Hydro-Meteorological Disasters – I

Cyclones · Floods · Urban Floods · Flash Floods — formation, structure, IMD classification, naming, Bay of Bengal vs Arabian Sea, flood types & urban flooding

📄 GS Paper 3🎯 Prelims + Mains⏱ 18 min read📅 Updated June 2026

Tropical Cyclones: The Big Picture

A tropical cyclone is an intense, low-pressure weather system with a warm core that develops over warm tropical oceans, characterised by violent winds spiralling around a calm centre (the eye) and very heavy rainfall. In the North Indian Ocean these systems are simply called "cyclones"; the same phenomenon is a hurricane in the Atlantic/East Pacific and a typhoon in the West Pacific. India faces an average of about five cyclones a year across the Bay of Bengal and the Arabian Sea, with the bulk forming in two seasons — the pre-monsoon (April–May) and the more destructive post-monsoon (October–December).

Why this matters: Cyclones are the single most damaging hydro-meteorological hazard for India's ~7,500 km coastline. Wind, torrential rain and — most lethally — storm surge combine to threaten the densely populated, low-lying east-coast deltas of West Bengal, Odisha, Andhra Pradesh and Tamil Nadu.

Conditions Required for Formation

Tropical cyclogenesis needs a specific combination of thermal, dynamic and locational ingredients. Their absence explains why cyclones never form near the equator or over cold oceans.

Warm sea-surface temperature (>26.5 °C) to a depth of ~50–60 m: supplies the latent heat (the "fuel") through evaporation that powers the storm.
Coriolis force: needed to impart spin to the converging air; it is zero at the equator, so cyclones do not form within ~5° of it.
Low vertical wind shear: minimal change of wind speed/direction with height lets the warm core build vertically instead of being torn apart.
Pre-existing low-level disturbance: a seedling low/ITCZ trough provides initial convergence and convection.
High mid-tropospheric humidity & upper-level divergence: sustains thunderstorm clusters and evacuates rising air aloft, lowering surface pressure.

Structure of a Tropical Cyclone

Eye: the calm, cloud-free, low-pressure core (20–50 km wide) with gentle subsiding air, clear skies and the lowest pressure.
Eyewall: a ring of towering cumulonimbus surrounding the eye — the zone of strongest winds and heaviest rain.
Spiral rainbands: curved bands of thunderstorms spiralling inward, producing squalls of wind and rain over a wide area.

Figure 1: Plan view of a tropical cyclone — a calm eye ringed by the violent eyewall and outward spiral rainbands.

IMD Classification by Wind Speed

The India Meteorological Department (IMD), as the Regional Specialised Meteorological Centre (RSMC), New Delhi for the North Indian Ocean, classifies systems on a graded scale of 3-minute sustained surface wind speed. This ladder is a high-yield Prelims fact.

Category	Sustained wind (km/h)
Low Pressure Area → Depression	31–49
Deep Depression	50–61
Cyclonic Storm (gets a name)	62–88
Severe Cyclonic Storm	89–117
Very Severe Cyclonic Storm	118–166
Extremely Severe Cyclonic Storm	167–221
Super Cyclonic Storm	≥ 222

Prelims hook: A system is given a name only when it reaches Cyclonic Storm stage (≥62 km/h). The IMD also issues colour-coded warnings — Green, Yellow, Orange, Red — and a four-stage cyclone watch/alert/warning/post-landfall bulletin system.

Figure 2: The graded IMD classification — naming begins at Cyclonic Storm; intensity rises to Super Cyclone.

Naming of Cyclones

Cyclones in the North Indian Ocean are named under a system coordinated by IMD as the RSMC, on behalf of the WMO/ESCAP Panel on Tropical Cyclones. Originally 8 member countries contributed names; the panel expanded to 13 nations — Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, UAE and Yemen — and a fresh list of 169 names was released in 2020. Names are assigned sequentially as each system reaches Cyclonic Storm intensity, and are not reused.

