Tropical cyclones

1. Nature, structure and energy source

A tropical cyclone is a non-frontal, synoptic-scale low-pressure system originating over tropical or subtropical waters, with organised deep convection and a closed surface wind circulation. It is called a hurricane in the North Atlantic and northeastern Pacific, a typhoon in the northwestern Pacific, and a cyclone in the Indian Ocean and South Pacific. These are regional names for the same basic phenomenon, not different categories of intensity.

A mature, intense cyclone commonly has an eye, an eyewall and spiral rainbands. The eye contains descending air and relatively light winds; skies may be partly clear. The surrounding eyewall contains vigorous rising air, deep cumulonimbus clouds, intense rainfall and usually the strongest winds. Rainbands extend outward and can produce heavy showers, squalls and local flooding. Weak or developing systems need not have a clearly defined eye.

The energy source is primarily heat and moisture transferred from the ocean. Moist air rises, cools and condenses, releasing latent heat that helps maintain a warm atmospheric column and low surface pressure. Air converges inward near the surface and diverges outward in the upper troposphere. Stronger winds increase ocean-to-atmosphere heat transfer, reinforcing the circulation under favourable conditions. Tropical cyclones therefore differ fundamentally from frontal systems driven mainly by horizontal temperature contrasts.

• Central pressure alone does not determine damage: storm size, wind distribution, translation speed and coastal exposure also matter.
• Calm conditions during eye passage are temporary; dangerous winds can return abruptly as the opposite eyewall arrives.

2. Conditions for formation and intensification

Formation usually requires warm ocean water, abundant lower- and middle-tropospheric moisture, atmospheric instability, a pre-existing disturbance and sufficient planetary rotation. A sea-surface temperature near 26.5°C or above is a useful textbook threshold. Warm water extending to roughly 50–60 metres or deeper helps prevent cyclone-driven mixing from quickly exposing colder water. Ocean heat content is consequently often more informative than surface temperature alone.

The Coriolis force helps organise inward-moving air into a rotating circulation. Because it is zero at the equator and weak nearby, tropical cyclones rarely originate within about 5° latitude of it. A pre-existing low-pressure area or tropical wave supplies initial convergence and relative vorticity. Moist middle levels limit the disruptive effects of dry-air entrainment, while upper-level divergence allows rising air to escape from the developing system.

Low vertical wind shear is especially favourable: a large change in wind speed or direction with height can tilt the vortex and separate thunderstorms from the surface centre. Warm water alone is therefore insufficient. Rapid intensification commonly means an increase in maximum sustained winds of at least 30 knots in 24 hours. It becomes more likely when high ocean heat content, weak shear and a favourable internal structure coincide.

• Cyclones usually weaken over land because moisture and heat supplies are reduced and surface friction increases.
• Cold water, dry air and strong wind shear can weaken a cyclone even before landfall.

3. Global distribution, movement and Indian seasonality

Major cyclone basins include the North Atlantic, northeastern and northwestern Pacific, North Indian Ocean, southwest Indian Ocean and the Australian–South Pacific region. Formation is concentrated broadly between 5° and 30° latitude. The northwestern Pacific is the most active basin. The southeastern Pacific and South Atlantic are generally unfavourable because of combinations of cool water, strong shear and an absence of suitable disturbances; however, South Atlantic occurrences are not impossible.

Cyclones are steered mainly by the surrounding atmospheric flow, including subtropical ridges and mid-latitude troughs. Many initially move westward with tropical easterlies, then turn poleward and sometimes eastward as they encounter westerlies. This recurvature is a common pattern, not a fixed rule. Some systems stall, loop or change direction. Landfall is the crossing of the coastline by the cyclone’s surface centre, although dangerous weather can begin much earlier.

The North Indian Ocean has two principal activity periods: April–June and October–December, with prominent peaks around May and November. Strong vertical shear during the southwest monsoon suppresses the development of many intense tropical cyclones, although monsoon depressions remain important rain-bearing systems. The Bay of Bengal generally supports more cyclone formation than the Arabian Sea. Moisture availability, disturbances, circulation and ocean conditions all contribute to this contrast.

• Odisha, Andhra Pradesh, West Bengal and Tamil Nadu are highly exposed on the east coast; Gujarat and other western coastal areas also face risk.
• Arabian Sea cyclone risk must not be treated as negligible: Tauktae in 2021 and Biparjoy in 2023 are important examples.

4. Hazards, coastal vulnerability and climate change

Cyclone hazards include destructive winds, wind-driven waves, storm surge, intense rainfall and inland flooding. Heavy rainfall can trigger landslides in hill regions far from the coast. A storm surge is the abnormal rise of sea level associated with a storm, above the predicted astronomical tide. Wind stress pushing water coastward is its main driver, while low atmospheric pressure also contributes. Storm tide combines the astronomical tide and storm surge.

Surge severity depends on storm intensity and size, approach angle, forward motion, coastal shape, shelf depth and tidal timing. Broad, shallow continental shelves and funnel-shaped bays favour large water accumulation. The northern Bay of Bengal is particularly vulnerable because of its coastal geometry, low-lying deltas, dense population and extensive settlements. Saline inundation damages crops, drinking-water sources and soils; transport disruption and power failures can magnify losses.

Climate change does not imply a uniform increase in cyclone numbers everywhere. Warmer oceans and greater atmospheric moisture can support higher rainfall rates and stronger peak intensities. Assessments project an increasing proportion of intense tropical cyclones globally, while changes in total frequency remain more uncertain and basin-dependent. Sea-level rise increases coastal inundation risk even without a stronger storm. The effects of warming must be distinguished from natural variability and growing coastal exposure.

• Storm surge is not a tsunami: tsunami waves usually originate from sudden displacement of water, commonly during submarine earthquakes.
• Track forecasts and intensity forecasts have different uncertainties; rapid intensification is a major operational challenge.

5. Monitoring, warnings and risk reduction in India

The India Meteorological Department is India’s principal cyclone-warning agency. Its Regional Specialised Meteorological Centre in New Delhi provides tropical cyclone advisories for the North Indian Ocean. Observations come from satellites, Doppler weather radars, ocean buoys, ships and coastal stations, supported by numerical models. Cyclone names are assigned from lists contributed by the WMO/ESCAP Panel’s member countries; naming improves communication and does not indicate intensity.

IMD’s four-stage warning system comprises a pre-cyclone watch, cyclone alert, cyclone warning and post-landfall outlook. These are normally issued about 72, 48, 24 and 12 hours respectively before the relevant anticipated onset or landfall stage. Forecasts support evacuation, fishing restrictions, port preparedness and emergency response. IMD’s wind-based intensity categories should not be confused with its colour-coded warning messages, which communicate expected impacts and required action.

Risk reduction combines timely evacuation with resilient buildings, shelters, reliable communication, protected utilities and appropriate coastal land-use planning. Mangroves and dunes can reduce some wave impacts but cannot replace engineered protection or evacuation. The National Cyclone Risk Mitigation Project has supported measures such as shelters, evacuation routes and early-warning systems. Effective preparedness must include fishers, persons with disabilities, migrants, livestock owners and isolated coastal communities.

• The Disaster Management Act, 2005 provides the institutional framework for national, state and district disaster-management authorities.
• A forecast becomes useful only when warnings reach exposed people in understandable language and lead to timely protective action.

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