Tropical cyclones: what they are and how they form

Detail

1. What a tropical cyclone is

• A tropical cyclone is a violent storm. It forms over warm tropical oceans. It then moves towards the coasts and brings:
• destructive winds
• very heavy rain

• storm surges (sea water piled up by the wind and pushed onto low coasts)

• It is an intense low: a small area of very low air pressure. Winds rush in towards its centre.

• Where it forms: only between 30°N and 30°S.
• Size: about 500–1,000 km across and 12–14 km high. That is roughly the full height of the troposphere in the tropics.
• A heat engine:
• Warm sea water evaporates. The moist air rises.
• As it rises it cools. The water vapour condenses into cloud.
• Condensation gives out latent heat (heat stored in water vapour and released when the vapour turns back into liquid water).
• This heat warms the core of the storm. Warm air rises faster, so more moist air is pulled in from the sea. The cycle keeps the storm going.
• Result: the storm grows only while it stays over warm sea. Over land or cold water its "fuel" is cut off, so it weakens.

2. "Cyclone": the general word and the regional names

• In meteorology, cyclone is also the general word for any intense low whose winds spiral inward:
• anticlockwise in the Northern Hemisphere

• clockwise in the Southern Hemisphere

• Class 7 view: a cyclone is an intense low that draws in moist sea air. This is why cyclones bring heavy rain to coastal India.

Regional 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

• IMD scale for the North Indian Ocean (by maximum sustained surface wind speed) [9][10]:

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) [9][10].

3. Conditions for tropical cyclone formation

Class 11 Ch 9 lists five conditions and Class 11 Ch 6 lists four. Taken together:

(a) A large, warm sea surface

• The sea must be warmer than about 26.5°C, and this warm water must reach down about 50 m [4]. (NCERT: above 27°C.)
• The 26.5°C threshold was first noted by Erik Palmén in 1948 [4].
• Why it matters:
• Above this temperature the air is unstable enough for deep convection (warm, moist air rising strongly to form tall thunderclouds).

• Below it the air is too stable, so few thunderstorms form [4].

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

(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.
• Why it matters:
• Without this turning, air would flow straight into the low.

• The low would "fill up", and no spinning storm would form.

• Coriolis force is zero at the equator and grows towards the poles. So no tropical cyclone forms within about 0–5° of the equator.

(c) Weak vertical wind shear (explained in section 4)

• Cyclones fail to form when the shear between the lower and upper troposphere (850–200 hPa) is more than about 20–25 knots [5].

(d) A pre-existing weak low or low-level cyclonic circulation

• A storm needs a "seed": an existing weak low or an area of spinning low-level winds.
• Examples include 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 in the troposphere.
• Why it matters:
• It carries away the air that rises in the storm.
• So surface pressure keeps falling, and more air keeps flowing in at the bottom.

(f) An unstable troposphere (added in Class 11 Ch 6)

• In an unstable atmosphere, 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 mid-level air weakens rising clouds.
• Formation peaks in late summer and autumn, when the seas are warmest.

4. Vertical wind shear

• Vertical wind shear is the change in wind speed or wind direction with height.
• Weak shear:
• Winds at all levels move the storm together.
• Its warm core (the warm central column of air) can build straight up.

• A deep, upright vortex forms and grows [5].

• Strong shear:

• Upper winds blow the top of the storm away from its base.
• The storm tilts and is torn apart.

• The heat released by condensation no longer sits over the low-level centre [5].

• The Indian case: why there are few cyclones during the south-west monsoon (June–September)

• Near the surface, strong monsoon westerlies blow over the North Indian Ocean.
• High up, the Tropical Easterly Jet blows from the east. A jet stream is a narrow belt of very fast winds high in the troposphere.

• The opposite directions create strong shear, so few cyclones form. Most monsoon systems stay as depressions.

• Result: India's cyclone season has two peaks (it is bimodal):

Season	Months	Shear	Cyclone activity
Pre-monsoon	April–May	Weak	Peak 1 (e.g. Amphan, May 2020) [8]
South-west monsoon	June–September	Strong	Mostly depressions
Post-monsoon	October–December	Weak	Peak 2 (the main peak in the Bay of Bengal)

5. Rapid intensification

• Rapid intensification (RI) means the maximum sustained winds of a tropical cyclone 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

• 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.

• Examples:

• Ockhi (2017): Lakshadweep Sea. It strengthened quickly close to the Kerala and Tamil Nadu coasts.
• Tauktae (2021): Arabian Sea, off India's west coast.
• Amphan (2020): Bay of Bengal. It was the first super cyclone of this century over the Bay of Bengal. Research by IMD scientists studied its RI and found that rings of convection, seen in 37 GHz microwave satellite images, appeared before the RI began [7]. It crossed the West Bengal coast across the Sundarbans as a Very Severe Cyclonic Storm; its life ran from 16–21 May 2020 [8].

• Hurricane Otis (2023): eastern Pacific. It struck Acapulco, Mexico, after an unexpected RI.

• Link to climate change: as oceans warm and the warm layer gets deeper, RI is expected to become more frequent. The Arabian Sea is a growing concern (Ockhi, Tauktae).

Prelims Hooks

• Tropical cyclones form only 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. This is why a cyclone weakens after landfall.
• Sea temperature threshold: 26.5°C down to about 50 m (NCERT: 27°C) [4].
• Strong vertical wind shear stops cyclones from forming. This explains the low cyclone count in the south-west monsoon months and the two-peak (April–May, October–December) season in the North Indian Ocean.
• Rapid intensification = a wind rise of ≥30 knots (≈55 km/h) in 24 hours [2].
• IMD Super Cyclonic Storm = winds of ≥120 knots (≥222 km/h). A cyclone is named from Cyclonic Storm stage (≥34 knots) [9].
• 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 [7][8].

Mains Points

• Formation needs several conditions at once: warm deep sea, Coriolis force, weak shear, a pre-existing low, upper-level divergence and an unstable atmosphere. This explains:
• where cyclones form: not at the equator; mainly over warm oceans in the tropics
• when they form: late summer and autumn; in India, pre- and post-monsoon

• 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).

• RI shortens lead time. This calls for better ocean observations (buoys, Argo floats), satellite microwave monitoring [7] and impact-based warnings.

• Disaster management: the IMD's graded classification [9] triggers graded warnings to coastal states. Storm surge on the low, flat deltas of Odisha and West Bengal (Sundarbans) means evacuation and cyclone shelters matter most.