Why name cyclones? Short, distinctive names ease public communication, media reporting, warning dissemination and inter-agency coordination — far easier than tracking numbers or latitude-longitude. Recent Indian-region names: Amphan (2020), Biparjoy (2023), Michaung (2023), Remal (2024) and Dana (2024).

Figure 3: The cyclone naming pipeline — from the 13-nation panel to IMD's RSMC assignment at Cyclonic Storm stage.

Bay of Bengal vs Arabian Sea

Cyclones in the North Indian Ocean are split unevenly between the two basins in roughly a 4:1 ratio, with the Bay of Bengal far more active. Understanding why is a favourite analytical theme.

Factor	Bay of Bengal (more cyclones)	Arabian Sea (fewer)
Sea-surface temperature	Warmer, more enclosed, higher SST	Relatively cooler, especially in the west
Freshwater inflow	Heavy from Ganga-Brahmaputra-Meghna & rain → stable warm surface layer	Limited river inflow; more mixing
Source disturbances	Receives remnants of Pacific typhoons + ITCZ lows	Fewer feeder disturbances
Wind shear	Generally lower	Higher shear often suppresses systems
Coastal exposure	Low, funnel-shaped deltas amplify storm surge	Steeper, less surge-prone coast

Why is India's east coast more cyclone-prone? Most systems form in the warmer Bay of Bengal and track west/north-west onto the east coast; the shallow, funnel-shaped head of the Bay greatly amplifies storm surge over the low-lying, densely populated deltas of Odisha, Andhra Pradesh, West Bengal and Tamil Nadu. Note: the Arabian Sea has shown a rising trend in cyclone frequency and intensity (e.g., Biparjoy 2023), linked to warming SSTs.

Storm Surge — the Biggest Killer

A storm surge is the abnormal rise of sea water above the astronomical tide, driven by a cyclone's winds pushing water onshore and the low pressure "sucking" the sea up. Historically it causes the majority of cyclone deaths (e.g., the 1999 Odisha Super Cyclone). India's improved early-warning and mass evacuation — as in Phailin 2013 and Fani 2019 — have drastically cut the death toll, a model cited globally.

Temperate (Extratropical) vs Tropical Cyclones

Although both are low-pressure storm systems, temperate (extratropical / frontal) cyclones differ fundamentally in origin and behaviour. In India they are felt as the Western Disturbances bringing winter rain/snow to the north-west.

Feature	Tropical Cyclone	Temperate (Extratropical) Cyclone
Location	Tropics (over warm oceans)	Mid/high latitudes (30°–60°), over land or sea
Energy source	Latent heat from warm ocean (warm core)	Temperature contrast along fronts (cold core aloft)
Fronts	No fronts	Associated with warm & cold fronts
Season	Summer / post-monsoon	Mainly winter
Size & intensity	Smaller, very intense, violent winds	Larger area, gentler, prolonged rain/snow
India example	Amphan, Fani, Biparjoy	Western Disturbances (winter rain in NW India)

Floods: Types & Causes

A flood is the temporary inundation of normally dry land by water. India is among the most flood-affected countries — about 12% of its land (~40 million hectares) is flood-prone, concentrated in the Ganga-Brahmaputra-Meghna and the Indus basins.

Types of Floods

Riverine (fluvial) floods: rivers overtop banks during heavy monsoon discharge — the classic Assam/Bihar floods.
Flash floods: sudden, high-velocity inundation within hours, typically from cloudbursts in hilly catchments or intense urban downpours; very short lead time.
Coastal floods: sea-water inundation from storm surge, high tides or tsunami along low-lying coasts.
Urban floods: waterlogging of cities when drainage cannot cope with intense rain — increasingly frequent.

Causes of Floods in India

Natural: intense/erratic monsoon rainfall, cloudbursts, glacial melt and GLOFs, low gradient in deltas, river course changes.
Human-induced: heavy siltation reducing channel capacity, deforestation in catchments, embankment failure, encroachment of floodplains & wetlands, poor reservoir/dam management, inadequate drainage.

