · 250 words Qtc-m1

Why is the east coast of India more prone to tropical cyclones than the west coast? Discuss how improved preparedness has changed the impact of cyclones in Odisha.

Solution

Introduction: The Bay of Bengal generates several times more tropical cyclones than the Arabian Sea, and the east coast — especially Odisha, Andhra Pradesh, West Bengal and Tamil Nadu — bears most landfalls.

Body — why the east coast:

Warmer, more humid Bay: higher sea-surface temperatures and moisture supply.
Pacific remnants: western Pacific typhoons sometimes cross South-East Asia and re-intensify over the Bay.
Steering winds: in the post-monsoon season, prevailing flow tends to move Bay systems towards the Indian and Bangladesh coasts.
Coastal geography: the funnel-shaped northern Bay, shallow shelf and low, flat deltas amplify storm surges.
Arabian Sea: generally cooler with more wind shear, though intense Arabian Sea cyclones have become more frequent as it warms (for example, Biparjoy in 2023).

Body — Odisha’s experience:

1999 super cyclone: limited warning reach, few shelters, a huge surge — thousands of deaths.
Reforms: Odisha State Disaster Management Authority, a network of multipurpose cyclone shelters (supported under NCRMP), trained community volunteers, better IMD forecasts and last-mile warning.
Results: Phailin (2013) and Fani (2019) caused far fewer deaths through mass evacuation — an approach often called “zero casualty”.
Remaining gaps: heavy economic losses (housing, power, fisheries, agriculture), salinisation of farmland, recovery time for the poorest.

Conclusion: Geography makes the east coast cyclone-prone; governance decides the toll. The next step is to reduce economic loss through resilient housing and infrastructure, mangrove restoration and risk insurance.

UPSC-pattern practice question

Quick Revision

Point	One-liner
Energy source	Latent heat of condensation
Formation	SST > 27 °C, Coriolis, low wind shear, pre-existing low, upper divergence
Rotation	Anticlockwise NH, clockwise SH
Eye / eye wall	Calm, descending air / strongest winds
Regional names	Cyclone (Indian Ocean), hurricane (Atlantic), typhoon (west Pacific)
IMD naming threshold	Cyclonic storm, 62 km/h
Super cyclone	222 km/h and above
Naming panel	13 countries; 169 names (2020); no reuse
Deadliest hazard	Storm surge
Indian seasons	April–May; October–December

Key Takeaways

A tropical cyclone is a heat engine: warm sea, rising moist air, latent heat — and the Coriolis force to spin it.
The Bay of Bengal’s warmth, shape and low deltaic coast make India’s east coast the most cyclone-prone.
Odisha shows that early warning, shelters and evacuation can cut deaths dramatically; reducing economic loss is the next frontier.

The Hook: Same Coast, Same Strength, Different Outcome

The Core Concept

Conditions for formation (NCERT)

Structure

Life cycle

Regional names

IMD classification (North Indian Ocean)

Naming of cyclones in the North Indian Ocean

What kills: the three hazards

India: why the Bay of Bengal gets more cyclones

Preparedness and mitigation

Memory Anchors

For Prelims

For Mains

Common Traps

In the News

Practice

Prelims

Mains

Quick Revision

Key Takeaways

Related Topics

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