Tropical Cyclone – Formation & Characteristics – UPSC (Geography Optional)

• A tropical cyclone is an intense, warm-core, non-frontal low-pressure system that forms over warm tropical oceans (mostly 5°–20° latitude). Winds spiral in anticlockwise in the northern hemisphere and clockwise in the southern, and IMD treats a system as a cyclone once sustained winds reach 62 km/h.
• It is the same phenomenon everywhere under different names: hurricane (Atlantic and north-east Pacific), typhoon (north-west Pacific) and cyclone (Indian Ocean and South Pacific).
• It is a heat engine run by the latent heat of condensation drawn from the sea, releasing about 200 times the world’s electricity-generating capacity. Its three killers are violent winds, torrential rain and the storm surge.
• The World Meteorological Organization uses the term for systems whose winds exceed gale force, a minimum of 34 knots (about 63 km/h), and they occur within about 30° of the equator.
  • It is a closed circulation of air around a low-pressure centre, and a squall (a sudden violent gust or local storm) and torrential rain (a torrent is a strong, fast stream) come with it. An anticyclone often lies between two cyclones.
• For India it is a leading coastal hazard: about 8% of the land and one-third of the population, in 13 coastal States and Union Territories, are exposed. About 5,700 km of the coast long measured at 7,516 km is rated prone, and the coastline is now reckoned at 11,098.81 km.

Origin and Development

Tropical Disturbances

• Tropical weather systems form a sequence of rising intensity:
  • Easterly waves: weak inverted-V troughs in the trades, 5°–20° from the equator, drifting west at 300–500 km a day; fair weather ahead (west) of the trough, where air sinks, and cumulonimbus showers behind it, where air converges. Most Atlantic hurricanes grow from waves leaving West Africa.
  • Tropical depression (winds under about 62 km/h, common on the ITCZ and monsoon trough) → tropical storm (62–118 km/h) → hurricane, typhoon or severe cyclone (119 km/h or more).
• Only a small fraction of disturbances ever reach cyclone strength.
• These storms have a thermal origin and grow over tropical seas in late summer (August to mid-November), advancing until they find a weak spot in the trade-wind belt.
• Small local differences in water and air temperature produce small low-pressure centres with a weak circulation, a few of which deepen rapidly into a true vortex.

Conditions for Formation: Gray’s Genesis Parameters

• William M. Gray (1968), from a global climatology of where storms form, set out six genesis parameters: three dynamic and three thermodynamic.
• Warm ocean (ocean thermal energy):
  • Sea-surface temperature must be at least 26.5 °C, and the warm layer must be about 50–60 m deep, so that the storm’s own churning does not bring cold water up and starve it. Older accounts gave 27 °C, with water of 26–27 °C down to 60–70 m.
  • The western parts of tropical oceans qualify (trades pile warm water there; warm currents); the eastern parts, cooled by cold currents and upwelling off Peru, Namibia and California, breed none.
• Coriolis force (planetary vorticity):
  • It is zero at the equator, so a cyclone rarely forms within about 5° of it. Most form between 10° and 20°, which hold about 65% of the activity.
  • Typhoon Vamei (2001), which formed at about 1.5° N near Singapore, is the rare exception.

• A pre-existing low-level disturbance (relative vorticity):
  • Thunderstorm clusters, an easterly wave, a low on the ITCZ or monsoon trough, or the cold-core remnant of an upper westerly trough.
  • The old upper troughs usually have cold cores, so the lapse rate beneath them is steep and unstable. Divergence on their eastern side encourages rising air and the thunderstorms that seed a vortex.
• Weak vertical wind shear:
  • The wind should change little with height, ideally under about 10 m/s between the lower and upper troposphere, so the warm core stays vertically stacked.
  • Strong shear tilts and ventilates the storm. This is why cyclones form equatorward of the subtropical jet stream, and why the Tropical Easterly Jet suppresses them over the north Indian Ocean in July and August.
• Moist middle troposphere: high humidity around 5 km lets cumulonimbus towers grow, while dry air entrained from deserts kills convection.
  • Relative humidity of about 50–60% in the mid-troposphere is commonly quoted. The western margins of oceans and the equatorial doldrums supply it, as the trades keep replacing saturated air.
• Conditional instability: the lapse rate must be steep enough for deep moist convection.
• Upper-level divergence: an anticyclone at about 200 hPa (9–15 km) must pump out the rising air, so that surface pressure keeps falling.
• Temperature contrast between air masses: older accounts add the meeting of trade winds of different temperature along the inter-tropical front, when the ITCZ lies farthest from the equator, as a source of instability.
• Evaluation:
  • These conditions are necessary but not sufficient.
  • Gray later combined them into a Seasonal Genesis Parameter. Kerry Emanuel and David Nolan (2004) built the Genesis Potential Index now used to map cyclone-favourable climates, including under global warming.