Flood-prone hotspots: The Ganga-Brahmaputra basin dominates — Assam (annual Brahmaputra floods, severe bank erosion) and Bihar (Kosi, the "Sorrow of Bihar"). Other states: West Bengal, UP, Odisha and the Punjab plains.

Flood Management Institutions

Rashtriya Barh Ayog (National Flood Commission, 1976): set up after the 1976 floods to evolve a national flood-management policy; recommended a comprehensive, basin-wise approach and a shift away from purely structural measures.
Structural measures: embankments, dams/reservoirs, channel improvement, drainage works.
Non-structural measures: floodplain zoning, flood forecasting (CWC), flood insurance, watershed management, wetland conservation.
Central Water Commission (CWC): nodal agency for flood forecasting and warning.

Urban Floods

Urban flooding is a distinct hazard where intense rainfall overwhelms a city's drainage, causing rapid, widespread waterlogging. It is fundamentally a human-aggravated, socio-natural disaster — the rainfall is natural, but the flooding is made worse by how cities are built.

Why Cities Flood

Encroachment of wetlands, lakes & natural drains: loss of the city's natural sponges and outfalls (e.g., Chennai's marshlands, Bengaluru's tanks).
Concretisation & loss of permeable surface: reduces infiltration; rain becomes runoff almost instantly.
Inadequate, clogged or outdated storm-water drains: designed for old, lower rainfall intensities.
Cloudbursts & very intense short-duration rain: increasingly common under climate change.
Unplanned development on floodplains and low-lying areas.

Major Urban Flood Events

City	Year	Key trigger / lesson
Mumbai	2005	~944 mm in 24 hrs; Mithi river encroachment; choked drains — the wake-up call
Chennai	2015	Loss of marshlands & lakes; Adyar river overflow; reservoir release timing
Bengaluru	2022	Encroachment of tanks & storm-water drains; IT corridor submerged
Delhi	2023	Yamuna at record levels; floodplain encroachment; drainage congestion

NDMA Urban Flooding Guidelines, 2010: India's first guidelines exclusively on urban flooding — recommended a National Hydro-meteorological Network, Urban Flooding Cells, Local Network of Doppler radars, real-time drainage modelling, design of drains for higher rainfall, and inclusion of urban flooding in city Master Plans. The DM (Amendment) Act 2025 further provides for Urban Disaster Management Authorities (UDMAs) in major cities.

Figure 4: How encroachment, concretisation and weak drainage convert heavy rain into urban floods.

Flash Floods

A flash flood is a sudden, rapid and high-velocity flood that occurs within hours (often <6 hours) of triggering rainfall, leaving very little time to warn or evacuate. They are especially deadly in Himalayan and hilly catchments, where steep slopes funnel water violently downstream, and in dense urban areas.

Cloudburst-driven: a cloudburst delivers ≥100 mm of rain in an hour over a small area, generating destructive flash floods and debris flows (e.g., Leh 2010, Uttarakhand/Kedarnath 2013, Amarnath 2022).
Short lead time: the chief challenge — unlike riverine floods that build over days, flash floods give minutes to hours, defeating conventional warning.
Early-warning challenges: sparse high-altitude observation networks, difficulty in forecasting localised convective cloudbursts, and last-mile communication gaps in remote terrain.
Mitigation: dense automatic weather/rain-gauge stations, Doppler radar coverage, impact-based forecasting, and IMD's flash-flood guidance system for South Asia.

Climate link: A warming atmosphere holds more moisture (~7% per °C, Clausius–Clapeyron), intensifying short-duration extreme rainfall and raising both cloudburst and flash-flood risk across the Himalaya and cities.

