Bay of Bengal cyclones — the most dangerous weather in South Asia
The Bay of Bengal produces the deadliest tropical cyclones on Earth. Science, history, seasons, warning systems, IMD categories, coastal risk and how to prepare.
Why the Bay of Bengal is so dangerous
The Bay of Bengal is responsible for around 5% of the world's tropical cyclones but more than 80% of all cyclone-related deaths in history. Three factors make it uniquely lethal. First, warm shallow waters (sea surface temperatures regularly above 28°C, peaking near 31°C in summer) provide abundant fuel for rapid intensification. Second, the funnel-shaped coastline of Bangladesh and the West Bengal sundarbans physically squeezes incoming storm surge into a wedge — water that would be 2-3 m on an open coast can pile up to 6-9 m here. Third, the deltaic plain is low-lying (less than 5 m above sea level for hundreds of kilometres inland), densely populated (over 1,000 people per km² in many districts) and historically had limited evacuation infrastructure. The same storm strength can kill 100 in Florida and 100,000 in Bangladesh — geography is destiny.
Historical disasters — the human toll
The 1970 Bhola cyclone killed an estimated 300,000-500,000 people in what was then East Pakistan (now Bangladesh) — the deadliest tropical cyclone in recorded history and one of the deadliest natural disasters of any kind. Cyclone Nargis (May 2008) killed over 138,000 in Myanmar's Irrawaddy delta. The 1999 Odisha super cyclone killed over 10,000 along India's east coast. Cyclone Amphan (May 2020) caused $13 billion in damages to West Bengal and Bangladesh — the costliest cyclone ever in the North Indian Ocean. More recent storms include Cyclone Sidr (2007, ~3,500 deaths), Cyclone Aila (2009), Cyclone Phailin (2013, mass evacuation), Cyclone Fani (2019, 89 deaths despite Category 5 strength) and Cyclone Mocha (2023, deadly impact on Myanmar's Rohingya refugee camps). Bangladesh alone has lost over half a million lives to cyclones since 1960.
How a cyclone forms — the science
Tropical cyclones need four ingredients: warm ocean water (above 26.5°C to a depth of at least 50 m), a pre-existing weather disturbance, low vertical wind shear (so the storm column stays vertical and organised), and Coriolis force (which requires being at least 5° from the equator — this is why cyclones don't form on the equator itself). When these align, warm humid air rises, condenses into thunderstorms, releases latent heat that further warms the column, and the air starts spinning. The storm grows in stages defined by the India Meteorological Department (IMD): Depression → Deep Depression → Cyclonic Storm → Severe Cyclonic Storm → Very Severe Cyclonic Storm → Extremely Severe Cyclonic Storm → Super Cyclonic Storm. Rapid intensification — the process where a storm jumps two or more categories in 24 hours — has become more common over the last 20 years due to warming waters.
IMD cyclone categories
India and Bangladesh use a 7-tier classification based on 3-minute sustained wind speed. Depression: 31-49 km/h. Deep Depression: 50-61 km/h. Cyclonic Storm: 62-87 km/h (this is where cyclones receive a name). Severe Cyclonic Storm: 88-117 km/h. Very Severe Cyclonic Storm: 118-165 km/h. Extremely Severe Cyclonic Storm: 166-220 km/h. Super Cyclonic Storm: 221+ km/h. Cyclone Amphan (2020) was a Super Cyclonic Storm at peak; Cyclone Fani (2019) was Extremely Severe at landfall. The international Saffir-Simpson scale is not used in South Asia, but for comparison: IMD's Very Severe ≈ Saffir-Simpson Category 2-3, Extremely Severe ≈ Category 4, Super Cyclonic ≈ Category 5.
Cyclone seasons — when and where
The Bay of Bengal has two distinct cyclone seasons separated by the southwest monsoon. The pre-monsoon season runs from April to May, with cyclones tending to track northward toward Bangladesh and Myanmar — this is when Cyclone Nargis, Mocha and Aila struck. The post-monsoon season runs from October through December and historically produces the more intense cyclones, with October and November as peak months. Post-monsoon storms often track westward, threatening India's east coast — Tamil Nadu (Chennai, Cuddalore, Nagapattinam), Andhra Pradesh (Visakhapatnam, Machilipatnam, Nellore), Odisha (Puri, Gopalpur, Paradip) and West Bengal (Kolkata, Digha). The Arabian Sea on India's west coast has its own (less intense) cyclone seasons; Cyclone Tauktae in May 2021 was a notable Arabian Sea storm.