How a Cyclone Intensifies

• Feedback loop:
  • Warm, moist air rises in thunderstorms, cools adiabatically and condenses, releasing latent heat.
  • The heated air column becomes lighter, so surface pressure falls.
  • More air rushes in, is deflected by the Coriolis force into a cyclonic vortex, and evaporates more water from the sea surface.
  • The upper outflow removes the rising air, so the cycle repeats as long as warm water lies beneath.
  • The rising air loses its moisture aloft, turns cold and dense, and sinks through the eye and at the edges of the storm.
  • On reaching land the moisture supply is cut off and the storm dissipates.
• Two explanations:
  • CISK (Conditional Instability of the Second Kind), proposed by Jule Charney and Arnt Eliassen (1964), treats the cyclone as a partnership between low-level convergence and cumulus convection.
  • The air–sea interaction (WISHE) theory of Kerry Emanuel (1986) is the current view: the storm grows because stronger winds draw more heat and moisture from the ocean, and its maximum potential intensity is set by the sea’s temperature and that of the outflow.
• Rapid intensification:
  • It is defined as a rise in sustained winds of at least 55 km/h (30 knots) within 24 hours.
  • It occurs over deep warm water with low shear and is the hardest part to forecast: Amphan (2020) went from cyclonic storm to super cyclonic storm in about two days.

Life Cycle

1. Formative: a disturbance deepens into a depression.
2. Immature: pressure falls fast, winds reach storm force, and rain organises into spiral bands around an eye.
3. Mature: peak intensity; the circulation widens and pressure stops falling.
   • Spiralling winds organise convective cells, with cumulonimbus rain bands alternating with calm zones of sinking air. Cloud is densest at the centre and thins outward, with nimbostratus and cumulus at the periphery and a cirrus overcast of ice crystals on top.
4. Decay: over land (moisture cut off, more friction), cooler water or strong shear; many recurve and undergo extratropical transition, acquiring fronts.
5. Most cyclones last about a week.
6. An average storm travels about 300–400 miles (about 480–640 km) a day, or about 3,000 miles (about 4,800 km) over its life.

Structure and Characteristics

Horizontal Structure

• Eye:
  • It is a roughly circular calm area at the centre, usually 30–65 km across (range about 8 to over 200 km), with light winds, clear or broken sky and the lowest pressure.
  • Air sinks in the eye and warms by compression, so the eye is the warmest part of the storm: up to 10 °C warmer than its surroundings at 12 km, but only slightly warmer at the surface.
  • Sinking stops 1–3 km above the sea, above an inversion. The eye forms because the spinning inflow cannot reach the centre and eyewall convection forces air down inside it.
  • Rotation throws the air outward, so the vortex keeps a calm centre, with little or no rain and sometimes blue sky or stars. Temperature is only 0–2 °C above the surroundings at the surface.
• Eyewall:
  • A ring of towering cumulonimbus (12–15 km) with the strongest winds and heaviest rain.
  • Intense storms undergo eyewall replacement cycles: an outer eyewall contracts and replaces the inner one, briefly weakening the storm.
• Spiral rainbands:
  • Bands of convection curving inward, with lighter rain and sinking air between them; most rain falls on only about 10% of the storm’s area.
  • Convergence is greatest along the bands and divergence above them. Warm, moist air rises through a band and sinks on both sides.
  • Sinking is concentrated on the inner side, so adiabatic warming is stronger there and pressure falls across the band. The tangential wind then strengthens, the band moves inward, and it can close into an eye and eyewall.
• Asymmetry:
  • In the northern hemisphere, the strongest winds and highest surge lie in the right-front quadrant relative to the storm’s motion, where its forward speed adds to the spin. In the southern hemisphere it is the left-front quadrant.

