Tropical cyclones and tornadoes

The first principle

A tropical cyclone is a heat engine that runs on water vapour. Moist air rising from a warm sea condenses in tall clouds, and the latent heat released warms the air column, making it lighter and lowering surface pressure; that low pressure pulls in more moist air, which the Coriolis force turns into a spinning vortex. The storm feeds itself while it stays over warm water and starves on landfall or over cold sea, which explains where cyclones form, why they are strengthening and why they die over land.

The core

Conditions for cyclogenesis

• A warm ocean above about 26.5°C to a depth of about 50 m, because the storm's winds churn up water from below and a shallow warm layer would soon be replaced by cold water.
• Coriolis force, which is zero at the equator and grows with latitude, so cyclones do not form within about 5° of the equator.
• Low vertical wind shear, since sharply different winds aloft tilt the storm and blow its heat away.
• A pre existing disturbance, such as a low or an easterly wave, and a moist middle troposphere, since dry air evaporates cloud drops and weakens updrafts.
• Upper level divergence, which removes rising air aloft so surface pressure keeps falling, like a chimney with a good draught.

Structure and life cycle

• The eye is a calm zone of sinking, compression warmed air 20 to 50 km across, while the eyewall around it has the strongest winds and heaviest rain, because inflowing air spins faster as it nears the centre, as a skater does on pulling in her arms.
• Rain bands spiral outward, and air flows out aloft. A storm forms, intensifies, matures and decays on landfall, where it loses moisture and meets friction.

IMD classification (maximum sustained wind)

The India Meteorological Department (IMD) grades systems as a depression (31 to 49 km/h), deep depression (50 to 61), cyclonic storm (62 to 88), severe cyclonic storm (89 to 117), very severe cyclonic storm (118 to 166), extremely severe cyclonic storm (167 to 221) and super cyclonic storm (222 km/h or more). Names are given from the cyclonic storm stage, from a list agreed by the thirteen countries of a regional panel of the World Meteorological Organization.

Dimensions

Why the Bay of Bengal gets more and stronger cyclones

Roughly four cyclones form in the Bay of Bengal for every one in the Arabian Sea. The Bay is warmer, and the huge freshwater outflow of the Ganga, Brahmaputra and Irrawaddy forms a light surface layer that resists mixing, so the sea stays warm even under a storm. Remnants of Pacific typhoons drift in from the east, and the funnel shaped Bay with its shallow shelf piles up high surges. The Arabian Sea has been cooler because of monsoon upwelling and dry air from Arabia, but it has warmed fast in recent decades, and a 2021 study in Climate Dynamics found a 52 per cent rise in cyclonic storms there in 2001 to 2019 compared with 1982 to 2000.

How sea surface temperature rise affects cyclones

Sea surface temperature rise is the long term warming of the top layer of the ocean as it absorbs most of the extra heat trapped by greenhouse gases. The Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6) states that global sea surface temperature rose by about 0.88°C between 1850 to 1900 and 2011 to 2020. Its effects on cyclones follow from the heat engine idea:

• More intense storms. Warmer water supplies more vapour and latent heat, and AR6 finds that the share of the most intense cyclones has likely increased over the last four decades.
• Rapid intensification, a rise of about 55 km/h in wind speed within 24 hours, becomes more common, as with Ockhi (2017), which grew from a depression to a very severe storm very fast near Kerala and Lakshadweep, and Tauktae (2021), which strengthened quickly off the Konkan coast before hitting Gujarat. Rapid growth leaves less time for warning.
• More rain, higher surge, slower and more poleward tracks, since warmer air holds about 7 per cent more vapour per degree and sea level is rising.
• The other side. Warming also changes wind shear and stability, so the total number of cyclones may stay the same or even fall while the strongest become stronger; a warm sea is necessary but not sufficient.

The Fujiwhara effect

When two cyclones come within about 1,400 km of each other, the circulation of each starts to steer the other, so the pair rotate around a common centre, anticlockwise in the northern hemisphere. It is named after the Japanese meteorologist Sakuhei Fujiwhara (1921). If one storm is much stronger, the common centre lies close to it, and the weaker storm orbits it and may be absorbed; two storms of similar strength may circle each other or merge. In 2017 Hurricane Hilary absorbed Hurricane Irwin in the eastern Pacific, and such interactions are seen most often among western Pacific typhoons, where many storms form close together. Tracks therefore bend, loop or stall unpredictably, and a storm may strengthen by taking in its neighbour or weaken under the other's outflow and shear.

