Tropical cyclone

Also called: Hurricane, Typhoon, Willy-willy · Topic: Air Masses, Fronts, Cyclones and Thunderstorms · NCERT: Class 11, Ch 4 "Climate"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"; Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"

Meaning

A tropical cyclone is a violent storm that forms over warm tropical oceans between 30°N and 30°S. It is an intense low (a small area of very low air pressure), and strong winds spiral in towards its centre. It is about 500–1,000 km across and 12–14 km high. It gets its energy from latent heat (heat stored in water vapour and released when the vapour turns back into liquid water), and it weakens once it moves over land or cold water.

It matters because it hits coasts with destructive winds, very heavy rain and storm surges (sea water piled up by the wind and pushed onto low coasts). India's east coast, especially the low deltas of Odisha and West Bengal, faces this danger again and again.

Explanation

How it works: a heat engine

• Cause: warm sea water evaporates
• Moist air rises from the sea surface.

• As it rises, it cools. Its water vapour condenses into cloud.

• Process: latent heat feeds the storm

• Condensation gives out latent heat. This heat warms the core of the storm.
• Warm air rises faster, so more moist air is pulled in from the sea.

• The cycle repeats and keeps the storm going.

• Result: it lives only over warm sea

• At landfall (when the storm reaches the coast), or over cold water, its "fuel" of moist air is cut off.

• So it weakens and dies out.

• Spin direction: winds spiral inward anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

• Height: 12–14 km is roughly the full height of the troposphere (the lowest layer of the atmosphere) in the tropics.

Conditions needed for formation

All of these must be present at the same time:

• (a) A large, warm sea surface
• The sea must be warmer than about 26.5°C, and the warm water must go down about 50 m [4]. (NCERT gives above 27°C.)
• Erik Palmén first noted the 26.5°C threshold in 1948 [4].
• Above this temperature, the air is unstable enough for deep convection (warm, moist air rising strongly to form tall thunderclouds). Below it, few thunderstorms form [4].

• The warm layer must be deep. The storm churns up the sea. If the warm layer is thin, cold water comes up and cuts off the energy.

• (b) Enough Coriolis force

• Coriolis force is the turning effect of the earth's rotation on moving air. It bends winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• Without it, air would flow straight into the low and "fill it up". No spinning storm would form.

• It is zero at the equator. So no tropical cyclone forms within about 0–5° of the equator.

• (c) Weak vertical wind shear

• Vertical wind shear is the change in wind speed or direction with height.
• Cyclones fail to form when the shear between the lower and upper troposphere (850–200 hPa) is more than about 20–25 knots [5].
• Weak shear: the storm's warm core (the warm central column of air) builds straight up into a deep, upright vortex [5].

• Strong shear: upper winds blow the top away from the base. The storm tilts and is torn apart [5].

• (d) A pre-existing weak low

• The storm needs a "seed": a weak low or an area of spinning low-level winds.

• Examples: easterly waves, or lows along the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the trade winds of the two hemispheres meet).

• (e) Upper-level divergence

• Divergence means air spreading out. Here it happens high up.

• It carries away the rising air, so surface pressure keeps falling and more air flows in at the bottom.

• (f) An unstable troposphere (from Class 11 Ch 6)

• Rising air stays warmer than the air around it. So it keeps rising and forms tall cumulonimbus clouds.

• Beyond NCERT: Gray's (1968) genesis parameters add moist air in the middle troposphere. Dry air at mid-levels weakens the rising clouds.

• Season: formation peaks in late summer and autumn, when the seas are warmest.

Classification and naming (IMD, North Indian Ocean)

IMD groups storms by maximum sustained surface wind speed [8][9]:

IMD category	Wind (knots)	Wind (km/h)
Depression	17–27	31–49
Deep Depression	28–33	50–61
Cyclonic Storm	34–47	62–88
Severe Cyclonic Storm	48–63	89–117
Very Severe Cyclonic Storm	64–89	118–166
Extremely Severe Cyclonic Storm	90–119	167–221
Super Cyclonic Storm	≥120	≥222

• A disturbance gets a name only when it reaches Cyclonic Storm strength (34 knots or more) [8][9].

Rapid intensification

• Rapid intensification (RI) means maximum sustained winds rise by at least 30 knots (about 55 km/h) in 24 hours (NOAA's National Hurricane Center definition) [2][3].
• Why it happens: very warm and deep warm water, weak shear and a moist middle troposphere.
• Why it is dangerous:
• A weak storm can become a severe one in a single day.
• Warning and evacuation time shrinks.

