Cyclones: Naming, IMD Categories and Warning Stages

A cyclone is a system of winds blowing spirally inwards towards a centre of low atmospheric pressure. The winds turn anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, the opposite of an anticyclone, which is a high pressure system with outblowing winds. Examiners ask three things from this topic and ask them repeatedly: the local name of a cyclone in each ocean, the wind speed band that earns each India Meteorological Department category, and the stages and colours of the warning that reaches a fisherman before landfall. In October 2026 the India Meteorological Department added a decision support system to its cyclone warning work, which is a good reason to revise the fixed parts of the subject.

How a tropical cyclone forms

A tropical cyclone is an intense low pressure system that develops over warm tropical seas. Five conditions must come together. The sea surface temperature has to be about 26.5 degrees Celsius or more, and the warm layer at least 50 metres deep, so that moist air keeps rising. The Coriolis force must be strong enough to give the rising air a spin, which is why no cyclone forms within about five degrees of the equator. Vertical wind shear must be weak, or the growing column is torn apart. There must be a pre-existing disturbance, and divergence in the upper atmosphere to pump the rising air away.

The mature storm has three parts. The eye is the calm core of descending air, ten to fifty kilometres across, where the pressure is lowest and the sky may be clear. Around it the eye wall carries the strongest winds and the heaviest rain. Beyond that, spiral rain bands feed the system. Latent heat released when water vapour condenses is the engine, so a cyclone weakens quickly after landfall, when that supply of moist air is cut off. The deadliest part is not the wind but the storm surge, the wall of sea water that the low pressure and the wind push on to a low coast.

What a cyclone is called where

IMD categories by wind speed

The India Meteorological Department grades a system by the sustained wind speed at its centre. The bands are fixed and are the single most asked set of numbers in the topic.

A system is given a name only when it reaches cyclonic storm strength, that is 62 km/h. In the Atlantic and the eastern Pacific the Saffir-Simpson scale is used instead, which runs from Category 1, beginning at about 119 km/h, to Category 5 at 252 km/h and above, and a hurricane of Category 3 or above is called a major hurricane.

How cyclones are named

Cyclones of the north Indian Ocean are named by the member countries of the World Meteorological Organization and Economic and Social Commission for Asia and the Pacific Panel on Tropical Cyclones. Naming in this basin began in 2004 with eight members, namely Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand. Iran, Qatar, Saudi Arabia, the United Arab Emirates and Yemen joined in 2018, taking the panel to thirteen members, and a fresh list of 169 names, thirteen from each member, was released in 2020. The names are taken in order, country by country in English alphabetical order of the country names, and in this basin a name is never reused. A name must be short, easy to say, neutral in religion, politics and gender, and not offensive to any member. Amphan, in 2020, was the last name from the 2004 list. The naming body for the region is the Regional Specialised Meteorological Centre at New Delhi, which is run by the India Meteorological Department and also issues cyclone advisories to all thirteen member countries.

The four-stage warning and the colour codes

• Stage one, pre-cyclone watch, issued 72 hours before the expected adverse weather.
• Stage two, cyclone alert, issued 48 hours before.
• Stage three, cyclone warning, issued 24 hours before.
• Stage four, post-landfall outlook, issued 12 hours before landfall.

The colour codes run green for no warning, yellow for be updated, orange for be prepared and red for take action. Area Cyclone Warning Centres work at Chennai, Mumbai and Kolkata, and Cyclone Warning Centres at Visakhapatnam, Bhubaneswar, Ahmedabad and Thiruvananthapuram. The National Cyclone Risk Mitigation Project builds shelters and early warning networks in the coastal states.

Where and when India gets cyclones

The north Indian Ocean season has two peaks, the pre-monsoon months of April to June and the post-monsoon months of October to December, of which November is the most active. The Bay of Bengal gets roughly four cyclones for every one in the Arabian Sea, because it is warmer, less saline at the surface and fed by the remains of Pacific systems, and because the funnel shape of its northern end raises the surge. The east coast, from Tamil Nadu through Andhra Pradesh and Odisha to West Bengal, is therefore the most exposed, while Gujarat and the Konkan face the Arabian Sea storms.

Temperate cyclones

Temperate or extratropical cyclones form in the middle latitudes where a warm air mass meets a cold one along a front, as the polar front theory of Bjerknes explains. They are larger, slower, have no eye, move from west to east, and bring the winter rain of north-west India as western disturbances.