Vertical Structure

• Inflow layer (lowest 1–3 km): air spirals in, gathering heat and moisture.
• Ascent layer (3–7 km): air rises in the eyewall and rainbands.
• Outflow layer (above 7 km, peak near 12 km): air spreads anticyclonically as a cirrus shield.
• Warm core:
  • Because the core is warmer than its surroundings, the pressure gradient weakens with height.
  • Herbert Riehl (1954) showed that the cyclonic circulation weakens above about 6 km and turns anticyclonic above about 9 km.

Main Characteristics

• Size: 150–1,000 km across, typically 500–600 km; far more compact than temperate cyclones. Older descriptions give an elliptical form with a 2:3 ratio of axes, about 80 km near the centre, growing to 300–1,500 km.
• Isobars: nearly circular and closely packed, so the pressure gradient is very steep. Central pressure lies 50–60 hPa or more below the margins.
• Wind: light in the eye, peaking at the eyewall and stronger on the poleward margin than at the centre. It is stronger over the sea than over land, where relief breaks it.
• Records:
  • The lowest sea-level pressure on record is 870 hPa, in Typhoon Tip (1979).
  • The highest reliable sustained wind is about 345 km/h, in Hurricane Patricia (2015).
  • The 1999 Odisha super cyclone (912 hPa, about 260 km/h) is the north Indian Ocean’s strongest on record.
• Weather sequence as a cyclone passes:
  • Falling pressure and a cirrus–cirrostratus halo, then gales, torrential rain and squalls under cumulonimbus.
  • The eye brings a sultry calm of about half an hour over a heaving sea.
  • The storm returns from the opposite direction as pressure jumps back; the violent phase lasts 12–24 hours.

Movement and Tracks

• Cyclones are steered by the surrounding flow: westward with the trades at about 15–30 km/h, then recurving near 20°–30° around the western edge of the subtropical high, and finally north-east in the westerlies, speeding up and weakening.
  • The usual path is parabolic, its axis parallel to the isobars: westward at first, northward near 20°, north-eastward near 25° and eastward near 30°, where cool water and westerly shear kill the storm. The Coriolis force and the easterlies and westerlies shape it.
• Over the north Indian Ocean:
  • Bay of Bengal storms move north-west towards Andhra Pradesh, Odisha and West Bengal, or recurve north-east towards Bangladesh and Myanmar.
  • Arabian Sea storms move north-west towards Oman, or north-east towards Gujarat and Pakistan.
  • Crossing the Peninsula: many storms striking the east coast south of 15° N cross into the Arabian Sea and can re-intensify there.
  • Genesis shifts with the season: Bay storms form north of about 16°–18° N and west of about 90°–92° E in June and July, north of 15° N in September, and between 8° and 14° N in October. Arabian Sea storms form at about 17°–20° N in June, 12°–17° N in October and 8°–13° N in November.
  • July storms run west along the monsoon trough, between about 20° and 25° N, and rarely recurve to the Himalayan foothills. About a quarter of post-monsoon Arabian Sea storms recurve north-east to Maharashtra or Gujarat.
• Slow storms flood worst; James Kossin (2018) found cyclones slowed by about 10% over 1949–2016.