Coastal cities at highest risk
Chennai (Tamil Nadu) faces northeast monsoon cyclones in October-December. Visakhapatnam and Machilipatnam (Andhra Pradesh) are exposed to nearly the full cyclone alley. Bhubaneswar and Puri (Odisha) experience some of the strongest landfalls — Fani in 2019 hit Puri with winds exceeding 240 km/h. Kolkata (West Bengal) is shielded somewhat by the Sundarbans mangroves but still vulnerable to large storms like Amphan. Chittagong, Cox's Bazar and Kutubdia (Bangladesh) sit directly in the path of pre-monsoon storms. Dhaka, despite being inland, often experiences cyclone tail effects with flooding. Yangon and the Irrawaddy delta in Myanmar are exposed but receive less coverage. Tourists planning coastal trips during peak season should monitor IMD and BMD bulletins and avoid beach resorts and houseboats when warnings are issued.
Warning systems — a global success story
India's IMD and Bangladesh's BMD issue cyclone warnings with progressively increasing lead time. The "Cyclone Watch" stage begins 72 hours before expected landfall, "Cyclone Alert" at 48 hours, and "Cyclone Warning" with mandatory evacuation orders at 24 hours. Modern satellite monitoring (INSAT, scatterometers) combined with numerical weather prediction (the ECMWF and IMD models among others) can now provide 3-5 day warnings of likely landfall — a dramatic improvement from the 1970s when warnings came hours before impact. Bangladesh's Cyclone Preparedness Programme (CPP), using more than 76,000 trained community volunteers, has reduced cyclone mortality by approximately 100-fold since the 1970s and is considered one of the great disaster risk reduction success stories of the 20th century.
How to prepare — 7-day checklist
Once IMD or BMD issues a Cyclone Watch, do the following without delay: (1) store at least 7 days of clean drinking water — minimum 3 litres per person per day; (2) stockpile dry, non-perishable food (biscuits, chura, jaggery, nuts); (3) keep a battery-powered radio, working torches and spare batteries; (4) fully charge mobile phones and power banks; (5) collect essential medicines for at least 14 days; (6) place important documents (ID cards, passport, ration cards) in a waterproof bag; (7) tape windows or board them with plywood; (8) cut overhanging tree branches and secure loose objects in your yard; (9) identify the nearest cyclone shelter (each district lists them — Bangladesh has more than 4,000); (10) agree a family communication plan in case you are separated.
During the storm — what to do
Never go outside during a cyclone. Stay away from windows — broken glass causes most injuries. Shelter in the strongest part of your home: an interior room, bathroom, or under a stairwell. If you live in a tin-roofed or kutcha (mud-brick) house, evacuate to the nearest concrete cyclone shelter before the storm makes landfall — do not wait. Turn off electricity at the main switch, close the gas connection. Listen to All India Radio or BMD updates on a battery-powered radio; do not rely on internet which often fails. The eye of the cyclone — the calm centre — can last 20-40 minutes. This is dangerous because the storm returns from the opposite direction with the same intensity. Do not go outside during the eye.
After the storm — hidden dangers
Many cyclone deaths occur after the storm passes. Downed power lines remain live and electrocute people wading through flood water. Floodwater contains sewage, snakes, broken glass and sometimes crocodiles in the Sundarbans region. Wait for local authorities to confirm safety before leaving your shelter. Boil all drinking water for at least 10 minutes before consuming. Do not walk through floodwater unless absolutely necessary. Watch for mosquito-borne diseases (dengue, malaria, chikungunya) which surge in the weeks after cyclones — eliminate standing water around your home. Damaged buildings can collapse for days after the storm; do not re-enter until inspected. Mental health support is also important after major disasters.
Cyclone naming — how they get their names
Since 2004, cyclones in the North Indian Ocean are named by a panel of 13 WMO/ESCAP member countries: Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, UAE and Yemen. Each contributes 13 names to a rotating list. India's contributions include Gati, Tej, Murasu and Aag. Bangladesh has contributed Niloy, Onkar and Probaho. Recent names include Biparjoy (2023, Arabian Sea), Tej, Hamoon, Midhili, Fengal, Mocha (2023, Myanmar landfall) and Remal (2024). Names are used in order so that warning communications are unambiguous — "Cyclone Fani is approaching" is clearer than "the cyclone in the Bay of Bengal".