Current Affairs Snapshot (up to June 2026)

Cyclone Dana (October 2024): made landfall near the Odisha–West Bengal coast as a Severe Cyclonic Storm; large-scale pre-emptive evacuation again kept fatalities very low — reinforcing India's zero-casualty approach.
Cyclone Remal (May 2024): first cyclone of the 2024 season over the Bay of Bengal, hitting the Bengal–Bangladesh coast; flagged early-season Bay activity.
Cyclone Biparjoy (June 2023): a long-lived Very Severe Cyclonic Storm in the Arabian Sea making landfall in Gujarat (Kutch) — emblematic of the rising Arabian Sea cyclone trend.
Cyclone Michaung (December 2023): triggered severe urban flooding in Chennai, re-exposing the city's drainage and wetland-loss problems.
Mission Mausam (2024): ₹2,000-crore push for denser radar/observation networks and AI forecasting to sharpen cyclone, cloudburst and urban-flood prediction.
DM (Amendment) Act, 2025: provides for Urban Disaster Management Authorities (UDMAs) in major cities — directly relevant to managing urban floods.
IMD upgrades: expanded Doppler Weather Radar network and impact-based, colour-coded warnings improving lead time for cyclones and heavy-rain events.

Previous Year Questions — Prelims PRELIMS

How to use: Prelims tests the physical geography and institutional facts — formation conditions, the IMD ladder, naming, and the Bay of Bengal vs Arabian Sea distribution.

UPSC Prelims 2020 Cyclone formation

Q. Consider the conditions favourable for the formation/intensification of tropical cyclones (warm SST, Coriolis force, low wind shear, pre-existing low). Which are correct? (Recurring Prelims theme on cyclogenesis conditions.)

Key Points to Remember

Warm SST >26.5 °C to ~50–60 m depth supplies latent-heat fuel.
Coriolis force imparts spin — hence no cyclones within ~5° of the equator.
Low vertical wind shear lets the warm core build; high humidity & upper-level divergence sustain it.
A pre-existing disturbance (ITCZ low) provides initial convergence.

UPSC Prelims 2018 Cyclone naming

Q. With reference to the naming of cyclones in the North Indian Ocean and the role of IMD — which statements are correct? (Prelims has tested the WMO/ESCAP panel and IMD's RSMC role.)

Key Points to Remember

IMD = RSMC New Delhi for the North Indian Ocean; assigns names.
Names are contributed by the 13-nation WMO/ESCAP panel (new 169-name list, 2020).
A system is named only at Cyclonic Storm stage (≥62 km/h).
IMD issues colour-coded (Green/Yellow/Orange/Red) warnings.

UPSC Prelims 2015 Cyclone distribution

Q. Which of the following best explains why the Bay of Bengal experiences more tropical cyclones than the Arabian Sea / why the east coast is more cyclone-prone? (Recurring distribution-and-causes theme.)

Key Points to Remember

Bay of Bengal has warmer, more enclosed waters and heavy freshwater inflow → stable warm layer.
It receives remnant Pacific typhoons + ITCZ lows; generally lower wind shear.
Most systems track west/north-west onto the east coast.
The shallow, funnel-shaped head amplifies storm surge over low-lying deltas.

Previous Year Questions — Mains with Model Answer Structures MAINS

How to use: Each model answer is a structured outline. Flesh out each point into 2–3 sentences in the exam. PYQs are covered up to UPSC Mains 2025.

UPSC GS3 2016 12.5 marks · 200 words

Q. "The frequency of cyclones in the Bay of Bengal and the Arabian Sea is increasing. What are the reasons for the higher frequency in the Bay of Bengal, and what measures should be taken to minimise the loss of life and property?"