Distribution, Classification and Naming

Breeding Grounds and Seasons

Basin	Local name	Peak season
North Atlantic: Caribbean, Gulf of Mexico	Hurricane	August–October
North-east Pacific, off Mexico	Hurricane	July–September
North-west Pacific: Philippines, South China Sea, Japan	Typhoon (Philippines: bagyo)	July–October
North Indian Ocean: Bay of Bengal, Arabian Sea	Cyclone	April–June and October–December
South-west Indian Ocean: south-east African coast, Madagascar, Mauritius	Tropical cyclone	January–March
Australia and South Pacific: Timor Sea, Coral Sea, Fiji	Tropical cyclone (old Australian willy-willy)	January–March

• About 80–90 tropical storms form each year. The north-west Pacific is the most active basin, with roughly a third of the total, and the north Indian Ocean produces only about 7%.
• Cyclones are virtually absent from the South Atlantic and south-east Pacific (cool seas, strong shear, ITCZ seldom south of the equator); Hurricane Catarina (2004), which struck Brazil, was the first recorded South Atlantic hurricane.

IMD Classification

• IMD classifies north Indian Ocean systems by 3-minute mean maximum sustained wind:

IMD category	Wind (km/h)	Wind (knots)
Low-pressure area	below 31	below 17
Depression	31–49	17–27
Deep depression	50–61	28–33
Cyclonic storm	62–88	34–47
Severe cyclonic storm	89–117	48–63
Very severe cyclonic storm	118–166	64–89
Extremely severe cyclonic storm	167–221	90–119
Super cyclonic storm	222 and above	120 and above

• The Saffir–Simpson Hurricane Wind Scale, devised by Herbert Saffir and Robert Simpson (1971), rates Atlantic and east Pacific hurricanes from Category 1 (119–153 km/h) to Category 5 (252 km/h or more), using 1-minute winds.
• Australia’s Bureau of Meteorology uses five categories based on typical gusts over open flat land:

Category	Class	Typical gusts (km/h)	Wind term
1	Tropical cyclone	90–125	Gales
2	Tropical cyclone	125–164	Destructive
3	Severe tropical cyclone	165–224	Very destructive
4	Severe tropical cyclone	225–279	Very destructive
5	Severe tropical cyclone	280 and above	Very destructive

Naming of Cyclones

• Storms are named by the six Regional Specialised Meteorological Centres (RSMCs) and five Tropical Cyclone Warning Centres under the World Meteorological Organization (WMO).
• IMD, New Delhi is the RSMC for the north Indian Ocean, serving the 13 members of the WMO/ESCAP Panel on Tropical Cyclones: Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, UAE and Yemen.
• Naming began in 2004 with an eight-country list that ended with Amphan (2020). A new list of 169 names (13 per member), agreed at the Panel’s 2018 session, was released in April 2020; Nisarga (June 2020) was its first.
  • Its second column closed with Ditwah (2025), and Arnab (October 2026) opened the third. Murasu and Akvan come next.
• Rules: a name is given at cyclonic-storm strength, taken in sequence, never reused, at most eight letters and neutral on politics, religion, culture and gender. India’s contributions include Gati, Tej, Murasu, Aag, Vyom and Jhar.

Tropical Cyclones and India

Bay of Bengal versus Arabian Sea

Aspect	Bay of Bengal	Arabian Sea
Frequency	About four times as many cyclones	Fewer, but rising sharply
Why	Warmer sea; river freshwater keeps the surface layer warm; remnants of Pacific typhoons and the monsoon trough supply disturbances	Cooler sea from upwelling off Somalia and Oman; dry air from Arabia; strong shear
Coast struck	Low, flat, funnel-shaped deltas with a shallow shelf	Many storms move away towards Oman and Pakistan; Gujarat is the main Indian target
Impact	Highest storm surges and death tolls in the world	Damage mainly from wind and rain
Recent examples	Amphan 2020, Yaas 2021, Michaung 2023, Dana 2024, Montha 2025	Tauktae 2021, Biparjoy 2023, Asna 2024

• Season:
  • The north Indian Ocean has a two-peaked season: April–June and October–December. Post-monsoon storms in the Bay are the most dangerous.
  • An average year brings 5–6 cyclones to these two seas, of which 2–3 turn severe. Older figures give the seasons as May–June and mid-September to mid-December, with May, June, October and November known for severe storms.
  • During the southwest monsoon strong vertical shear keeps cyclones rare, but monsoon depressions from the Bay bring much of central India’s rain.
• Frequency, 1897–2014 (India Meteorological Department):

