A rotating low-pressure weather system with organised thunderstorms that forms over warm tropical waters. Called a typhoon, hurricane or cyclone depending on the ocean basin.
phenomenaWhat is a Tropical Cyclone?
A tropical cyclone is a rapidly rotating storm system that forms over warm tropical or sub-tropical ocean waters. It is characterised by a low-pressure centre, strong winds, organised thunderstorms, and torrential rain. In the North Indian Ocean — which includes the Bay of Bengal and the Arabian Sea — these systems are called cyclones. The same phenomenon is called a hurricane in the Atlantic and northeast Pacific, and a typhoon in the northwest Pacific.
The Bay of Bengal produces around 5% of the world’s tropical cyclones but accounts for over 80% of cyclone-related deaths, mainly because of the dense population on the low-lying coast of Bangladesh and eastern India.
How a tropical cyclone forms
Four ingredients are needed for a tropical cyclone to develop:
- Warm ocean water — Sea surface temperature must be at least 26.5°C to a depth of around 50 m. The Bay of Bengal regularly exceeds 30°C in May and October.
- A pre-existing disturbance — Usually a tropical wave or low-pressure area that organises the convection.
- Low vertical wind shear — The wind must blow in roughly the same direction at all altitudes, allowing the storm column to stay intact.
- Coriolis force — The storm needs to be at least 5° from the equator to spin. This is why cyclones never form on the equator itself.
Once these conditions align, warm moist air rises, condenses into thunderstorms and releases latent heat that further warms the column. The whole system begins to rotate around a low-pressure centre — anti-clockwise in the Northern Hemisphere.
IMD classification
The India Meteorological Department (IMD) classifies North Indian Ocean cyclones into seven stages based on 3-minute sustained wind speed:
| Stage | Wind speed (km/h) |
|---|---|
| Depression | 31–49 |
| Deep Depression | 50–61 |
| Cyclonic Storm (named at this point) | 62–87 |
| Severe Cyclonic Storm | 88–117 |
| Very Severe Cyclonic Storm | 118–165 |
| Extremely Severe Cyclonic Storm | 166–220 |
| Super Cyclonic Storm | 221+ |
For comparison with the international Saffir-Simpson hurricane scale: an Extremely Severe Cyclonic Storm is roughly equivalent to a Category 4 hurricane, and a Super Cyclonic Storm matches Category 5.
Cyclone seasons in South Asia
Unlike the Atlantic or Pacific, the North Indian Ocean has two distinct cyclone seasons separated by the monsoon:
- Pre-monsoon: April–May. Cyclones tend to track northward toward Bangladesh and Myanmar. Examples: Cyclone Nargis (2008), Mocha (2023), Sidr (2007).
- Post-monsoon: October–December. Historically the most intense season, especially November. Cyclones often track westward toward India’s east coast — Tamil Nadu, Andhra Pradesh, Odisha and West Bengal. Examples: 1999 Odisha super cyclone, Phailin (2013), Fani (2019), Amphan (2020), Remal (2024).
The Arabian Sea is far less prolific but produces increasingly intense storms — Cyclone Tauktae (May 2021) was a notable Arabian Sea event affecting Gujarat and Maharashtra.
Most destructive aspects
Most cyclone deaths historically come not from wind but from three secondary hazards:
- Storm surge — A wall of seawater pushed inland by the cyclone’s winds. The funnel-shaped Bay of Bengal can amplify surge to 6–9 m in Bangladesh. The 1970 Bhola cyclone killed an estimated 300,000–500,000 people, almost entirely from surge.
- Inland flooding — Heavy rain (sometimes 300+ mm in 24 hours) overwhelms drainage, especially in deltaic regions.
- Wind damage and debris — Modern concrete buildings survive most cyclones, but kutcha (mud-brick) houses, tin roofs and trees suffer catastrophic damage.
Cyclone preparedness
The most encouraging story in South Asian disaster management is Bangladesh’s Cyclone Preparedness Programme, which has reduced cyclone mortality by approximately 100-fold since 1970 — from 300,000+ deaths in Bhola (1970) to under 30 in Cyclone Remal (2024). Key elements:
- Early warning: 72-hour Cyclone Watch, 48-hour Alert, 24-hour Warning
- 76,000+ trained volunteers for door-to-door evacuation
- 4,000+ raised concrete cyclone shelters across the coast
- Coastal mangrove restoration in the Sundarbans
India has built similar capacity along the east coast since the 1999 Odisha super cyclone, evacuating over a million people each time a major cyclone approaches.
Frequently asked questions
When is cyclone season in the Bay of Bengal? Two seasons: pre-monsoon (April–May) and post-monsoon (October–December). Post-monsoon, especially October–November, historically produces the most intense storms.
What is the difference between a cyclone, hurricane and typhoon? They are the same phenomenon — tropical cyclone — named differently by ocean basin. North Indian Ocean = cyclone; Atlantic / northeast Pacific = hurricane; northwest Pacific = typhoon.
How are cyclones named? A panel of 13 WMO/ESCAP member countries (Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, UAE and Yemen) contributes 13 names each. Names are used in rotation. Recent examples: Biparjoy, Tej, Hamoon, Midhili, Fengal, Mocha, Remal.
Which Indian states are most exposed? On the east coast: Odisha, Andhra Pradesh, West Bengal and Tamil Nadu. Odisha has experienced the strongest single landfalls. On the west coast, Gujarat occasionally faces Arabian Sea storms.
Where can I track an active cyclone? India’s IMD (mausam.imd.gov.in), Bangladesh’s BMD (bmd.gov.bd), Joint Typhoon Warning Center (US), and Mausam Online’s live wind/pressure data on every coastal city page. See our complete guide: Bay of Bengal cyclones.