Model Answer Structure

Intro: North Indian Ocean basins split roughly 4:1 in favour of the Bay of Bengal; situate within hydro-meteorological hazard risk.
Reasons — Bay of Bengal: warmer enclosed SST, freshwater-driven stable warm layer, remnant Pacific typhoons + ITCZ lows, low wind shear, funnel-shaped surge-prone coast.
Rising trend: warming SSTs are also intensifying Arabian Sea cyclones (e.g., Biparjoy 2023).
Structural mitigation: cyclone shelters, embankments, resilient coastal infrastructure (NCRMP), mangrove shelterbelts.
Non-structural: IMD early warning & colour-coded alerts, mass evacuation (Phailin/Fani model), CAP/SACHET, coastal zoning.
Community & way forward: CBDM, Aapda Mitra, "zero-casualty" approach, climate-resilient planning.
Conclusion: India's warning-plus-evacuation model has slashed deaths; the next frontier is reducing economic damage.

UPSC GS3 2016 12.5 marks · 200 words

Q. "The frequency of cloudbursts and consequent flash floods in the Himalayan region has increased. Discuss the reasons and suggest measures for mitigation."

Model Answer Structure

Intro: Define cloudburst (≥100 mm/hr over a small area) and flash flood (sudden, <6 hr lead time); Himalayan fragility.
Reasons: warming atmosphere holding more moisture (Clausius–Clapeyron), orographic lifting, steep slopes, GLOFs, fragile geology.
Aggravating human factors: deforestation, unplanned hill construction, hydropower & road-cutting, encroachment of river channels.
Examples: Leh 2010, Kedarnath 2013, Amarnath 2022, recurrent Uttarakhand/HP events.
Mitigation — forecasting: dense AWS/rain-gauge networks, Doppler radar, flash-flood guidance system, Mission Mausam.
Mitigation — planning: hazard zonation, regulation of construction, slope stabilisation, GLOF monitoring, community early warning.
Conclusion: Combine technology-led short-lead warning with eco-sensitive Himalayan development.

UPSC GS1/GS3 2021 10 marks · 150 words REPRESENTATIVE

Q. "Urban flooding is increasingly a man-made disaster. Critically examine the causes with reference to recent Indian cities and suggest a framework for resilient cities." (Representative question framed on a recurring urban-flooding theme; not tied to a specific PYQ year.)

Model Answer Structure

Intro: Urban flooding as a socio-natural disaster — natural rain, human-made vulnerability.
Causes: wetland/lake encroachment, concretisation reducing infiltration, clogged/undersized drains, cloudbursts, floodplain development.
Case studies: Mumbai 2005, Chennai 2015, Bengaluru 2022, Delhi 2023.
Institutional gap: uneven implementation of NDMA Urban Flooding Guidelines 2010; weak storm-water master planning.
Framework: sponge-city/blue-green infrastructure, restore wetlands, redesign drains for higher rainfall, real-time modelling, Urban Flooding Cells.
Governance: Urban Disaster Management Authorities (UDMAs) under DM (Amendment) Act 2025; integrate with city Master Plans.
Conclusion: Resilience needs ecology-led planning, not just engineering.

⚡ Quick Revision

Formation: SST >26.5 °C · Coriolis · low shear · seed low
Structure: Eye → Eyewall → Rainbands
Named at: Cyclonic Storm (≥62 km/h)
Super Cyclone: ≥222 km/h
Bay : Arabian Sea ≈ 4 : 1
Storm surge = biggest killer

📌 Must-Know Facts

IMD = RSMC New Delhi
13-nation WMO/ESCAP naming panel
~12% of India flood-prone (~40 m ha)
Rashtriya Barh Ayog: 1976
CWC = flood forecasting nodal body
NDMA Urban Flooding Guidelines: 2010

🌀 Recent Cyclones

Amphan — 2020 (Bengal)
Biparjoy — 2023 (Arabian Sea, Gujarat)
Michaung — 2023 (Chennai floods)
Remal — 2024 (Bengal)
Dana — 2024 (Odisha)

✍️ Answer Keywords

Warm-core latent-heat engine
Funnel-shaped surge amplification
Zero-casualty / Fani model
Socio-natural urban flooding
Sponge city / blue-green infra
Clausius–Clapeyron rainfall intensification