Season	Bay: cyclonic storm	Bay: severe	Arabian Sea: cyclonic storm	Arabian Sea: severe
January–February	4	3	2	0
March–May	28	48	6	19
June–September	117	31	16	15
October–December	107	109	25	34
Total	256	191	49	68

• The Bay therefore produced about four times as many storms as the Arabian Sea, and severe storms cluster in May and in October–November.
• Maximum destruction falls within about 100 km of the centre and on the right of the track.

Storm Surge

• A storm surge is an abnormal rise of sea level above the predicted tide, driven ashore by a cyclone. It causes most cyclone deaths.
• Causes:
  • Wind set-up: strong onshore winds pile water against the coast.
  • Low pressure: sea level rises about 1 cm for every 1 hPa fall in pressure.
  • Wave set-up adds to the rise, and the surge is worst when it coincides with high (spring) tide.
  • Amplifiers: a shallow continental shelf, a concave or funnel-shaped coast and low-lying deltas. The head of the Bay of Bengal combines all three.
  • The angle and speed of approach of the storm also matter, and estuaries amplify the rise.
• Deadliest examples:
  • The Bhola cyclone (1970) in East Pakistan, now Bangladesh, killed an estimated 3,00,000–5,00,000 people.
  • The 1991 Bangladesh cyclone and Nargis (2008) in Myanmar each killed about 1,38,000.
  • The 1999 Odisha super cyclone raised the sea 5–6 m and killed about 10,000.
  • Bakarganj (1876): a surge reported at about 13 m near the Meghna estuary, among the highest known.
• How a surge behaves on land:
  • It is a moving mass of water, not a wave, travelling at the speed of the cyclone until the ground rises to its height. It reaches 10–20 km inland on flat coasts and may hold the land under for 45 minutes or more.
  • The rush stresses walls, turbulence scours foundations, and debris acts as a battering ram. Water pressure can float a house off its footing.
• Probable maximum surge around India: about 12–12.5 m at the head of the Bay, about 11 m on the southern Tamil Nadu coast, mostly 3–6 m along Odisha and Andhra Pradesh, and about 2.4–5 m on the west coast.
• Surge and seiche:
  • A surge is a one-time rise driven by wind and low pressure.
  • A seiche is a standing oscillation that sloshes back and forth in an enclosed or semi-enclosed basin.

Hazards and Management

• Hazards: wind wrecks houses, power lines and plantations; rain floods cities (Michaung, 2023, in Chennai); surge inundates and salinises farmland.
  • Wind does not stop at the shore: structures around Vijayawada, about 100 km inland, have been damaged. Speeds of 50 m/s are not uncommon on these coasts, and building codes for coastal areas allow for them.
  • Rain of up to 50 cm a day for several days saturates catchments and floods rivers. Large, hard raindrops erode soil, and soaking weakens tank embankments and utility poles.
  • Most deaths come from coastal inundation, then rain floods. Collapsing buildings, falling trees, flying debris, electrocution and post-storm disease add to them.
• Vulnerable States:

State	Exposure	Marker events
Andhra Pradesh	Long flat coast from Srikakulam to Nellore; Krishna and Godavari deltas; 71 cyclones from 1892 to 2014	November 1977, May 1990, Montha 2025, Arnab 2026
Odisha	Low delta coast, from Ganjam to Balasore	Super cyclone 1999, Phailin 2013, Fani 2019
Gujarat	Kachchh and Saurashtra coasts; rated very high damage risk for wind	June 1998, Tauktae 2021, Biparjoy 2023
West Bengal	Funnel-shaped head of the Bay; Sundarbans	Amphan 2020, Remal 2024
Tamil Nadu and Puducherry	Post-monsoon strikes on the Coromandel coast	Chennai floods of Michaung 2023, Fengal 2024