Climate change impact
While the total annual number of Bay of Bengal cyclones is not clearly increasing, the proportion of intense cyclones (Category 4-5 equivalent on Saffir-Simpson, or Extremely Severe / Super Cyclonic in IMD terms) appears to be rising. Sea surface temperatures in the Bay of Bengal have warmed by about 1°C since 1980, providing more energy for rapid intensification. Cyclone Fani (2019), Amphan (2020), Mocha (2023) and Remal (2024) all underwent rapid intensification, jumping multiple categories in 24-36 hours and shrinking the warning window. Sea level rise (about 3-4 mm per year in the Bay) also amplifies storm surge impacts on low-lying coasts. Adaptation — stronger shelters, mangrove restoration, embankment upgrades, improved evacuation — remains the most effective short-term response.
Tracking cyclones — official sources
India: IMD's Regional Specialised Meteorological Centre in New Delhi issues the authoritative bulletins at mausam.imd.gov.in/imd_latest/contents/cyclone.php. Bangladesh: BMD at bmd.gov.bd. Myanmar: Department of Meteorology and Hydrology. Global cross-checks: the Joint Typhoon Warning Center (US Navy/Air Force) and NOAA's Climate Prediction Center provide independent tracks. Helpful private trackers include Zoom Earth and Tropical Tidbits but always follow official local advisories for evacuation decisions. Our city pages (Chennai, Visakhapatnam, Kolkata, Dhaka, Chittagong) show live wind, pressure and rain — useful precursors before a named storm forms.
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. The southwest monsoon period (June-September) suppresses cyclone formation.
Has Bangladesh really reduced cyclone deaths? ▾
Dramatically — from over 300,000 in the 1970 Bhola cyclone to under 200 in comparable-strength Cyclone Sidr (2007), and under 30 in Cyclone Remal (2024). This is due to early warning systems, the 76,000-volunteer Cyclone Preparedness Programme, raised concrete cyclone shelters (over 4,000 nationwide), improved evacuation coordination and mangrove restoration in the Sundarbans.
Which Indian states are most exposed? ▾
On the east coast: Odisha, Andhra Pradesh, West Bengal and Tamil Nadu — in roughly that order of historical impact. Odisha has experienced the strongest single landfalls (1999 super cyclone, Phailin 2013, Fani 2019). On the west coast, Gujarat and Maharashtra face occasional Arabian Sea cyclones (Tauktae 2021).
Are cyclones a risk for tourists in India? ▾
The risk is real but manageable. Avoid India's east coast (Odisha, Andhra Pradesh, Tamil Nadu) during the October-November peak if possible. If a cyclone warning is issued, follow local evacuation orders. Modern concrete hotels in cities are generally safe; beach resorts, houseboats and tin-roofed buildings are not. Check IMD forecasts (mausam.imd.gov.in) before any coastal trip.
What is the "eye" of a cyclone? ▾
The calm centre of the storm, where winds drop and skies briefly clear for 20-40 minutes. This is dangerous — many casualties happen when people leave shelter during the eye, only to be caught when the storm returns from the opposite direction with full intensity. Stay inside until officials confirm the storm has fully passed.
How do I prepare a cyclone emergency kit? ▾
7 days of drinking water (3 L/person/day), 7+ days of dry food (biscuits, chura, jaggery, nuts), battery-powered torch and radio, spare batteries, charged power banks, 14 days of essential medicines, important documents in waterproof bag, basic first aid kit, mobile phones charged, cash in small denominations. Tape windows or board them up; cut overhanging tree branches before the storm.
Are cyclones getting stronger because of climate change? ▾
Yes — the proportion of intense (Category 4-5 equivalent) cyclones has risen since the 1990s. Sea surface temperatures in the Bay of Bengal are about 1°C warmer than in 1980, supplying more energy. Rapid intensification — jumping multiple categories in under 24 hours — has become more common, shrinking the evacuation warning window.
Where can I track an active cyclone? ▾
Official sources: IMD (mausam.imd.gov.in) for India, BMD (bmd.gov.bd) for Bangladesh, DMH for Myanmar. Cross-check with Joint Typhoon Warning Center and NOAA. Our city pages show live wind, pressure and AQI — useful for monitoring conditions before and during a named storm. Always follow local evacuation orders rather than relying on social media.