• RI is still among the hardest things to forecast.

• Link to climate change: as oceans warm and the warm layer gets deeper, RI is expected to happen more often.

Where it occurs

• World belt: between 30°N and 30°S, over warm oceans, but not within 0–5° of the equator.
• Regional names:

Name	Region
Cyclones	Indian Ocean (Bay of Bengal, Arabian Sea)
Hurricanes	Atlantic Ocean and eastern Pacific
Typhoons	Western Pacific and South China Sea
Willy-willies	North-western Australia

• India: a season with two peaks (bimodal)
• Pre-monsoon (April–May): shear is weak, so this is Peak 1 (e.g. Amphan, May 2020) [7].
• South-west monsoon (June–September): monsoon westerlies blow near the surface, while the Tropical Easterly Jet blows high up. A jet stream is a narrow belt of very fast winds high in the troposphere. Winds blowing in opposite directions create strong shear, so most systems stay as depressions.

• Post-monsoon (October–December): shear is weak, so this is Peak 2, the main peak in the Bay of Bengal.

• Named examples:

• Amphan (2020), Bay of Bengal: the first super cyclone of this century over the Bay of Bengal. IMD scientists found rings of convection in 37 GHz microwave satellite images before its RI began [6]. It crossed the West Bengal coast across the Sundarbans as a Very Severe Cyclonic Storm. Its life ran from 16–21 May 2020 [7].
• Ockhi (2017), Lakshadweep Sea: strengthened quickly close to the Kerala and Tamil Nadu coasts.
• Tauktae (2021), Arabian Sea: off India's west coast.
• Fani (2019) and Biparjoy (2023): recent cyclones in India.
• Hurricane Otis (2023), eastern Pacific: struck Acapulco, Mexico, after an unexpected RI.

Don't confuse with

• Temperate (extra-tropical) cyclone: it forms in the mid-latitudes along fronts (boundaries where warm and cold air masses meet). It does not depend on warm sea water. A tropical cyclone has no fronts and runs on latent heat from a warm ocean.
• Anticyclone: a high-pressure system with winds blowing outward, bringing calm, dry weather. A tropical cyclone is an intense low with winds spiralling inward.
• Depression (IMD): a weaker stage at 17–27 knots, with no name. A storm is named only from Cyclonic Storm stage (≥34 knots) [8].
• Hurricane / Typhoon / Willy-willy: these are not different storms. They are regional names for the same phenomenon. Only the ocean region changes.

Prelims Hooks

• Tropical cyclones form between 30°N and 30°S but not within 0–5° of the equator, because Coriolis force there is almost zero.
• Typhoon: western Pacific and South China Sea. Hurricane: Atlantic and eastern Pacific. Willy-willy: north-western Australia. Cyclone: Indian Ocean.
• Energy source: latent heat of condensation, which is why a cyclone weakens after landfall. Sea temperature threshold: 26.5°C down to about 50 m (NCERT: 27°C) [4].
• Rapid intensification = a wind rise of ≥30 knots (≈55 km/h) in 24 hours [2]. IMD Super Cyclonic Storm = ≥120 knots (≥222 km/h) [8].
• Trap: cyclones spin anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
• Amphan (May 2020): first super cyclone of the 21st century in the Bay of Bengal; landfall across the Sundarbans in West Bengal [6][7].

Mains Points

• Formation needs many conditions at once (GS-I): a warm, deep sea, Coriolis force, weak shear, a pre-existing low, upper-level divergence and an unstable atmosphere. This explains:
• where cyclones form: over warm tropical oceans, but not at the equator
• when they form: late summer and autumn; in India, pre- and post-monsoon, because strong monsoon shear blocks them from June to September

• why the Bay of Bengal gets more cyclones than the Arabian Sea

• Rapid intensification and climate change (GS-III, disasters):

• Warmer, deeper warm water makes RI more likely (Ockhi 2017, Tauktae 2021, Amphan 2020). The Arabian Sea is a growing concern.

• RI cuts warning time. India needs better ocean observation (buoys, Argo floats), satellite microwave monitoring [6] and impact-based warnings.

• Disaster management (GS-III):

• IMD's graded classification [8] sets off graded warnings to coastal states.
• On the low, flat deltas of Odisha and West Bengal (Sundarbans), storm surge is the biggest killer. So timely evacuation and cyclone shelters matter most.

Related concepts

• Cyclone
• Conditions for tropical cyclone formation
• Vertical wind shear
• Rapid intensification

Read more

• Tropical cyclones: what they are and how they form