• Andhra Pradesh: over 2,500 coastal villages and some six million people on an undivided-State count, many in thatched houses of fishing and weaker households. A flat delta lets a storm carry 20 km inland.
• Odisha, 29 October 1999: over 13 million people were hit. Khurda, Puri, Cuttack, Jagatsinghpur and Kendrapara took the storm; Jajpur, Bhadrak, Keonjhar, Balasore and Mayurbhanj took the floods.
• Phailin, 12 October 2013: it crossed near Gopalpur with sustained winds of 200–210 km/h, gusting to 220 km/h (India Meteorological Department). Ganjam, Gajapati, Khurda, Puri and Jagatsinghpur took the worst, with coastal Andhra Pradesh also damaged, and about a million people moved to safety.
• Gujarat, 9 June 1998: the Kachchh district lay on the track. A parliamentary reply of July 1998 gave 1,178 lives lost across 12 districts, with later official figures of 1,173 dead and 1,774 missing, and media accounts higher still.
  • Migrant labour in salt pans and port areas, in temporary shelters, bore much of the toll. Rural walls of unburnt brick or stone were at most risk, as in Jamnagar (50%) and Kachchh (over 70%).
  • Kachchh, Jamnagar, Porbandar, Junagadh, Amreli and Bhavnagar sit in the top damage class for wind and cyclone.
• Warning:
  • IMD’s four-stage warning runs: pre-cyclone watch (72 hours ahead), cyclone alert (48 hours, yellow), cyclone warning (24 hours, orange) and post-landfall outlook (12 hours, red).
  • It draws on INSAT-3D/3DR satellites, a coastal network of Doppler weather radars, numerical models, surface and upper-air stations, a world network that reaches the Arctic and Antarctic, ships and commercial aircraft.
  • Detection rests on three parameters: fall in pressure, rise in wind speed and the track. Islands are valuable watch points, and satellite radiometers in visible and infra-red bands image the cloud by day and night.
  • Three Area Cyclone Warning Centres (Chennai, Mumbai, Kolkata) and four Cyclone Warning Centres (Ahmedabad, Thiruvananthapuram, Visakhapatnam, Bhubaneswar) issue warnings. New Delhi issues advisories to neighbouring countries.
  • Warnings go out by radio, television, police wireless and the satellite-based Cyclone Warning Dissemination System and Disaster Warning System.
  • Older practice gave at least 48 hours’ warning, with a 12-hour track forecast still imprecise. Warnings allow ports to close, fishing to stop, people to move, and food, water and shelter to be stocked.
• Response:
  • Odisha’s model of early warning, cyclone shelters (under the National Cyclone Risk Mitigation Project) and mass evacuation cut deaths from about 10,000 in 1999 to under 50 in Phailin (2013); over a million people were evacuated before Fani (2019).
• Buffers: mangroves such as the Sundarbans and Bhitarkanika reduce wind and surge damage.
• Mitigation:
  • Cyclone shelters: community buildings such as schools and hospitals, on raised ground, with their short side or a circular plan facing the wind and earth berms and green belts in front. Odisha built many after 1999.
  • Engineered housing: avoid thatched kachcha houses, build on stilts or mounds, anchor roofs and avoid large overhangs.
  • Coastal shelter belts: deep-rooted trees planted in rows cut the force of the wind, and also hold back soil erosion and sand drift.
  • Flood measures, since rain floods follow every major landfall.
• Post-disaster assistance: evacuation, shelter, search and rescue, medical aid, food and water, water purification, disease surveillance, reopening of roads and communications, debris clearance, damage assessment and seed for replanting.