In practice

IMD colour coded warnings

IMD issues cyclone warnings for coastal areas in four stages. A pre cyclone watch, about 72 hours before bad weather, alerts senior officials. A cyclone alert (yellow) comes at least 48 hours before, a cyclone warning (orange) at least 24 hours before, and a post landfall outlook (red) at least 12 hours before landfall, describing what inland districts will face. District level warnings have also moved to impact based colour codes: green means no action, yellow means be aware and keep updated, orange means be prepared to move and protect property, and red means take action, evacuate and follow official orders. People respond better to what a storm will do to them than to its wind speed.

Cases

• Odisha super cyclone (1999) killed about 10,000 people; Odisha then built shelters and a state disaster authority, and mass evacuations kept deaths low in Phailin (2013) and Fani (2019).
• Biparjoy (2023) lasted about 13 days in the Arabian Sea, one of its longest lived storms.

Schemes: the National Cyclone Risk Mitigation Project (Ministry of Home Affairs, from 2011) built shelters and warning networks in coastal states, under National Disaster Management Authority (NDMA) cyclone guidelines (2008).

Tornadoes or twisters

A tornado, or twister, is a violently rotating column of air that reaches from a thunderstorm cloud down to the ground, usually tens to a few hundred metres wide, with winds that can exceed 300 km/h. Most strong tornadoes come from supercells, thunderstorms with a rotating updraft called a mesocyclone. Rotation begins with wind shear: winds that strengthen and turn with height roll the low level air like a pencil between two hands, and a strong updraft tilts this roll upright and stretches it into a narrow spinning column. Damage is rated on the Enhanced Fujita scale from EF0 to EF5.

Why most twisters occur around the Gulf of Mexico

The United States records about 1,000 to 1,200 tornadoes a year, more than any other country, according to the National Oceanic and Atmospheric Administration (NOAA). Warm, moist air flows north from the Gulf of Mexico at low levels, dry air from the Rockies and the Mexican plateau rides over it, and cold, dry air comes down from Canada. The dry layer acts as a lid under which heat and moisture build until storms break through explosively, and the turning of wind with height supplies rotation. No east to west mountain range separates these air masses on the flat plains, so the belt from Texas to Nebraska is called Tornado Alley. Bangladesh and eastern India also see deadly pre monsoon tornadoes, when moist Bay of Bengal air meets dry north westerlies.

Cyclone and tornado compared

A cyclone is hundreds of kilometres wide, forms over warm seas, lasts days and is forecast days ahead; a tornado is under a kilometre wide, forms over land from one storm, lasts minutes and gives minutes of warning, yet its winds can be stronger.

Issues and way forward

• Rapid intensification shortens warning time, as Ockhi showed when fishermen were caught at sea; more ocean buoys and better models under the Ministry of Earth Sciences' Mission Mausam (2024) are the remedy.
• Last mile communication. Warnings fail if they do not reach fishermen and coastal villages, so the Common Alerting Protocol based SACHET system and NavIC messaging on boats need full coverage.
• Rising Arabian Sea risk. The west coast has fewer shelters than Odisha, so the Odisha model of shelters, drills and volunteers should be copied there.
• Coastal protection. Mangroves such as Bhitarkanika and the Sundarbans cut surge and wind, so mangrove planting under the Mangrove Initiative for Shoreline Habitats and Tangible Incomes (MISHTI, 2023) and Coastal Regulation Zone rules need enforcement.

Common confusions

• Cyclone, hurricane and typhoon are the same phenomenon named by region: cyclone in the Indian Ocean, hurricane in the Atlantic and eastern Pacific, typhoon in the western Pacific.
• Tropical and temperate cyclones. Tropical cyclones have a warm core fed by latent heat; temperate cyclones form along fronts between air masses and have no eye.
• Storm surge and tsunami. A surge is sea level pushed up by wind and low pressure; a tsunami is set off by an undersea earthquake or landslide.

For the exam

• Keyword bank: latent heat, warm core, Coriolis force, wind shear, rapid intensification, storm surge, Fujiwhara effect, impact based warning, pre cyclone watch, supercell, mesocyclone, Enhanced Fujita scale, Tornado Alley, dry line.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction (data): IPCC AR6 records that global sea surface temperature has risen by about 0.88°C since pre industrial times, and the Arabian Sea has seen a sharp rise in cyclonic storms since 2001.
  • Introduction (report): IMD's shift to impact based colour coded warnings tells people what a storm will do to them, which is why Odisha's evacuations during Fani in 2019 kept deaths low.
  • Conclusion: A warmer ocean is a richer fuel tank for cyclones, so India must pair better forecasting of rapid intensification with shelters, mangroves and last mile warnings.
  • Conclusion: From the Fujiwhara effect to Tornado Alley, the behaviour of storms follows from simple physics of heat, pressure and rotation, and understanding it is the basis of every early warning.