Recent Trends

• Climate change:
  • The IPCC Sixth Assessment Report (2021) finds that the share of major (Category 3–5) cyclones has likely risen over four decades.
  • It projects more intense storms with heavier rain and higher surges on a higher sea, though the total number may not rise.
• Arabian Sea:
  • Comparing 2001–2019 with 1982–2000, the study by Medha Deshpande, Roxy Mathew Koll and colleagues (2021) found a 52% rise in Arabian Sea cyclones, a 150% rise in very severe cyclones, and a near-tripling of their energy.
  • Over the same period, Bay of Bengal cyclones fell by 8%.
• Recent Indian cyclones:
  • Amphan (2020), a super cyclonic storm, hit the Sundarbans; Tauktae (2021) struck Saurashtra near Diu and Yaas (2021) hit Balasore.
  • Biparjoy (2023) lasted 13 days 3 hours, the longest in the Arabian Sea since 1977, before landfall near Jakhau, Kachchh.
  • Asna (August 2024) was rare: it formed over land in Kachchh and then intensified over the Arabian Sea.
  • Remal (2024) hit West Bengal–Bangladesh, Dana (2024) Odisha’s Bhitarkanika coast, Fengal (2024) Puducherry and Montha (2025) Narsapur, Andhra Pradesh; Ditwah (2025) devastated Sri Lanka.
  • Other storms of 2025: Shakhti (October, Arabian Sea, severe cyclonic storm, dissipated off Oman) and Senyar (November, Strait of Malacca, landfall in Sumatra).
  • Arnab (October 2026), the first named storm of that year, became a very severe cyclonic storm and crossed the Andhra Pradesh and Yanam coasts south-west of Yanam, about 20 km east-north-east of Narsapur, early on 28 October, with winds of about 125–135 km/h.

Tropical and Temperate Cyclones Compared

Basis	Tropical cyclone	Temperate cyclone
Latitude	5°–30°	35°–65°
Origin	Thermal: latent heat over warm sea, in a single air mass	Dynamic: meeting of polar and tropical air masses on the polar front
Fronts	None	Warm, cold and occluded fronts
Core	Warm core	Cold core
Where formed	Only over warm oceans; dies on land	Over land and sea
Size	150–1,000 km	Up to 2,000–3,000 km
Isobars and winds	Circular, steep gradient; winds often above 119 km/h	Elliptical or V-shaped, gentle gradient; winds 30–60 km/h
Movement	East to west, recurving poleward	West to east with the westerlies
Calm centre	Eye	No eye
Rain	Torrential, a few hours	Steady and widespread at the warm front, showers at the cold front, over days
Life	About a week	5–7 days, in families; older accounts up to 15–20 days
Indian example	Bay of Bengal and Arabian Sea cyclones	Western disturbances bringing winter rain and snow

• Both have closed isobars, low centres and cyclonic circulation, and a tropical cyclone that recurves into the westerlies can become a temperate cyclone.

Tropical Cyclone and Tornado

• A tornado is a violently rotating column of air reaching from a thunderstorm to the ground. Changes of wind speed and direction start a horizontal spin inside a storm cell, and rising air tips it upright.
• Ingredients: shear, lift, instability and moisture, with wind shear the most important. Most come from rotating supercell thunderstorms.
• Damage: winds in the funnel may exceed 500 km/h, and pressure near 800 mb at the centre, against about 1013 mb at sea level, can make structures collapse outward.
• Timing: commonest in spring, with a smaller peak in autumn and fewest in winter, and tied to the hour of the day through solar heating.
• Distribution: on every continent except Antarctica, rare in polar regions and infrequent beyond 50° latitude. The United States has the most violent, Canada reports the second largest number, and Bangladesh is the most prone country in the subcontinent.
  • About 1,800 thunderstorms are in progress across the world at any moment.

Basis	Tornado	Tropical cyclone
Scale	A few yards to over a mile wide; winds 40–300+ mph	Hundreds of kilometres across; closed low over warm sea
Rotation	Counter-clockwise in the north, clockwise in the south	Same
Precipitation	Rain	Rain, sleet and hail
Frequency	About 1,200 a year in the USA; the Netherlands has most per unit area	Older count of 10–14 a year in the South Pacific and Indian Ocean
Where	Where cold and warm fronts converge, almost anywhere	Warm tropical seas
India	Rare and weak, in the north-west and north-east	Bay of Bengal and Arabian Sea

• Cyclones of the north-west Pacific that exceed 74 mph are typhoons.